This page is for answers to many possible questions on storm chasing, storms in general, plus any science, mathematics, and technology that can be related to such in any way. I have received many emails concerning my site on storms and I am delighted to provide FAQ answers and definitions like the ones below on-line. I will begin with a glossary of weather, earth science, physics, electronics, and ofcourse, storm chasing terms even including storm chasing "slang". I will then go into detail about the different main types of storms and the methods used to chase them. I will also cite information on chase safety regarding this subject. If you do not wish to scroll up and down this page, try using the "quick link" area on the start of this page to jump right to a letter for the glossary or sections! This information was gathered by myself from many sources mainly through knowledge and experience. I hope the information you find here is helpful.

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STORM CHASING INFORMATION "PRIMER"

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WEATHER AND CHASING TERMINOLOGY

123 RULE - Weighted average of tropical cyclone forecast errors (in nautical miles) based on historical information. This rule implies a "1" for the first forecast period, 24 hours, as a 100-mile error. A "2" is for 48 hours and has a 200-mile error. A "3" is for 72 hours with a 300-mile average error. This rule is used alot in the shipping and marine forecast areas.

34 KNOT RULE - A respective and cautionary rule where ships and marine interests try to avoid the 34-Knot (37 MPH) wind fields of a tropical cyclone. This is the threshold of just under GALE forced (tropical storm forced winds) and should be avoided by any sea going vessels.

3DC0 - Three dimensional unconfirmed mesocyclone. Doppler radar storm attribute assuming presence of a mesocyclone in more than one scan tilt but not having the structure to flag it as "MESO".

3 BODY SCATTER - Radar reflectivity phenomina caused when a radar beam strikes a target in the air, and some energy bounces off that object downward, reflects off the ground, then back off the object, and back to the radar, showing up as false information. This is important because thunderstorms with exceptionally large hail exhibit this effect, due to the shape of the large stones, where a "false" reflectivity band shows up behind the precipitation core (high reflectivity region). This streak is called a HAIL SPIKE.

ABSOLUTE ZERO - A temperature at which there is no heat present in matter. This is 0 degrees K (Kelvin) or -459.67 degrees F (Fahrenheit) or -273.16 degrees C (Celsius). Such cold is very difficult to achieve.

ACCESSORY CLOUD - A cloud or cloud feature that is dependant on a larger weather system for its formation. Beavers tails, wall clouds, and scud are all examples of accessory clouds.

ACCAS - AltoCumulus CAStellanus. A variant of altocumulus that forms in unstable environments where the height of each cloud element is about the same as its width. Often seen with elevated storms or mid level instability that may precede severe thunderstorm development later on.

ACE INDEX - Accumulated Cyclone Energy Index. An averaged measure of the total amount of "energy" released by tropical cyclones based on their wind speeds and duration during the life cycle of each cyclone. Very busy tropical seasons, such as 1995 and 2005 in the Atlantic have very high ACE indices while quieter seasons such as 1983 and 1997 have a lower ACE index.

ADO - Atlantic Decadal Ocillation. A change in the ocean currents in the North Atlantic Ocean, occuring about every ten years or so, that influences other large scale anomalies such as rainfall in nearby continents and general tropical cyclone activity.

ADVECTION - Transportation of clouds or one type of weather system to another place by prevailing winds or steering currents. For example, advection makes it possible for cold Canadian air to move into the tropics, as with a strong winter cold front. Specifically Cold Air Advection (CAA) or Warm Air Advection (WAA) is also used for this entry.

AEROLOGY - A branch of meteorology dealing with the FREE ATMOSPHERE, that is, the upper air dynamics at least 1 mile above the earth's surface. This concerns jet stream and wind patterns aloft and is very important for weather forecasting.

AEROSOL - A suspension of solid or liquid material in a gas (or solid in a liquid) due to small particle or droplet size. Clouds are examples of aerosols (liquid water suspended in air).

AEROSPACE - The field of navigation in the atmosphere, including all operations of aircraft.

AEROSPHERE - Another name for atmosphere, but much less widely used. See AEROSPACE.

AEROSTAT - A stationary observation platform suspended in the air, commonly as a tethered balloon.

AGL - Above Ground Level. Commonly used for altitude.

AIR - A mixture of gases that makes up most of the atmosphere around the earth. It is roughly 78% nitrogen, 21% oxygen, and 1% argon and other trace gases.

ALOFT - Above the ground in the atmosphere.

ALTITUDE - Height above the surface of the earth or elevation. Can be measured as AGL or MSL. If pressure units are used, it is called PRESSURE ALTITUDE, for example, the 500 MB level (18,500 feet above MSL).

ALTO - Prefix applied to cloud types to designate their heights. Usually mid level clouds. Common usage is ALTOSTRATUS and ALTOCUMULUS. "Alto" is a Latin word meaning tall or high.

ALTOCUMULUS - Mid level convective clouds resembling cumulus. Composed of water droplets and / or ice crystals. Common in unstable atmospheres. Abbreviated as AC.

ALTOSTRATUS - Mid level layered clouds composed of either water droplets and / or ice crystals. Common around thunderstorm anvils and blowoff or ahead of an advancing low pressure system. May produce virga. Abbreviation is AS.

AMPERE - A unit of the amount of electrical current (flow of electrons). This is about 6.28 x 10^18 electrons flowing through a conductor in a seconds time. Abbreviated as AMPS.

AMPLIFICATION - The strengthening of a low pressure trough (or of a high pressure ridge), usually along the axis of that trough (or ridge). This is the exact opposite of DEAMPLIFICATION.

ANCHOR CELL - A thunderstorm cell just below a break in a storm cluster, line, or bow echo. Cells in this region can behave like supercells.

ANEMOMETER - A weather instrument for measuring wind speed. There are many types, such as the common CUP ANEMOMETER and digital anemometer.

ANOMYLOUS PROPOGATION - False reflectivity showing up on a radar image caused by refraction of the radar beam in the atmospshere. Abbreviated as AP.

ANTICYCLONE - The opposite of a cyclone. Usually associated with a high-pressure system. ANTICYCLONIC rotation is any rotation opposite to that of a cyclone. This rotation is clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere.

ANNULAR HURRICANE - Also may be "ANNULAR TYPHOON" or "ANNULAR CYCLONE" depending on location. An intense tropical cyclone with one main dominant eyewall around a relatively large eye, and with limited convection (such as feeder bands) outside that main eyewall. These storms are often very intense (even category-5 strength) and do not change or fluctuate in intensity as readily since there is only the one main eyewall. Eyewall replacement does not occur in an annular storm unless it changes to a storm with concentric eyewalls first. Great examples of annular hurricanes in the Atlantic were hurricane Isabel in 2003 and hurricane Katrina in 2005. See also EYEWALL.

ANVIL - The top of a towering cumulonimbus (thunderstorm) cloud reaching the equilibrium altitude where the air ceases vertical motion and flattens out in a formation very similar to a black smith's anvil. A spreading anvil is also called a CROWN.

ANVIL ROLLOVER - The upwind side of a thunderstorm anvil rolling into the upper level wind flow. Sometimes called a BACK SHEARED ANVIL.

ALGORITHM - A series of steps which lead to the execution of a task. Commonly used with computer based software and modeling systems. The WSR-88D uses software algorithms for radar data processing.

ARCTIC - Polar geographical region of Earth at latitudes greater than 66.5 degrees south. This region also experiences about 6 months of night followed by six months of day due to the Earth's tilt of 23.5 degrees. The middle of the 6 month "day" in this region is in late December.

ARCTIC CIRCLE - The latitude line of 66.5 south latitude.

AOA - Acronym used for "At Or Above" in abbreviated weather message products.

APPROACHING - Term used to describe an intensifying thunderstorm based on warning and intensity thresholds. Commonly used as APPROACHING STRONG (or SEVERE) limits.

ARCHAMEDES PRINCIPLE - The effect of a displaced volume of gas or liquid causing an upward buoyancy of the displacing object. A great example is that a boat floats because of the amount of water displaced produces upward lift (buoyancy), even though the boat maybe much heavier than water.

ARCTIC - Polar geographical region of Earth at latitudes greater than 66.5 degrees north. This region also experiences about 6 months of night followed by six months of day ("midnight sun") due to the Earth's tilt of 23.5 degrees. The middle of the 6 month "day" in this region is in late June.

ARCTIC CIRCLE - The latitude line of 66.5 north latitude.

ARCUS - The "left over" low-level clouds along the outflow regions of a dissipated thunderstorm.

ARMSTRONGS LINE - The altitude at which the critical pressure of body fluids at 98.6 degrees F is reached, normally around 63,000 feet above sea level (MSL). At and above this altitude, blood may boil at body temperature.

ASPECT ROLLS - Horizontally rotating air currents along a wind axis. See also HCR.

ATTM - Abbreviation for "At This Time" in abbreviated weather message products.

AOA - Acronym used for "At Or Above" in abbreviated weather message products. ATTITUDE - The position and orientation of an object in the air or space. Commonly applied to headings of aircraft and orientations of satellites in space.

AWIPS - Advanced Weather Information Processing System. New weather instrumentation and computers developed in the late 1980's for improved data aqcuisition and forecasting.

AXIS - The axis or center of a trough of low pressure, which can be followed along the trough to its ends where the trough can no longer be discerned. For example, a low pressure area can be over Canada but the trough axis, or AXIS TROUGH can extend southward into the southern US as the entire wind flow responds to the low pressure disturbance.

AZIMUTH - The degree measure of direction from 0 to 360 degrees in a clockwise direction where 0 or 360 degrees is due north and 180 degrees is due south. due north

BACK BUILDING - Development of a thunderstorm on the upwind side of the storm with storm propagation in the direction opposite to that of the steering winds in which it is embedded. Commonly associated with LEFT MOVING storms. May also occur along the flanking line of a thunderstorm.

BAG - Slang term for "catching" a storm event while storm chasing, such as a tornado.

BAIL OUT - To intentionally abort or end a storm chase.

BALL LIGHTNING - A fireball (or plasma ball) created from a bolt of lightning. This is rare and not much is known about this type of lightning.

BALLOON - An expandable structure filled with a partial pressure of a gas. If the gas is lighter than air, such as helium or hot air, the gas envelope will provide lift and carry a payload. Balloons are very important for weather research and upper-atmosphere observations and weather forecasting. Large balloons can also carry people.

BALLUTE - A combination of a balloon and parachute. This special balloon structure is normally used to slow high speed flight or free-fall like a parachute and function as a balloon after (if filled with a ligher-than-air gas). See also PARACHUTE.

BAM MODEL - Beta and Advection Model. Modeling for storm tracks and intensity based on winds and temperatures in several layers of the atmosphere. Variants of this model are the VICBAR MODEL (nested baratropic model) and the LBAR MODEN (Limited-Area Barotropic Model).

BARBER POLE - A corkscrew pattern on a rotating supercell updraft tower. Sometimes this is called a CORKSCREW.

BAROCLINIC - Processes which involve changes in winds, air pressure, or temperature for cyclone genesis and support. Not to be confused with LATENT HEAT, which supports convective systems such as tropical cyclones.

BAROCLINIC LEAF - A cloud formation observed on a satellite picture resembling a leaf. Commonly used to identify possible cyclogenesis.

BAROCLINIC ZONE - A region where there is a large horizontal change in temperature, humidity, and or pressure, such as across a frontal zone. Baroclinic zones often form along regions of large temperature and humidity change and may precede the formation of a frontal system if those differences become sharper. The opposite of BAROCLINIC is BAROTROPHIC, where there is very little or no temperature variations across the horizontal distance.

BAROMETER - Instrument used to measure the pressure of the air. Common units are MILLIBARS (MB) and INCHES of mercury (IN/Hg).

BAROMETRIC PRESSURE - The air pressure at a given point in the atmosphere. Normal sea level pressure is 1016 MB or 30.00 inches or mercury. Barometric pressure also can be used to measure altitude or elevation above sea level, where 500 MB is about 18,500 feet.

BARREL - A visible funnel type tornado that has a similar with at the top near the cloud base as at the bottom near the ground. May also be called a TUBE.

BEACON - A storm which is usually isolated or significant, attracting chasers from a wide area. Causes a chaser convergence.

BEARS CAGE - The rain-free region on the backside of a supercell thunderstorm, often wrapped with the rain hook, and an extremely dangerous place to be chasing into because the constant threat of a tornado. In some HP storms, this region is completely enclosed with precipitation. The "bear" is the analogy of the "tornado" which is possible in this region of the storm.

BEAUFORT SCALE - A wind scale and flag system used for sustained wind speeds. Scale is mainly from 0 to 12. This is a rather nostalgic scale used by mariners for many years.

BEAVERS TAIL - A cloud feature on the inflow side of an active thunderstorm, usually a supercell, which resembles the tail of a beaver. This is a type of inflow band or accessory cloud.

BERMUDA TRIANGLE - An imaginary triangle formed by lines stretching from Miami, Florida, Bermuda, Puerto Rico, and back to Miami. This area has a very high occurance of tropical cyclones and powerful oceanic storms, and is very infamous for many lost ships and aircraft.

BERNOULLIS PRINCIPLE - Important aerodynamic and hydrodynamic property where the flow of a fluid caused regions of high and low pressure to develop around an object immersed in it. This property causes destructive forces in cases of strong winds. It also can be destructive in the case of fast moving water, as in storm surges and floods.

BLACKOUT - Loss of electrical power to lights, appliances, buildings, or entire cities. Storms often knock out power causing blackouts. A BROWNOUT is when the electrical power drops but does not go completely out. Loss of conciousness can also be called a BLACKOUT.

BLIP - A point or blotch indicating an object on a radar display screen.

BLOCKING - The inhibition of the forward progression of one weather system by another. Commonly a high pressure system that blocks or deflects the motion of an advancing low pressure area is called a BLOCKING HIGH.

BLOWOFF - The anvil and cloud debris from active (or previously active) thunderstorms being blown downwind ahead of the storm system by upper level winds. Composed of mid and especially high level clouds such as cirrus.

BOBS ROAD - A slang term for an unpaved road through rural areas, usually unnamed.

BOILING - The rapid transformation of a liquid into a gas. Fresh water boils at about 212 degrees (F) or 100 degrees (C).

BOMB - A low pressure area undergoing rapid or explosive intensification (more than 1 MB per hour). The term is most commonly applied to rapidly intensifying hurricanes and extratropical storms.

BOUNDARY LAYER - A region of turbulence and drag created between a surface and a fluid flowing past it (slipstream). Such phenomena exist with winds aloft and the earth's surface. Sometimes this is called a FRICTION LAYER or GEOTROPH. This layer often causes winds to decrease especially at night in the lowest levels of the atmosphere, with the stronger winds only a slight distance above the layer.

BOW - A radar image showing a region of a squall line or outflow boundary that pushes ahead of the rest of the system, forming a bow-shaped feature. Weather near the center of this bow pattern is dominated by strong and damaging outflow winds. A series of bows along a squall line is called a LINE ECHO WAVE PATTERN or LEWP. May also be called a BOW OUT.

BOX - A weather watch box (areal region). This can be a tornado watch or severe thunderstorm watch indicating conditions are favorable for these types of storms. It does not mean that are occurring or WILL occur, it just means the atmosphere has the dynamics for producing them if the right conditions occur. See also WATCH. Often abbreviated WW.

BR - Base Reflectivity. The amount of radar reflectivity from a target, such as a thunderstorm. Unit of measure is a DBZ.

BROAD - General term applied to the size of a weather system. Often used as BROAD LOW with tropical disturbances that have a weak but closed circulation.

BTU - An english system based unit of energy, based on the amount of energy to raise one pound of water one degree Fahrenheit. A BTU is equal to about 1,055 Joules of energy. See also JOULE.

BULK RICHARDSON NUMBER - A number indicating converctive storm types based on CAPE values and vertical wind shear. This number is critical in determining the behavior and evolution of such convective storms (such as supercell vs multicell storms). The number is derived by dividing CAPE by the half-square of the vertical wind shear in the lower 6 km of the atmosphere. The number is often abbreviated as BRN. Values from 10 to 45 suggest supercells, less than 10 suggests storms that can evolve into supercells, and greater than 50 suggests multicell storms.

BUOY - An anchored or floating marker, navigation, or instrument package in a body of water. DATA BUOYS relay important aspects on water temperature and wave information in oceans.

BUOYANCY - Lift produced when either an object displaces fluid (gas or liquid) or when a parcel is less dense than its surrounding environment. This rule is VERY important in developing convection. If the force is negative, or opposite of lift, the term NEGATIVE BUOYANCY may be used.

BUST - A failed storm chase. A storm chase aborted due to vehicle failure, traffic problems, roadblocks, or even because the conditions in the atmosphere did not organize and "nothing" happened.

BWER - Bounded Weak Echo Region. Radar feature showing a region on the inflow side of a thunderstorm, especially a supercell, where the RFD and FFD precipitation surrounds the area forming an "opening". Common on the southeast side of such storms. Visually this region appears as the NOTCH or BEARS CAGE in tornadic storms. May also be called a VAULT.

CALIFORNIA CURRENT - A cold ocean current in the North Pacific Ocean that flows southward from near the Alaskan coast south to just off California. Influences extratropical storm systems in the area.

CALORIE - A metric system unit of energy, related to heat. It is based on the amount of energy required to raise the temperature of one gram of pure water one degree Celsius. A calorie also is equal to 4.184 Joules. See also JOULE.

CAMCORDER - A video camera and video tape recorder in a single, stand-alone unit.

CAP - A dry and warmer air layer at mid altitudes that prevents any convection from rising above that region. A cap can prevent development of thunderstorms, however, if the cap is removed or overcome, especially late in the day when CAPE values are highest, thunderstorms may develop explosively. A cap is sometimes called a LID.

CAP CLOUDS - Convective clouds developing in neutral or de-stabilizing air over a pre-existing thermal from a forest fire or other similar heat source.

CAPACITANCE - Ability to hold a static charge of electricity (stored electrons not in motion) at a certain voltage. Usually measured in FARADS. Electrical components that store such charges are called CAPACITORS. See also FARAD.

CAPE - Convective Available Potential Energy. A measure of computed heat energy in the lower atmosphere available for convection. Often measured in Joules per Kg. Solar heating usually increases the CAPE values during the day.

CAPROCK - A plateau (enscarpment) in west central Texas extending into eastern New Mexico. This is also an important orographic factor for storm development. It is also a beautiful place to storm chase, especially on the edge of the caprock, where there are cliffs and canyons.

CEILING - The height from the ground to the cloud base. Commonly used in the aviation community.

CELSIUS - A metric unit of a degree for measurement of temperature, based on the boiling point of water of 100 degrees (Celsius). Also called CENTIGRADE.

CELL - A precipitation element, usually associated with convective precipitation such as thunderstorms. The term is also applied to an individual storm or weather system component as a low-pressure (or high-pressure) cell.

CELLULAR - A communication network allowing wireless portable phones to be used. In meteorology, term applied to convection where air parcels rise and are separated by non-rising air. A cumulus field is a result of CELLULAR convection.

CG - Acronym for Cloud to Ground lightning. Lightning striking the ground, water, or surface structure such as a building or antenna are all considered CG's.

CHAFF - Radar reflecting material in the form of shredded "Christmas tree like tinsel" or metallic material often used to test radar equipment or create a false radar blip. Often used and deployed by military operations.

CHAIN LIGHTNING - A common form of lightning resembling zig-zag or forked patterns. Most lightning bolts are of this type. Cloud to ground is the most dangerous type of this lightning. Also called ZIG-ZAG or FORKED lightning.

CHASE POSITION - The area around or nearby a storm where storm chasers perfer to make their observations. For most tornado-producing supercell thunderstorms moving towards the northeast, the best chase position is about 2 miles southeast of the storm core and therefore the safest for observing a tornado and avoiding large hail, winds, etc. Not to be confused with the chase TARGET AREA.

CHASE TEAM - A group of storm chasers working together.

CHASE VEHICLE - Car or truck used in a storm chase. The type of storm here is irrelevant. The type of vehicle also does not have to be road based. The Air Force hurricane hunters use a WC-130 aircraft as their "chase vehicle".

CHASER CONVERGENCE - A storm chaser or chase team coming across another chaser or chase team. This can be 2 people or hundreds of chasers on a storm chase in a given area, such as around a big supercell event.

CHINOOK - A warm and dry downslope wind on the lee of a mountain range. May reach high speeds under the right conditions. See also FOEHN.

CIGAR - A cigar shaped inflow cloud, usually the shape of a "beaver's tail" but viewed from below. Can also be used for the shape of a tornado that resembles a cigar, where the tornado with is less than its height, like a "stove pipe" tornado but its bottom portion "rounded".

CIN - Convective INhibition. The amount of energy required to overcome forces inhibiting free convection in the atmosphere. Like CAPE, the units are often measured in Joules per KG.

CIRRUS - High level clouds composed mainly of ice crystals. There are many cirrus type clouds such as CIRROSTRATUS and CIRROCUMULUS. The word "cirrus" comes from the Greek word meaning wispy. Cirrus clouds are common around low pressure systems where abundant moisture exists at high altitudes or after convective clouds dissipate at these levels. A CIRRUS SHIELD is commonly used for the tops of tropical systems reaching great heights as wells as with blowoff active thunderstorm anvils. Abbreviations for the three main variants are CI, CS, and CC.

CISK - Conditional (or Convective) Instability of a Second Kind. Organized cellular convection leading to the development of a convective vortex (such as a tropical cyclone or polar low).

CLASSIC SUPERCELL - A supercell with "text-book" like features such as a well-defined hook echo on a radar scope and the precipitation core downwind of the wall cloud and rain free storm base. Classic supercell tornadoes are often un-obstructed from view if being viewed from the backside of the storm.

CLEAR AIR MODE - Weather radar mode operating with higher gain for reflected signal. Often a radar operating in this mode displays information on its screen, which is actually smoke, dust, even birds and insects.

CLEAR AIR TURBULENCE - Air turbulence with little or no clouds experienced by a penetrating aircraft. Abbreviated as CAT.

CLIPER MODEL - CLImatology and PERsistence model. A statistical model for determining storm tracks based on linear regression and statistical track data.

CLOSED CIRCULATION - A weather system having a circulation forming a complete circle. To be a true cyclone and not a trough, a system must have a completely closed circulation. Commonly applied to tropical systems.

CLOUD - A region of suspended droplets of a liquid or very small solid particles in another medium. Commonly seen as a suspension of water droplets in the air (water clouds). See also AEROSOL.

CLOUD SEEDING - Release of silver iodide or dry ice into clouds to trigger precipitation. Often done in drought areas to trigger rainfall.

CLOUD SUCK - Slang aviation term for strong updrafts developing in or under convective clouds, such as cumulonimbus or cumulus clouds. In thunderstorms, cloud suck can be strong enough to lift a glider or parachutist to be lifted to great heights! Cloud the suck effect can also create strong inflow winds beneath the developing updraft at ground level (such as thunderstorm inflow).

COL - A saddle like reqion between two low pressure systems where a shallow pressure gradient exists. This region may also be found when high pressure systems around the two low pressure areas are situated on each side of the col region.

COLLOID - A suspension of very small solid or liquid particles or droplets in a gas (or solid in a liquid). The droplets or particles are much smaller than that of cloud droplets in an aerosol. Haze is an example of a colloid (very tiny solids suspended in air).

COLORADO LOW - An infamous low pressure system that originates in southeastern Colorado. In winter, it often rapidly develops and causes problems for a large portion of the eastern United States. In Spring, it helps with severe weather development in the Plains. Commonly develops from the LEE TROUGH east of the Rocky Mountains. See also LEE TROUGH.

CONDENSATION FUNNEL - The visible pendant cloud of a tornado. This is caused by air pressure and wind speed changes associated with vortex allowing the vapor in the air to condense into water droplets. This region does not have to touch the ground to have a tornado. The circulation is only air and is sometimes completely invisible if a condensation funnel is not present. Sometimes called a VISIBLE FUNNEL.

CONE - The visible funnel of a tornado or funnel cloud which is wide at the top and narrow at the bottom, resembling the flared shape of a tuba or ice-cream cone.

CONFLUENCE - Flow convergence of a fluid stream towards the axis of the stream. Commonly used for convergence environments and upper air analysis. May also be called CONVERGENCE.

CONTRAIL - A visible trail of vapor and / or ice crystals caused by a high-flying aircraft. The hot exhaust from an aircraft engine contains various gases, including water vapor. The extreme cold at high altitudes often cause this water vapor to condense into liquid droplets (or sublime into ice crystals), especially if the air has a high moisture content. The formation and persistance of a contrail depends on the temperature and moisture content of the air. Contrails can form at lower altitudes given the conditions are correct.

CONVECTION - Air rising and falling in the atmosphere due to density changes from heating or cooling. The term more commonly used to signify rising air.

CONVECTIVE CHIMNEY - Term applied to the deep convection associated with a tropical cyclone. This region is commonly found at the core of the system and is characterized by strong updrafts. Thunderstorms in this region can be called HOT TOWERS or PRIMARY ENERGY CELLS. Most of the low pressure associated with such systems is a result of the air being removed through this convection. The hurricane eyewall is also part of the convective chimney.

CONVECTIVE INITIATION - The start of thunderstorm convection. Thunderstorms need a triggering mechanism such as a front or mountain in order to get going.

CONVECTIVE VORTEX - Any vortex or rotating low-pressure region associated with rising air. This is a very general term, covering a category that tornadoes, dust devils, thermal lows, and tropical cyclones all fit into. Sometimes referred to as a CV.

CONVERGENCE - Air that comes together at a given point in the atmosphere. Important for convective initiation.

CORE - The center of a storm system. Thunderstorm precipitation cores usually contain the strongest non-tornadic wind gusts, largest hail, and heaviest rains.

CORE PUNCH - Driving your vehicle through the precipitation core of a thunderstorm. Can also be described as a path through the center of core of any kind of storm. Core punching an active thunderstorm is very dangerous.

CORIOLIS FORCE - Tendency for low pressure systems, as well as high pressure systems, to exibit circulations in the same sense as the rotation of the earth. This is a dominant influence on many large scale storm systems such as tropical cyclones and extratropical systems. The coriolis force is caused by the momentum and inertia a parcel of air experiences when moved across latitudes (North or South). It is weakest at the earth's equator.

COULOMB - A unit of the amount of a stored electrical charge equal to the potential of one AMPERE. This is about 6.28 x 10^18 electrons at rest (like on the plates of a capacitor). See also AMPERE.

COUPLET - Two opposite values (or features) close to one another. Commonly refers to doppler radar signatures where a region of fast-moving particles moving one way is close to a region moving the other way. A TVS signature is a type of velocity based COUPLET. In fluid dynamics, two counter-rotating vortices interacting with each other is calso called a COUPLET.

CREPUSCULAR RAYS - Effect of sunlight shining through clouds, smoke, or haze where the rays and shadows become visible as lines or streaks through the sky. Commonly seen under breaks in clouds where the sun shines through into hazy air.

CRYOGENIC - A substance that is at extremely low temperatures, such as -250 degrees (F) or colder. Liquid nitrogen is used for most scientific applications with cryogenics at -320 degrees (F).

CSI - Acronym for Conditional Symmetric Instability.

CUMILIFORM - Cellular formation of clouds and / or convection. Cumulus type clouds with vertical development such as CUMULUS and CUMULONIMBUS. Derived from Greek and Latin words meaning "heaped-up".

CUMULONIMBUS - A cumulus cloud that has precipitation associated with it. The term CUMULO is Latin derived meaning "heaped up" and NIMBUS is Greek derived word for "rain". Cumulonimbus clouds can occur at any altitude and often vertically developed. They are resposible from anything from a light rain shower to a tremendous supercell thunderstorm. Abbreviation is CB.

CUMULUS - Puffy clouds resembling cotton, associated with moist thermal convection. Towering cumulus is when vertical development of these clouds occurs. Towering cumulus is also called CUMULUS CONGESTUS. The name "cumulus" means heaped or piled up. Flattened cumulus clouds with bases wider than their heights are sometimes called STRATOCUMULUS. Cumulus is abbreviated as CU. Stratocumulus is abbreviated SC.

CURL - A curling formation of dust produced by the edge of thunderstorm outflow. Edges of microbursts, may curl up as the air impacts the ground and spreads out. CURL also is a scientific measure of the "turning" of a fluid similar to vorticity.

CYCLOID - A geometric path consisting of "half circles" with each end of those circles joined at a point or small "loop". A bouncing ball takes the path of a parabolic CYCLOID if thrown with horizontal speed. Often used to describe steep wind waves and seas. A boat (or ship) bouncing in (or jumping) waves also produces a CYCLOID. A HYPERCYCLOID involves small loops at the connecting points of the circles. Common with damage paths caused by sub-vortices within in large tornadoes.

CYCLONE - A rotating region of low atmospheric pressure. In the northern hemisphere, cyclones rotate counter-clockwise. They rotate clockwise in the southern hemisphere. The word has Greek origins.

DBZ - Decibels returned power from a radar beam "echoing" back from a precipitation area. The higher the DBZ value, the heavier the precipitation is at the radar target.

DEEP - Term usually applied to convection. Vertical convection from the lower atmosphere all the way to the upper air. Sometimes called HARD or HOT convection, especially when referring to tropical systems. The term can also be applied to low pressure systems where the deeper the storm, the lower the pressure and more intense it is.

DEEPENING - Term applied to the pressure drop associated with an intensifying low pressure system.

DEFORMATION ZONE - A region where convergence and divergence in the atmosphere along two separate axes. Such regions occur between high and low pressure systems and low pressure col areas.

DENDRITE LIGHTNING - Cloud to cloud flashes of lightning, often resembling webs or nets on the underside of a high altitude cloud base. Common under anvils of dissipated thunderstorms. Sometimes called ANVIL CRAWLER, STIGMA, or SPIDER lightning.

DENVER CYCLONE VORTICITY ZONE - A region of mesoscale low pressure created by the front range of the Rocky Mountains west of Denver, Colorado. Usually a diurnal feature created by the sun heating the ground in the afternoon, and may lead to land-spout type tornado development if moisture is present for convection. Abbreviated as DCVZ.

DEPOSITION - Phase change from a gas directly to a solid, without going through the liquid phase. This is the opposite of SUBLIMATION. Snow flurries sometimes develop in very cold moist air from deposition (water vapor directly to ice). See also SUBLIMATION.

DERECHO - A widespread event of very damaging straight-line winds. Usually associated with large squall lines.

DEVELOPMENT - The formation and strengthening of a storm system. Thunderstorm development is the initiation and building of cumulus and cumulonimbus clouds.

DEVIANT MOTION - Storm motion different from the storm mean motion vector.

DEWPOINT - The temperature at which the air becomes saturated and can no longer hold water, thus condensation or dew occurs. Dewpoints equal to the temperature means the air is saturated. Cloud bases are regions where the dewpoint and temperatures are equal due to altitude. The dewpoint cannot be higher than the temperature. Used to compute RELATIVE HUMIDITY.

DIFLUENCE - Flow separation of a fluid stream away from the axis of the stream, like the water from a showerhead. Difluent jet stream winds over a thunderstorm can cause the storm updraft to strengthen and the storm to become more severe.

DIG - Action of a trough of low pressure to amplify in a southward manner in the northern hemisphere.

DILLOCAM - A device designed by storm chaser Charles Edwards resembling a bowl or "armadillo's shell", into which cameras or weather instrumentation can be placed. The unit is designed to record data from inside a tornado, and has seen an indirect hit with a large tornado during its service. Similar to other devices such as HITPR and the bulkier TOTO.

DISSAPATION - The demise or end of a storm system. Thunderstorm dissipation occurs when downdrafts are dominant in the entire storm cell and the storm quickly disorganizes thereafter.

DISTRAIL - The dissipation of a cloud along a path of a vehicle, such as an aircraft. Air turbulence and hot engine exhaust often cause cloud droplets to evaporate along the path of the craft, forming a distrail. This is the opposite of a CONTRAIL, where condensation is encouraged by the aircraft, and occurs at higher altitudes. Distrails often form at lower altitudes through a deck of clouds.

DISTURBANCE - Another name given to a region of weather in the atmosphere suggesting low pressure, such as a trough.

DIURNAL - An event that occurs daily, such as daytime heating due to solar insolation. Convective systems, such as sea / land breeze development, which develop in the late afternoon and dissipate at night, are DIURNAL events.

DIVERGENCE - Separation of air from a given point in the atmosphere. Common in high-pressure regions.

DOLDRUMS - Very light winds that persist over the equilateral regions of the earth. The normally blow from an Easterly direction.

DOME - An overshooting top of convection past the equilibrium level above the top of a thunderstorm. This is usually the top of the updraft plume. Also used as the name given to a high pressure cell.

DOPPLER INDICATED - A term used to describe a certain weather anomaly that was indicated by using a doppler radar. Doppler indicated tornadoes are sometimes acronymed DITOR (Doppler Indicated TORnado).

DOPPLER RADAR - Radar device measuring a frequency shift in returned radio waves indicating the speed the target is moving toward and away from the antenna. This can return information on motions inside storm clouds, such as supercell thunderstorms.

DOW - Doppler On Wheels. A portable Doppler radar on a truck used in storm research.

DOWNDRAFT - Sinking air in the atmosphere. Commonly associated with precipitation regions of thunderstorms.

DOWNGRADE - A demotion in status of a storm system, usually a tropical system. Example, the hurricane was DOWNGRADED to a tropical storm.

DOWNSLOPE - A wind blowing down an incline. This is from a higher terrain to a lower elevation. Such a wind becomes dry and stable. Chinooks and Foehn winds are downslope winds. See also CHINOOK, FOEHN.

DRAG - Aerodynamic force on an object moving through a fluid medium or from the action of a fluid medium on a stationary object. For example, a tree being blown down by strong winds is damage due to the DRAG force of the moving air against the tree.

DROP SONDE - An instrumentation package dropped from an aircraft to gather information on conditions from the release altitude to the surface. Often used in hurricane research operations to gather wind-drift data, pressure, and temperature in tropical systems.

DROUGHT - A lengthy period with little or no precipitation. Droughts cause a water defecit and can kill plants and wildlife, especially through higher incidences of fires.

DRY ICE - Frozen carbon dioxide gas. Appears as a white solid around -110 (F) that sublimes to a gas at normal atmospheric pressure. The absence of any "liquid" present during this sublimation gives the name "dry ice". Used as a deep-freeze coolant and scientific applications. Dry ice fog is also produced with dry ice placed in tubs of warm water. The carbon dioxide is more than twice as heavy as air and causes the fog to stay close to the ground (this technique used mainly for theatrical purposes).

DRY LINE - Region of drier, and sometimes cooler air from the Southwest US that pushes eastward. If the air to the east of the dryline is moist and unstable, the dryline will trigger thunderstorms. A DRYLINE PUNCH is when a portion of the dryline pushes out into the moist air to the east in response to a low-pressure system.

DRY SLOT - A section or sector of drier air entrained in a low-pressure system. Often seen with subsident air behind an extratropical cyclone.

DSL - Digital Subscriber Loop. A high speed Internet service using fiber optics for the network infrastructure but normal phone lines (copper) between the fiber network and homes / businesses. May also be called ADSL for Asymmetric DSL because upload speed is often less than download.

DUATS - Electronic flight breifing and weather services for the aviation community. Fee-based system provided in conjunction with the FAA.

DUST DEVIL - A small convective vortex of warm dry air, common in hot and dry regions. These are characterized by blowing leaves or dust that reveal the vortex presence. The normally are not very dangerous, except to low level aviation concerns. Dust devels do not require organized convection for their formation. May also be called a DUSTER. Commonly abbreviated as DD.

DUST WHIRL - A cloud of dust and or debris picked up when a tornado is on the ground. Sometimes the visible funnel does not reach the ground, but the presence of the dust whirl often confirms the contact. Called a DEBRIS CLOUD in stronger tornadoes.

DVORAK METHOD - Method developed in the early 1980's using detailed satellite imagery with known scales to compute the estimated wind speeds in tropical systems by the movement of the clouds over a given time-frame.

DYNAMICS - The study of physics where motion and acceleration is involved (work is done).

DYNAMIC FETCH - Wave-generating winds over a body of water in-phase with the movement of that region of winds. See also TRAPPED FETCH.

DYNAMIC PRESSURE - Force created by a fluid medium (gas or liquid) acting on a given surface area. See also BERNOULLIS PRINCIPLE.

EAS - Emergency Alert System. Originally part of civil defense, a government based alert system where emergency broadcasts can be made to the public. This can be used for both weather warnings as well as warfare preparation.

EASTERLY JET - A region of strong winds at the jet stream level but from an easterly direction. This is dominant only in the tropical regions during the summer months when high pressure is present at subtropical latitudes aloft.

EBR - Enhanced Base Reflectivity. See also BR.

ECMF - European Computer Model Forecast.

EL NIÑO - A periodic shifting of warm waters of the Pacific ocean toward the East towards South America. El Niño causes major disruptions in the earth's weather.

ELECTROCUTION - A person or animal being killed by electricity. A direct hit by lightning often causes electrocution by arresting the heart muscles and stopping breathing in the victim.

ELEVATED - Term used to describe higher elevations, such as mountains. With storms, particulary thunderstorms, the term is applied to describe HIGH BASE clouds. Elevated high base thunderstorms are sometimes given an improper slang name by storm chasers as ALTOCUMULONIMBUS. The term also can be used to describe and upper-level system, opposed to a SURFACE BASED weather system.

ELEVATION - Height of a mountain or terrain above sea level or MSL. Also a degree measure from 0 to 90 degrees of an apparent position above the horizon where 0 degrees is the horizon.

ENERGY-HELICITY INDEX - An important instability and directional wind-shear index for determining the severity of thunderstorms based on the CAPE and helicity present in the storm environment. This index, also called EHI, is simply the helicity multiplied by the CAPE and the result divided by 160,000. An EHI of more than 2.0 presents the possibility of supercell storms. An EHI of more that 4.0 presents a risk for significant tornadoes.

ENGLISH SYSTEM - A non-metric system of weights and measures, used widely in the United States in non-medical and non-scientific applications such as manufacturing. Some units include and inch, pound, ounce, gallon, yard, mile, etc. Converting between these units is often cumbersome, opposed to the Metric System, which is based on units of 10. This is a basis for SAE. See also SAE.

ENHANCED F SCALE - New tornado damage rating scale developed in February 2006. It is a newer replacement for the FUJITA PETERSON SCALE used since 1971. The scale is rated EF0 to EF5, but has different wind ratings. EF0 is for 65-85 MPH winds, EF1 is for 66-110 MPH, EF2 is for 111-135 MPH, EF3 is for 136-165 MPH, EF4 is from 166 to 200 MPH, and EF5 is over 200 MPH. These wind speeds are also based on a 3 second gust rather than peak winds. See also FUJITA PETERSON SCALE.

ENHANCED V - Name given to a well defined V NOTCH feature on a radar (or even a satellite) image of an often supercell thunderstorm.

ENSO - El Niño Southern Oscillation. Periodic shifting of both lower level and upper air winds along the equator in the Pacific ocean. The ENSO allows El Niño and La Niña cycles to occur.

ENTRAINMENT - The drawing of air or moisture into a weather system.

EQUILLIBRIUM LEVEL - The point in the atmosphere when a rising air parcel is the same temperature as the surrounding air. The parcel, in theory, will lose its buoyancy and stop rising at this level. Common with convective clouds with a spreading, flat top such as a thunderstorm anvil. Also called EL.

ESCAPE ROUTE - A pre-planned or considered route to get away from a dangerous area while storm chasing. Often such a route away from a supercell thunderstorm moving to the northeast is a road going to the southeast.

ETA - Older computer forecast model, not supplanted by the NAM model. See also NAM.

EVAPORATION - Change of a liquid to a gas but not as rapidly as with BOILING.

EVAPORATIVE COOLING - Cooling of air through evaporation of water in or near the air. Occurs often in precipitation evaporating in drier air it falls through. Heat energy of the air is absorbed by the liquid water and turns it into a vapor. Since no increase in the water temperature changes it to a gas because of the LATENT HEAT effect, the surrounding air is much cooler because the heat energy was transferred to the water. Often causes negative buoyancy and downdrafts.

EVOLUTION - Dynamic changes from one type of storm to another, or a storm undergoing changes during its lifetime. Commonly used with supercell thunderstorms. The study of such phenomena is also called STORM MORPHOLOGY.

EXPLOSIVE - Extremely rapid development of a convective cloud such as a thunderstorm. Supercells in high CAPE environments often develop explosively and thus contain very strong updrafts and producing severe weather. In low pressure systems, EXPLOSIVE DEEPENING is a term used for systems with a rapid drop is pressure in a 12 to 36 hour period, such as about 2.5 MB per hour. In tropical, hybrid, and extratropical storms, explosive deepening causes a BOMB storm.

EXTRATROPICAL - Simply means not in the tropics. Usually applied to large-scale storm systems that originate in mid and upper latitude regions, such as frontal systems.

EYE - The center of an intense storm system, usually applied to severe tropical cyclones such as hurricanes, characterized by light winds, no rain, and a break on the clouds. Sinking air in the storm center causes a hurricane eye.

EYEWALL - The region of strong winds and precipitation surrounding the eye of an intense storm such as a hurricane. This region is usually characterized by the strongest winds and deepest thunderstorm convection in the case of severe tropical systems.

EYEWALL REPLACEMENT - Cyclic re-structuring of the eyewall in a tropical cyclone, usually in an intense hurricane / typhoon. Often an outer eyewall may form and shrink thus "choking" the inner eyewall. After that, the smaller outer eyewall becomes the "main" eyewall. This cycle could repeat several times during a peak intensity episode in a tropical system.

FAA - Federal Aviation Administration. Federal agency to enforce aircraft safety and law abidement as well as regulate all air traffic within US airspaces.

FARAD - A measurement of a static charge with the number of volts its potential is at. A charge of one COULOMB (enough electrons for one AMPERE for a second, or 6.28 x 10^18 electrons) at a potential of 1 VOLT. The number of FARADS is basically COULOMBS multiplied by the number of VOLTS (sort of like a "WATT in storage"). Commonly used to rate capacitance in electronics. See also AMPERE, COULOMB, VOLT.

FAHRENHEIT - A non-metric measurement of temperature in degrees. To convert degrees Celsius to Fahrenheit, multiply by 9, divide the result by 5, then add 32 to it.

FEEDER BAND - A line or band of clouds that draws into a storm system, associated with storm inflow. Commonly used with tropical cyclones, such as hurricane spiral bands.

FEMA - Federal Emergency Management Administration. Government agency for disaster relief efforts as well as disaster awareness.

FETCH - A distance a wind blows across a body of water for the generation of waves or transport of moisture or clouds.

FFD - Forward Flank Downdraft. A region of strong downdrafts and outflow winds in the precipitation area, usually on the downwind side of a thunderstorm. The gust front is also ahead of this region.

FILLING - Term applied to the weakening phase of a low pressure system where the central pressure of the system is rising.

FIRING - Initiation of convective storms, or initial development of a mesoscale weather system.

FLANKING LINE - The main inflow clouds on the upwind side of a thunderstorm, usually a supercell. This appears as a line of towering clouds that builds towards the main updraft tower, and has a flat, rain-free base. Sometimes storms may back-build along this line.

FLASH FLOOD - A sudden flood of water caused by heavy rainfall in a short period of time or sudden snowmelt. Flash floods could occur as ponding of water in low areas or a destructive wall of water. Watches and warnings are issued for this type of danger.

FLUID HAMMER - A physics term for aerodynamic / hydrodynamic theory when a moving flow of fluid produces a spike of greater force when set in motion (or stopped) due to the inertia / momentum of that fluid. Rattling pipes after suddenly shutting off a faucet is due to the FLUID HAMMER effect. Importance in drainage and oceanographic structures engineering.

FLYING EAGLE - A radar configuration where a hook-echo and v-notch signature are present, resembling the wings of an eagle with the front of the "bird" being at the hook-echo portion of the image. A flying eagle configuration often denotes a well-defined supercell thunderstorm.

FLYING SAUCER - A cloud formation in the lower and middle level portions of supercell thunderstorms with very strong mesocyclones extending to the limit (or beyond) the cloud boundary. It has a striking visual appearance with layers and striations resembling a stack of saucers or one big "flying saucer". Usually circular in shape. Also called MOTHER SHIP.

FNMOC - Fleet Numerical Meteorology and Oceanography Center. Government based center for numerical modeling of ocean and atmospheric phenomina such as ocean waveheight and storm tracks. Strictly uses the NOGAPS modeling in many wave forecasts.

FOEHN - A drier and warmer downslope wind on the lee side of a mountain range. Marked by air subsidence.

FOEHN GAP - Absence of clouds on the lee side of a mountain range caused by sinking air (subsidence).

FOG - A low cloud that is at or near ground level. Resembles stratus clouds and is formed by moisture condensation close to the earth's surface.

FORCING - Lifting of air by "mechanical" means such as an advancing front or mountain range. The air being forced aloft may or may not be capeable of rising by itself convectively. Forcing often causes precipitation events from air that may otherwise be stable. Forcing also occurs in regions of strong convergence such as in tropical systems as well as with thunderstorm processes such as outflow boundaries. DYNAMIC FORCING occurs due to air convergence, as with a front or low pressure system's inflow. OROGRAPHIC FORCING occurs over mountains or land masses which force air to rise.

FREE CONVECTION - Region where air in a rising air parcel is always warmer, and thus less dense, than the surrounding air. The air parcel will continue rising as long as it is lighter that the surrounding air.

FREEZING - The point at which liquid water becomes solid (ice). This is about 32 degrees (F) for fresh water or 0 degrees (C).

FREEZING LEVEL - The altitude or elevation in the atmosphere where the air temperature is cold enough for liquid water to freeze and become ice at 32 degrees (F).

FRONT - A boundary separating two air masses. An advancing cold air mass is bounded by a COLD FRONT. An advancing warm air mass is bounded by a WARM FRONT. A GUST FRONT is the leading edge of cold air from thunderstorm outflow.

FRONT RANGE - The eastern slope of the Rocky Mountains in the United States, typically in New Mexico and Colorado. Very important for upslope type convection and / or lee trough dynamics.

FRONTAL SYSTEM - A weather system involving one or more fronts. Commonly used with extratropical storms having warm and cold front regions.

FUJITA PERTERSON SCALE - An F0 to F5 scale for tornado wind-speeds and inflicted damages. Developed by scientist Dr Fujita and structural engineer Dr Peterson at the NSSL in 1971. F0 is for 40-71 MPH winds, F1 is for 72-112 MPH, F2 is for 113-157 MPH, F3 is for 158-206 MPH, F4 is from 207 to 260 MPH, and F5 is over 260 MPH. In February 2006, this scale was supplemented and revised by the ENHANCED F SCALE.

FUJIWHARA EFFECT - The rotation and influence of two vortices on each other. Commonly observed when two tropical cyclones of similar intensity come within 700 miles of each other. Circulations of both influence one another with the two systems rotating around a common point between them in a cyclonic manner. If one system is larger than the other, that system will be more dominant of the two. Sometimes the smaller vortex may even become entrained into the larger one! More than two vortices can also influence one another on a similar basis with a common central "point" of rotation. The vortices can be tropical cyclones, hybrid or extratropical storms, even multiple vortex tornadoes or dust devils. Two vortices affecting one another can also be conceived as a VORTEX COUPLET. This phenomenon was discovered by Dr Fujiwhara to study hurricane and typhoon behavior. May also be called the FUJIWHARA DANCE.

FUNNEL CLOUD - A rotating vortex, signified by a condensation funnel, but not qualifying as a tornado. These do not reach the ground, neither does the circulation itself reach the ground.

GALE FORCED - Sustained winds from 38 to 54 MPH. If a tropical cyclone is involved, the term TROPICAL STORM is used and its upper limit for the winds is extended to 73 MPH.

GALE PROJECT - Government funded operation in the 1980's to study the dynamics of extratropical storms causing infamous Nor' Easters along the US East coast.

GENERAL THUNDERSTORM - A thunderstorm not meeting the criteria for a strong thunderstorm, with winds less than 38 MPH. These storms still can produce lightning and heavy rainfall. Also called a GARDEN VARIETY THUNDERSTORM.

GENESIS - The initial formation of any kind of weather anomaly. Common uses are TORNADO-GENESIS or CYCLO-GENESIS.

GEOGRAPHIC - Pertaining to features of the surface of the earth, such as continents, mountains, coastlines, oceans, etc.

GEOSTATIONARY - Term used to describe a satellite that remains above the same place on the surface of the earth at all times due to the orbital velocity sychronized with the rotation of the earth at orbit altitude.

GEOTROPIC - The effect of the earth's surface on the lower atmosphere.

GFDL - Acronym form Geophysical Fluid-Dynamics Laboratory. A government body responsible for computer modeling of climate and weather information using numeric modeling.

GOES - Geostationary Orbital Environmental Satellite. A weather satellite in geostationary orbit above the equator at an altitude of about 22,000 miles in space. Very important for weather and tropical meteorology.

GOM - Slang abbreviation for Gulf Of Mexico.

GORILLA HAIL - Slang name storm chasers use for giant, and usually very destructive hail falls. Hail sizes are usually those larger than baseball sized.

GPS - Global Positioning Satellite. Navigation based by fixation on three or more satellites in orbit to calculate position of a receiver on earth. It is invaluable for all types of navigation and readily available to anyone at low cost.

GREENAGE - Slang name for green hue to the sky in a thunderstorm. Very common with hail producing storms due to the refraction of light at a certain angle with ice in the clouds. Often chasers try to avoid areas of GREENAGE, which often means damaging hail and high winds.

GREYOUT - Slang name given to zero visibility due to heavy precipitation, such as rain.

GROUND CLUTTER - False reflectivity showing up on a radar image caused by reflection of the radar beam by nearby ground obstacles. Abbreviated as GC.

GRUNGE - Haze or low clouds surrounding a region of storms which limits visibility to storm chasers. Sometimes this term is used to describe low clouds, drizzle, and fog common when there is no good storms to chase. Sometimes may also be called GUNGE, especially if it is haze or smoke.

GULF STREAM - A warm ocean current in the Atlantic Ocean that flows from near the east coast US to Europe. It has profound influences in the weather such as influencing tropical and extratropical cyclone development.

GUST FRONT - The leading edge of cooler outflow air from a thunderstorm. A thunderstorm dying from downdrafts killing its updraft components is sometimes said to be GUSTING OUT. A left over gust front can also be called an OUTFLOW BOUNDARY.

GUSTNADO - A rotating vortex associated with the leading edge of thunderstorm outflow or along a cold front. They are normally short lived but can be destructive. Sometimes associated with thunderstorm gust fronts, especially if the outflow is dry and moving over dusty terrain.

HABOOB - A dust storm kicked up by a thunderstorm gust front. Common in the southwest United States during the monsoon.

HADLEY CELL - A large circulation pattern between major wind belts in the Earth's atmosphere. Separates the equator, horse latitudes (30 degrees north or south latitudes, including the tropical wind belts and trade winds), the westerlies, up to 60 degrees north or south latitudes, and polar regions). Often bounded by opposing wind flow at high altitudes than at the Earth's surface.

HAIL - A form of frozen precipitation consisting of balls or lumps of ice. Hail can be as small as a pea or larger than a grapefruit in extreme cases. Most hail events occur with convective clouds which bring ice and water up and below the freezing level until the updraft can no longer support the weight. Large hail forms in extreme updraft cases, and can fall at over 100 MPH causing damage. Large hail is very dangerous to storm chasers who penetrate the precipitation core of a thunderstorm.

HAIL ROAR - Sound caused by large falling hail stones resembling a "waterfall" or "faint jet airplane" sound. Common around supercell thunderstorms with large or giant hail. The sound is produced by many hail stones falling through the air at high speed making sound as the air passes over each of them. Sometimes the sound may be produced by colliding hail stones during their free-fall out of the storm.

HAIL SPIKE - A radar reflectivity feature showing a streak or spike-shaped reflectivity region behind (away from the radar site) a high reflectivity region. It is caused by very large hail in the core of the storm and the 3-body scatter of radar energy off the hail stones. See also 3 BODY SCATTER.

HAIL STREAMERS - Streaking or striations in a cloud caused by large hailstones falling through the cloud structure. Occasionally the hail streamers may take on a greenish hue.

HALO - A circle or arc containing the primary colors of the light source, such as the sun, caused by the refraction of different wavelenghts of the source light off particles or droplets suspended in a medium. Most common with clouds and sunlight, or spray droplets from waterfalls. Size of the circle is affected by the size of the refracting drops or particles, as well as the distance to them. A RAINBOW is a type of halo. Also may be called a GLORY. A PILOTS GLORY is a halo effect caused by the refraction of light around the shadow of an aircraft on an underlying cloud deck.

HAM RADIO - A practice of amateur radio licensed by the FCC. Often becomes useful in emergency and storm spotting operations.

HAZARDOUS WEATHER OUTLOOK - A National Weather Service product outlooking expected weather related hazards for a given day. Such hazards may be convective, marine, and non-convective such as high winds or fog.

HCR - Horizontal Convective Roll. A "tubelike" area of rotation caused by a convective and unstable atmosphere along the axis of the direction of the prevailing winds. Often develops at the top of the boundary layer between layers of opposing winds (such as where directional shear exists). May also be referred to as an ASPECT ROLL.

HEAT BURST - A hot sinking downdraft caused by convection. It is different than a microburst because all the precipitation has evaporated from the sinking air long before it reached the ground. The evaporation makes a downdraft cooler, but since the sinking air continues downward for some time AFTER all its water has evaporated, the sinking air becomes very dry and warmer. Heat bursts can cause strong winds and warmer temperatures, and occur usually in the late evening. They are much rarer than microbursts. See also MICROBURST.

HEAT ENERGY - Stored kinetic energy of any matter as a function of temperature. In water, heat energy is stored as one calorie per gram of water for each degree (Celsius). LATENT HEAT is different because it is exchanged only during melting (or freezing) and boiling (or condensing). See also LATENT HEAT.

HEAT INDEX - Apparent temperature air feels depending on the humidity. Higher humidities make the air "feel" warmer because perspiration does not evaporate as readily to cool the skin in higher humidities. For example, in Phoenix, AZ, the air can be 100 F with 20% relative humidity. Miami, FL can be 85 F with 90% relative humidity. The person in Phoenix may not feel as "hot" as one in Miami. Also called HUMITURE.

HEAT LIGHTNING - A form of lightning, usually sheet type lightning caused by a distant storm, seen on warm summer nights. This is just a name and it probably does not exist because there is no evidence of lightning occuring in clear air without any kind of storm clouds in place. Technically it is simply caused by a distant flash of lightning that cannot be seen where the sky is illuminated as sheet lightning.

HEAT WAVE - A period of hot, and usually dry weather.

HEAVY SURF ADVISORY - National Weather Service marine product warning of dangerous marine conditions due to wave action such as large breaking waves and / or swell along the coast.

HECTOPASCAL - A metric unit, equal to a millibar, to measure air pressure. Abbreviated "HPa"

HELICITY - Measure of the tendency of a rising parcel of air to twist as it rises in the atmosphere. Wind patterns which change direction with height in the lower atmosphere result in high helicity values. In unstable, thunderstorm environments where the helicity values are high, supercell thunderstorms are possible. Applied to storms as Storm Relative Helicity or SRH.

HELIOMETER - An instrument for measuring the amount of sunlight for a given day.

HERBERT BOX - A region of the North Atlantic Ocean to the northeast of Puerto Rico formed by the imaginary latitude and longitude lines. Historically, an alarmingly high percentage of westward moving hurricanes moving through this "box" have struck either Florida, the Gulf of Mexico region, or the lower US East Coast. The (in)famous Labor Day hurricane of 1935, hurricane Andrew of 1992, and the Great Miami Hurricane of 1926 all passed through the HERBERT (Pronounced "hay-bay") BOX.

HERF - Acronym for High Energy Radio Frequency (high energy RF).

HERTZ - A unit of measurement for the number of cycles (such as a wave repeating, or electrical frequency) in one second. Very important in vibration measurement, electronics, and radio communications. Abbreviated as Hz.

HF - High Frequency. Radio communications from about 520 KHz to 50 MHz.

HIGH PRESSURE - A region of high air pressure in the atmosphere. Commonly associated with fair weather. Strong high-pressure regions may cause strong outflow winds.

HITPR - Acronym for Hardened In-situ Tornado PRobe. A small device designed by storm researcher Tim Samaras and his colleages in-which cameras or weather instrumentation can be placed, that is designed to be placed in the path of a tornado. The prototype is basically a shell of 1/4" ballistic steel in the shape of a "Chinese hat". This device has recorded both video and conditions in and near tornadoes! It does the same basic thing as the TOTO unit used back in the 1980's, but much smaller, and within the limits of today's technology. Also similar to the DILLOCAM, another small tornado probe.

HOOK - A region of precipitation being pulled around the rotation associated with a supercell thunderstorm. On a radar scope, this feature appears as a hook on the backside of the storm in the sense of the rotation of the storm. A hook echo often signifies a TVS (Tornadic Vortex Signature) and therefore a tornado is possible on the ground under that storm.

HORIZONTAL VORTEX - A vortex that is nearly horizontal to the ground. Most common near the inflow regions of strong to violent tornadoes, such as those F4 or higher.

HORSE LATITUDE - The latitudes of 30 degrees north (or south). Dominated by high pressure, between the westerly and trade wind belts. Term derived from early sailors (relying solely on wind power to fill their sails) abandoning horses overboard to lighten their shipment during light winds found near these latitudes.

HORSEPOWER - A unit of power in physics (work done per unit of time) equal to approximately 745.7 Watts. Abbreviated HP.

HOSE - A slang name for a photogenic "elephant-trunk" type tornado.

HOSED - Slang name for broken or not working. Usually applied to computers or electronics. In chasing, it can also be used to describe a hopeless situation with catching up with a storm or being delayed due to road options, traffic, judgement, etc.

HOSEFEST - Slang name for a cluster of high-precipitation (non-supercell), flooding storms.

HP SUPERCELL - A high precipitation supercell. These supercells have a large precipitation core with heavy rains and possible hail. The rain can completely wrap an HP supercell tornado and obscure it from view on any side of the storm. Sometimes called HP'er.

HURLEY BURLEY - Slang name given to turbulent or rough in Britain. Sometimes used for a thunderstorm.

HURRICANE - A tropical cyclone with winds at or over 74 MPH. May also be called TYPHOON in other parts of the world.

HURRICANE HUNTERS - Famous name given to the US Air Force 53rd weather reconaissance squadron. Their mission is to fly 4 engine turboprop aircraft called WC-130's into the core of tropical systems, including hurricanes and typhoons, to gather vital information on the storm's intensity and movement. Not to be confised with NOAA's aircraft operations who use a modified WP3 Orion to carry out similar missions.

HYBRID - A storm having both characteristics of two types of storm systems. A supercell thunderstorm having LP and HP characteristics can be called a LP-HP HYBRID SUPERCELL. A low pressure system having both extratropical and tropical features can also be called a HYBRID STORM.

HYDRAULICS - The study of physics involving the behavior of fluids as long as they are not compressed changing their volume. This is important for studying the effects of running water such as rivers, floods, and ocean waves.

HYDROLOGY - The study of water budgeting, rainfall totals, and moisture statistics for a given area.

HYDROPHERE - The portion of the earth comprised of water, such as oceans, seas, lakes, etc.

HYPERCANE - A slang name given to an extreme hurricane-like tropical cyclone of incredible intensity. A "hypercane" has never been seen by man and its existance is only in theory. It develops in the same way as tropical cyclones do, but only over exceoptionally warm pools of oceans, like 120 degrees F, for example. In theory, they will be tightly wound, like the most intense "compact" hurricanes much like Hurricane "Andrew" back in 1992, but extend to over 20 miles in height and have maximum sustained winds around 500 MPH! In comparison, the most intense hurricanes reach 200 MPH, and are about 12 miles in height. Hypercanes in theory will be about 1/5 the size of an intense hurricane, but will weaken quickly once removed from the ocean "hot-spot", or even evolve into weaker "regular hurricanes". The intense hot-spot in the sea-surface temperature required for a hypercane would most likely be from an asteroid impact or massive undersea volcanic activity. The closest storm known to resemble a "hypercane" (a very large one) is the big red spot on the planet Jupiter.

ICE - Water in a solid state.

INERTIA - Physics term where a mass of an object in motion tends to stay in motion (or remain at rest) until a force is applied to it (discovered by Sir Issac Newton). If a person stops short in a car, INERTIA causes you to lunge forward into the seat belt (you tend to stay in motion until the restraint stops you via a force). The CORIOLIS effect is driven by INERTIA.

INFLOW - Influx of air into a storm system of any kind.

INFLOW JETS - Local jets of air feeding into the base of a tornado or thunderstorm with a powerful inflow structure.

INFRARED - Electromagnetic waves between the radio waves and visible light spectrums. They are invisible and commonly given off by hot objects. Infrared heating is abundant from the sun. Also called IR.

INSOLATION - Influx of solar energy and radiation through the atmosphere to the earth's surface. Also called SOLAR INFLUX.

INTERCEPT - The "meeting" of a storm chaser or team of chasers with the storm they were looking for.

INTERNET - A wide area global network for communications, information, and file sharing. Based on IP (Internet Protocol) technology. Also called WWW (World-Wide Web). INTER means BETWEEN (other networks) in this case.

INTRANET - A network limited to a corporate or local domain (network "area") but utilizing the same technology as that used on the world-wide INTERNET (Internet Protocol, or IP used). INTRA means WITHIN (a network) in this case.

INVERSION - A region of the atmosphere where the temperature increases with height rather than decreased. Such a lapse rate causes air to sink and may prevent any convective activity (subsidence). Sometimes inversions prevent mixing at low levels causing smog and pollution to be trapped beneath it, common in Los Angeles, California, for example.

INVERTED TROUGH - A trough of low pressure appearing as an "upside down" trough on a weather map. These are troughs where the flow they are embedded in is from an Easterly direction in the Northern hemisphere.

IONOSPHERE - Upper portion of the atmosphere above the mesosphere. Extremely thin air at these altitudes is exited by the solar radiation causing the gas to become ionized. AURORAS occur when these ions are affected by the earth's magnetic field near the poles. The NORTHERN LIGHTS are an example of an aurora.

IR SATELLITE - A satellite imagery using the temperature of cloud tops as an indication of their height. The higher the cloud top, the colder. Since such images use the infrared spectrum, clouds can be seen at night when a visible satellite will not show anything.

IRIDESCENCE - Glowing or bright regions caused by sunlight reflecting off of or being diffracted by a cloud. Commonly seen around ice clouds such as cirrus.

ISENTROPIC LIFT - Lifting of unsaturated air as it moves over a boundary such as a front or dryline or even an upslope such as high terrain.

ITCZ - Inter-Tropical Convergence Zone. A region of convergence between the trade wind systems from the southern and northern hemispheres. Causes thunderstorm convection in tropical regions near the equator. See also TRADE WINDS.

JETLET - A small, local speed maximum, smaller in scale to a full-fledged "jet stream". Basically a narrow and small area of strong winds aloft.

JETMAX - A local wind speed maximum in a jet stream. Vertical wind shear increases abruptly beneath the jet max. Also called JET CORE.

JET STREAK - A strong and narrow stream of very high velocity winds in a jet stream. Called so because this region often is marked by high clouds in a narrow fast moving band as viewed on a satellite loop. See also JETMAX.

JET STREAM - A narrow band of high altitude winds that circles the earth in the mid and upper latitudes of the earth. The altitude, usually from 20 to 40 thousand feet, and poleward displacement, usually from 30 to 60 degrees north (or south) latitude depends on the season and path of smaller disturbances moving along its path. There are usually two jet streams per hemisphere, a POLAR and SUBTROPICAL jet, and the polar one being the one at a higher latitude. Wind speeds in the jet stream can be up to 250 MPH and its position is very important for weather forecasting because certain types of storms often develop just under it.

JOG - A movement of a storm system characteristic of a temporary deviation or "wobble" from its general track. Commonly used with tracks of tropical cyclones.

JOINT TYPHOON WARNING CENTER - Tropical weather forcasting company, in part by the US Navy, for the western Pacific Ocean. Acronym is JTWC.

JOULE - A fundamental metric system unit of energy in physics. A watt is the work of one JOULE in one second. CAPE is measured in J/kg, or JOULES per kilogram (kg), of air. About 4.184 Joules make up a CALORIE. See also CALORIE.

KATABATIC WIND - A wind that develops from cooler sinking air, usually down a slope, such as a mountain.

KELVIN - System for measuring temperature, in degrees, based on absolute zero (0 degrees K). Widely used in science and engineering. Water freezes at about 273.16 degrees K and boils at 373.16 degrees K. To convert Kelvins to degreees Celsius, simply subtract 273.16!

KELVIN HELMHOLTZ EFFECT - Instability, usually forming waves or eddies, between two fluid mediums moving at different speeds and / or directions. KELVIN HELMHOLTZ WAVES can sometimes be seen above a cloud deck / layer in regions of strong wind shear.

KELVIN WAVE - A surge of a fluid caused by differing densities in that fluid. Mixing of warm and cold air at different densities can produce such waves. The El Niño effect is characterized by a large kelvin wave of warm Pacific ocean water off South America.

KILOMETER - A thousand (1000) meters for metric measurement of length. This is about 3,300 feet.

KNOT - One nautical mile per hour. A nautical (air or sea) mile is about 6,076 feet and is about 1.15 times a SATUTE (land) mile of 5,280 feet. One KNOT is about 1.15 MPH.

KNUCKLES - The upwind side of a thunderstorm anvil top with mamma-like protrusions along the edge that resemble the knuckles on a hand. These are not mammatus clouds.

KPH - Kilometers Per Hour (1000 meters per hour). Metric system measurement of speed.

LABRADOR CURRENT - A cold ocean current in the North Atlantic Ocean that flows southward from near the Canadian coast south to New England.

LAMINAR - A smooth or non turbulent flow of a fluid.

LA NIÑA - A periodic shifting of warm waters of the Pacific ocean toward the West towards Asia resulting in cooler waters near South America. La Niña can cause increased tropical cyclone activity in the Atlantic ocean.

LAND SPOUT - A type of tornado denoted by a nearly transparent "tube" appearance developing under a convective cloud, such as a thunderstorm. Sometimes, the vortex is detached from any cloud features above it but differs from a dust devil because the parent convection is required to support its development. Landspouts are common near thunderstorm boundaries, under moisture-deprived or high-base (such as elevated) thunderstorms. They have all the dangers of "regular" tornadoes.

LAPSE RATE - The rate a variable such as temperature changes with an increase of altitude above the ground. High lapse rates for temperature decreasing with height near the ground can make the atmosphere unstable, and lead to convection.

LARGE SCALE - A scaling term for large scaled weather systems. Often depicts a size of 100 miles or more. Sometimes called MACROSCALE.

LATENT HEAT - The heat energy stored by a particular substance, such as water, as a result of a phase change. Water requires a calorie of heat to raise the temperature of one gram of it 1 degree C (heat energy). However, this is different for a phase changes. To convert liquid water at 100 degrees C to vapor, about 540 calories per gram is required. To melt ice (water at 0 degrees C), about 80 calories per gram is required. When the water condenses or freezes, the stored calories are "given up" by the water back to the air. It is this property that tropical cyclones and most convective clouds derive their source of energy from. For sublimation, when water goes from ice to a vapor directly, even more energy is required, about 720 calories, whether or not it goes through the liquid phase. The latent heat theory is also used to determine the CAPE values of a parcel of air.

LCL - Lifted Condensation Level. A measure, in pressure altitude above sea level, at which the condensation level will be located. This is the approximate level, on a well mixed convective atmosphere (no inversions or caps), one will expect cloud bases to be at.

LEADER - Small bolt or streamer that occurs just before or near a hit of lightning. Often seen atop high structures or moving objects such as cars. Caused by a strong static electricity field. Main leader for lightning strike is called a STEP LEADER.

LEE TROUGH - A low pressure trough that develops in the lee (east) side of the Rocky Mountains as faster moving westerly winds aloft interact with the mountains. This often causes LEE CYCLOGENESIS if a low pressure area develops from this trough. It is very important for weather forecasting.

LEFT MOVING STORM - See LEFT TURNING STORM.

LEFT TURNING STORM - A thunderstorm (or supercell) which makes a turn to the left relative to its movement axis. Such storms can also be referred to as the storm's "LEFT SPLIT" and often increase in movement due to the cell becoming more "embedded" in the storm relative winds.

LEMON METHOD - A method used to analyze the vertical radar structure of thunderstorms, taking into account important radar features such as weak echo regions and overhangs. This method can discern between special thunderstorm varients, such as multicell and supercell thunderstorms.

LFC - Acronym for Level of Free Convection. This is the altitude where a rising parcel of air enters the region of free convection (usually above the cap and boundary layer). See also FREE CONVECTION.

LIDAR - LIght Designation And Ranging. A radar which uses RF frequencies near that of infrared light. The "RA" in the word RADAR was replaced with "LI" to get LIDAR. Used mainly for research and water vepor analysis.

LIFT - Upward (buoyant) force given to make a parcel of air rise. Also describes the action of a trough of low pressure de-amplifying and moving back northwards in the northern hemisphere.

LIFTED INDEX - A measurement of the instability of the atmosphere based on the temperature difference between a parcel of air from the surface and the ambient air at 500 Mb pressure altitude (18,500 feet). If this number is negative, there is enough instability for thunderstorms. Also called LI. There are also similar K, SHOWALTER, and TOTAL-TOTALS indices.

LINEAR - Name given to a storm complex or multi-cell storm cluser in the form of a line or squall line.

LITHOSPHERE - Portion of the earth pertaining to its solid makeup, such as the earth's crust.

LLJ - Low Level Jet. A strong wind aloft but confined to the lower levels of the atmosphere.

LND - Acronym for Level of Non-Divergence. The level above the earth where air in a weather system is neither converging or diverging. Most commonly applied to tropical cyclones where air converges at low levels and diverges aloft after being lifted. It is the altitude between these processes where the air is neither diverging or converging that is considered the level of non-divergemce or LND.

LOADED GUN - A slang name for a sounding, profile, or atmospheric condition that indicates explosive thunderstorm development once the right conditions are met. Loaded gun profiles usually contain a moderate to weak cap, high CAPE and helicity values, and lapse rates encouraging convection.

LONG WAVE - In radio, see VLF. In meteorology, refers to a large-scale low pressure / jet stream trough. See also WAVE.

LOOP CURRENT - Name given to a few popular ocean currents. The Gulf loop current is a mild clockwise flow of water in the Gulf Of Mexico. The Atlantic loop current is a large system of currents, the Gulf Stream to the west, Azores cool current to the east, the North Atlantic Drift to the north, and the Antilles Current forming the south side of the loop.

LORAN - A long wave navigation service that works very similar to GPS. Unlike GPS, the transmitters (3 or more) are positioned along a coastline, and allow boat and ship traffic to use the 3 stations to calculate their position. It also uses much lower frequencies in the VLF range.

LOWERING - General term for a portion of a cloud base with a lowered formation. Lowerings such as wall clouds are important features with bases of thunderstorms.

LOW LEVEL - Region of the atmosphere from the earth's surface to 6,500 feet.

LOW PRESSURE - A region of low air pressure in the atmosphere. Commonly associated with storms and cyclones.

LP SUPERCELL - A low precipitation supercell. These supercells have little VISIBLE precipitation and most regions of the storm base are visible. The word VISIBLE is stressed here because these storms often contain very large hail which does not block light as much as rain as if issues from the cloud. LP storms usually have a striking visual appearance with corkscrew or "barber-pole" striations on the updraft tower. LP supercell tornadoes are often un-obscured from viewing around the storm. Sometimes called LP'er.

MACKER WAVE - A surfer's expression of a very large ocean wave. The term comes from Mack trucks, because wiping out or being hit with such a wave is like being hit with a Mack Truck!

MAMMATUS - Underside of a turbulent thunderstorm anvil containing rounded protrusions resembling women's breasts. These clouds are a result of severe air turbulence and strong winds aloft. These clouds are harmless but often these clouds precede severe or tornadic thunderstorms.

MARINE CONTINENT - Islands and regions of the Western Pacific ocean. Also called OCEANIA. Most of these regions sport a tropical environment.

MASS TRANSPORT - Term commonly applied to oceanography where strong winds blowing across a body of water create a strong surface current. Also occurs in high surf zones along shorelines where a strong inward current develops due to the energy of the breaking waves or swell. In tropical systems, can cause storm surges and even push ocean vessels such as ships well inland.

MATURE - The main sequence of a storm system. Thunderstorms mature when precipitation reaches the ground from the base of the cloud and continues until the updraft column has completely abated.

MAX-Q - Aeronatical term for the maximum drag force on an aircraft or rocket depending on its altitude and speed it is moving through the atmosphere at. Commonly used by NASA during rocket launches when the speed of the rocket and density of the atmosphere it is moving through creates maximum stresses on the vehicle.

MCC - Mesoscale Convective Complex. A large multicell thunderstorm cluster covering an area of several hundred square miles.

MCS - Mesoscale Convective System. A thunderstorm complex covering an area less than that of an MCC. This is just a fancy name for a multicell cluster or line of thunderstorms.

MERGING STORM - A thunderstorm (or supercell) where two or more distinct (discreet) cells consolidate into a single thunderstorm. Often a multicell storm cluster MERGES together into a single storm, and evolves into a supercell once a mesocyclone forms within the now single updraft.

MESO - Doppler radar storm attribute assuming presence of a mesocyclone.

MESOCYCLONE - A broad-scale rotation associated with a thunderstorm. This rotation may or may not be of tornadic velocities, but is often several miles wide and is confined to the portions of the cloud above the cloud base. It is detectable on Doppler radar and visually if striations are seen in the storm updraft tower. Also referred to as MESO (especially with doppler radar storm attribute usage).

MESONET - A collection of real time weather instruments throughout an area, currently in use in the state of Oklahoma. A portable MESONET consists of weather instrument packages placed atop chase vehicles to gather information around a storm or particular area.

MESOSCALE - A scaling term for medium sized weather systems. May also be used as a MESO prefix with other types of words such as in MESOCYCLONE. Often depicts a size of a couple of miles to about a hundred miles. Sometimes called MID or MEDIUM SCALE. The term can also be used for something that is between two others, as in the term MESOSPHERE.

MESOSCALE DISCUSSION - A summary product from the Storm Prediction Center (SPC) issued when severe weather is possible for a given area. Often issued a few hours and up to the issuance of a weather watch (WW) for the same area. Also applied to winter weather.

MESOSPHERE - Upper portion of the above the atmosphere above the stratosphere. Characterized by decreasing temperature with height but extremely low moisture. Also is the coldest point in the atmosphere at about -135 degrees (F).

METEOROLOGY - Branch of science pertaining to the study or the earth's (or any planet's) atmosphere. May also be called ATMOSPHERIC SCIENCES (a branch of earth science).

METER - A device for indicating a measured scalar quantity. A fundamental unit of measure for the metric system.

METRIC SYSTEM - A measurement system of weights and measures based on units of 10. Widely used in Europe, Australia, and Latin America.

MICROBURST - A violent sinking of a parcel of air in the atmosphere, commonly associated with thunderstorm downdraft regions. When such a downdraft hits the ground it spreads out forming a brief and violent wind gust, which can be very destructive. There are DRY (having little precipitation) and WET microbursts (having a lot of precipitation).

MICROSCALE - A scaling term for small scaled weather systems. May also be used as a MICRO prefix with other types of words such as in MICROBURST. Often depicts a size of a couple of miles or less. Can also be called SMALL SCALE.

MICROWAVE - Radio comminucations at frequencies above 1.2 GHz. Sometimes called Extremely High Frequency, or ELF. Commonly used in RADAR and satellite applications.

MID LEVEL - Region of the atmosphere from 6,500 feet to 20,000 feet.

MINI SWIRL - A tornado-like vortex, brief and violent, embedded in the eyewall of an intense tropical system. These were discovered by Dr Fujita in 1992 during study of damage patterns of hurricane Andrew. These vortices were very much like the suction vortices found in violent tornadoes. They are very destructive.

MIXING - The exchange of air vertically in the atmosphere as a result of convection (dry or moist). A MIXED LAYER has a minimal temperature and / or moisture lapse rate with increasing altitude and is most common below the boundary layer. The mixed layer also is the most turbulent. Also called ML. In oceanography, the ocean is warmest at the surface and decreases with depth (thermocline). This lapse rate is the least in the mixed layer just below the sea surface. Referred to as MIXED LAYER DEPTH or MLD.

MONSOON - A cyclic or seasonal changing of wind patterns causing an increase (or decrease) in moisture and precipitation. Monsoons are common in tropical regions that cause a rainy and a dry "season".

MORPHOLOGY - Study of change of form or shape. Commonly applied to the EVOLUTION of a storm system, such as a supercell thunderstorm. Storm morphology is very important for any storm related research and forecasting.

MPH - Statute Miles Per Hour (5,280 feet per hour). Do not confuse this with the KNOT, which is 1.15 MPH (6,076 feet per hour).

MRF - Medium Range Forecast. Forecast models for the weather patterns from 36 hours to several days into the future. These often have high error tolerances and should only be used for rough guidance.

MS - Meters per Second. Metric speed measurement, common with weather and science. One MS is about 3.6 KPH (about 2.3 MPH).

MSL - Mean Sea Level. Often used to describe altitude or elevation. Pressure altitude is measured in barometric units and is relative to MSL.

MSLP - Mean Sea Level Pressure. See also SEA LEVEL.

MULTIPLE VORTEX - Term applied to a tornado containing multiple smaller vortices within the main vortex.

NADER - Slang name given to TORNADO. Comes from the southern or country accent applied to the word.

NAM - North American Model. Long range computer model, similar to the ETA.

NAO - North Atlantic Oscillation. A 20 to 30 year cyclic change in the speed and trajectory of the North Atlantic current, which flows clockwise around the basin of North Atlantic Ocean. Slight changes in this current can have dramatic climate effects.

NASA - National Aeronautics and Space Administration. Federal agancy for the design, research, testing, and evaluation of aircraft and spacecraft, as well as space exploration and satellite deployment.

NAUTICAL MILE - An "air or sea" mile, which is about 6,076 feet. Not to be confused with the "land" statute mile of 5,280 feet. There are about 1.15 statute miles to a NAUTICAL MILE. See also KNOT.

NCAR - National Center for Atmospheric Research. Branch of NOAA dealing with the study of atmospheric phenomina.

NCEP - National Center for Environmental Prediction. Branch of NOAA dealing with short term weather related anomalies, such as storms and droughts, and their impacts.

NEEDLE - Name given to a very thin visible funnel type tornado or funnel cloud resembling a needle extending from the cloud base.

NEXRAD - NEXt generation RADar. New and more reliable weather radar with computer enhancement and filtering abilities for tracking storms.

NGM - US Navy forecast model. See NOGAPS.

NHC - National Hurricane Center. US Government tropical weather specialist firm. Responsible for all tropical cyclone monitoring and forecasting in the Atlantic and Eastern Pacific basins.

NHC90 MODEL - A statistical and dynamic model using the AVIATION MODEL (AVN) to forecast future tropical cyclone tracks four times a day. Used and developed by the National Hurricane Center. Updated version may be called NHC98 MODEL.

NIGHT VISION - An electonic device allowing viewing and / or photography in near total darkness. Commonly uses infrared imaging or light amplification technologies. Some high end camcorders are equipped with night vision.

NIMBOSTRATUS - Low to mid level, depending on the environment, layered clouds producing precipitation. They are not cumiliform like the cumulonimbis clouds. Clouds usually composed of water droplets that may or may not be mixed with ice crystals. Often produces steady precipitation. Abbreviated as NS.

NIMBUS - Greek derived word meaning rain. Most precipitation producing clouds are called CUMULONIMBUS or NIMBOSTRATUS. Both are abbreviated CB and NS, respectively.

NOAA - National Oceanographic and Atmospheric Administration. Government weather and oceanography forecast and research firm for government, military, and civilians. Part of the Department Of Commerce.

NOGAPS - Navy Operational Global Atmospheric Prediction Systems. Numerical meteorological model provided by the US Navy's numeric modeling division (FNMOC). Also called NGM for short. See also FNMOC.

NOR' EASTER - An infamous storm system, usually an extratropical storm with gale or even hurricane forced winds, that affects the US East coast. The name is given because the western side of the cyclone, with its prevailing North and East winds, are encountered as the storm passes, often with heavy precipitation. The storms can be especially dangerous during the winter, with heavy snow and damaging winds, including coastal damage from large waves, swell, and storm surge.

NORTH ATLANTIC DRIFT - The termination point of the Gulf Stream current in the North Atlantic Ocean off the British and Irish countries in Europe. This warm current is responsible for Western Europe's warm climate despite the high latitudes (50 degrees N).

NORWEGIAN STORM MODEL - The basic configuration of a mid-latitude / wave cyclone consisting of a cold front, warm front, any occlusions, and the low-pressure center.

NOTCH - A rain-free area, dominated by inflow, into the updraft regions of a atrong thunderstorm, usually a supercell. The region may appear as a rain free clear slot below the rain-free base bounded by the RFD and FFD regions, if any. Not to be confused with the V NOTCH radar feature. Causes part of the BWER radar feature. May also be called a BEARS CAGE in tornadic storms.

NSSL - National Severe Storms Laboratory. Government weather research firm (Department Of Commerce) specializing in severe weather. Located in Norman, Oklahoma.

NWS - National Weather Service. Government weather forecasting firm (Department Of Commerce) for most civilians. Has many offices around the nation, particularly in major cities. Also called NWSFO for NWS Forecast Office.

OCCLUSION - The overunning of one weather system over another. May also be called STACKING when used with low pressure systems at different altitudes. More commonly used when two weather fronts meet and one front rides up and over the other. If a cold front rides up and over a warm front, it is a WARM OCCLUSION. If a cold front slides under the warm front, it is a COLD OCCLUSION. In a supercell, an RFD downdraft that wraps around the storm and cuts off incoming warm air is said to be an OCCLUDED MESO.

OHM - A unit of resistance measurement (restricting the flow of electricity). An OHM can block an AMPERE (6.28 x 10^18 electrons per second) at a potential of one VOLT. OHM's law is a fundamental basis for electronics based on the equation VOLTS equals AMPERES multiplied by the OHMS. See also AMPERE, VOLTS.

OLYSIS - A suffix to describe the degrading or dissipation of a storm system or weather anomaly. Commonly used in CYCLOLYSIS or FRONTOLYSIS.

OMEGA BLOCK - A blocking upper level high pressure ridge where two upper level low pressure areas are on each side of it. The high pressure area has a flow mimicking an upside-down horseshoe pattern similar to the greek alphabetic character omega. Any weather systems follow this flow around the edges of the omega block high.

OPEN CIRCULATION - A circulation of a weather system that does not form a complete circle. Commonly applied to troughs and shortwave disturbanced as well as tropical waves.

OROGRAPHIC - Effects of mountains or high terrain on the weather. Orographic lift can result from a wind blowing against a mountain ridge forcing it upwards. This can take relatively stable air and force in aloft causing convective clouds.

OTIS - Optimal Themal Interpretation Software. Computer based software coupled with bouy and satellite information for the measurement of sea surface temperatures. Developed by the United States Navy.

OUTFLOW - Air moving out of and away from a storm or other type of weather system. Thunderstorms produce outflow because of downdrafts. High pressure areas are also dominated by outflow.

OZONE - A tri-atomic form of oxygen (molecule of three oxygen atoms). Caused by electrical discharges and ultraviolet light when oxygen atoms are ionized where some of the atoms re-arrange themselves as three (ozone) instead of one or two (oxygen). Ozone is toxic, unstable, and even explosive in high conventrations. It has a "fresh air" smell that is most common after a thunderstorm. In the upper atmosphere, ozone is beneficial in trace amounts for blocking the sun's harmful short untraviolet rays.

PAKWASH EVENT - An extreme damaging wind event, usually associated with HP supercell outflow or derecho type events. Originated in Pakwash, Ontario (Canada) when a forest was blown down by extreme winds in July 1991. See also DERECHO, XDW.

PALMER DIVIDE - Important geographic feature of higher terrain in eastern and southeastern Colorado. Very important for upslope convection.

PARABOLA - A geometric path where one axis is related to the square of another axis (in 2 dimensions). Falling objects, such as those dropped from an aircraft, fall in a PARABOLA before encountering air resistance. The shape of a water fountain is a parabola.

PARACHUTE - A device for slowing a descent of an object by imposing additional drag. The term comes from the French word meaning "Break Fall". Normally, a parachute is a large surface of strong fabric which is stored in a container and deployed to slow the descent rate of a falling body, such as a payload or skydiver. When a parachute is deployed, it catches the air and creates large amounts of drag which slows the falling body down. Parachutes will only function in the atmosphere.

PDO - Pacific Decadal Ocillation. A change in the ocean currents in the Pacific Ocean, occuring about every ten years or so, that influences other large scale anomalies such as the ENSO and El Ñino.

PDS - Particularly Dangerous Situation. A special wording applied to high-risk severe weather watches. Commonly referred to as ENHANCED WORDING. Example: "This is a particularly dangerous situation with the possibility of very damaging tornadoes."

PENCIL - A narrow visible funnel tornado which is rather narrow but not thin as with a rope type tornado. Resembles the trunk of an elephant.

PERFECT STORM - Name given to the infamous extratropical (or hybrid) storm off New England at the end of October 1991. This storm was a combination of a tropical and extratopical system and had hurricane forced winds over a huge area. The storm also produced some of the highest waves and swell ever seen in the Atlantic. This storm was a NOR' EASTER which remained at sea. Most damage was to shipping and fishing interests, as well as coastal damages from waves, swell, and storm surge. It even inspired a book and movie.

PERIOD - The time it takes, in seconds, for a full cycle (such as a wave or frequency) to complete. Any frequency given in HERTZ units can be converted to PERIOD (in seconds) by simply dividing 1 by the number of HERTZ. See also HERTZ.

PHASE - A current state of matter, such as a gas, liquid, or solid. A PHASE CHANGE refers to a change in such a state of matter, such as melting or boiling water. In wave analysis, PHASE refers to the current position of a wave with respect to time, usually measured in degrees or radians.

PHASING - The consolidation of two or more atmospheric waves, such as a short wave, into a single wave. Also used to describe two atmospheric anomanies coming together to produce a weather event (such as an upper air disturbance moving over the air ahead of a dry-line for severe thunderstorm development).

PHOTOGENIC - Any scene which presents a good photo opportunity. In storm chasing, a tornado in high contrast with a sunset in the background is a photogenic example.

PLASMA - A fourth state of matter, other than a solid, liquid, or gas, comprised of ionized material. Usually, matter with 50 percent, or more, ions is plasma. Plasmas make up our stars, including the sun, and can be found on earth inside lightning bolts.

POES - Polar Orbiting Environmental Satellite. Satellite for studying and imaging of polar regions of the earth.

POH - Probability Of Hail. Weather radar storm attribute based detection algorithm based on VIL and BR determining the probability, from 0 to 100 percent, or hail in a given storm.

POLAR - Regions of the earth where the latitude is greater than 66.5 degrees north or south. These regions have vague seasons except for near the lower latitudes. Often there is 24 hour sunlight or 24 hour night seasons depending on the time of the year. Weather is often cold and dry throughout the entire year. Polar conditions also occur at extreme elevations such as in the Himalayan mountains in Asia.

POLAR EASTERLIES - Winds with an easterly component at the surface near the Earth's poles, usually at latitudes more than 60 degrees. In the north polar regions, these winds are from the northeast. In the south polar regions, they are from the southeast. They are very cold and dry winds.

POLAR LOW - A small but intense vortex that develops at very high - near arctic (or low - near antarctic) latitudes as relativly cold air passes over warmer water. Winds in these shallow systems can approach wind speeds found in tropical cyclones in extreme cases. They develop mostly over water, and tend to weaken over land. Life span is usually no more than a couple of days. Abbreviation is PL. May also be called ARCTIC / POLAR HURRICANE.

POLAR VORTEX - Circulation of air around the north and south poles of Earth. This includes the POLAR EASTERLIES.

POSH - Probability Of Severe Hail. Same as storm attribute POH, but for hail 3/4 of an inch or larger. See POH.

POWER FLASH - A flash of light caused by a powerline or transformer as a storm knocks out power. Common in strong wind events and tornadoes, especially at night.

PRECIPITATION - Any form of water that falls out of the atmosphere, such as rain, snow, and hail.

PRECIPITATION MODE - Weather radar mode where lower gain is used on returned signal. Radar switches to this mode when very high reflectivity values (from precipitation) are encountered in clear-air mode.

PRECIPATABLE WATER - The total amount of water vapor in the air, expressed in liquid equivalent in inches.

PRIMED - A name given to a atmospheric conditions with many ingredients in place for severe thunderstorm and / or tornado developnment. Often applied to a LOADED GUN sounding.

PROFILE - The distribution of temperature, dewpoint, winds, pressure, and other qualities from the earth's surface to the upper atmosphere. Weather balloons and satellites are responsible for data acquisition in this area. Often referred to as WIND PROFILE with winds aloft or LAPSE RATE with temperatures aloft.

PROFILER - A device for measuring the winds speeds and directions aloft. Also called a VAD or Velocity Azimuth Display. A VAD wind profiler or VWP is a combination of the wind profiler instrument, similar to a vertical clear-air doppler radar, and the VAD display.

PROG - Short name for PROGnosis, which is a forecast.

PROGRESSIVE - Sucessive weather systems following a narrow path. With mesoscale precipitation, my also be called TRAINING.

PROJECT STORMFURY - An effort conducted by the US military to control tropical cyclone strength by cloud seeding experiments.

PROPAGATION - Generation or development of a weather system or convective complex of thunderstorms in a general direction. For example, a line of thunderstorms may develop towards the Southeast but each cell in the line may move to the Northeast before dissipating. This line can be said to propagate to the SE.

PSYCHROMETER - An instrument for measuring dry and wet bulb temperatures for the computation of relative humidity.

PVA - Positive Vorticity Advection. Increased upward lift and vorticity in advance of an upper level disturbance / short wave.

PYROCLASTIC CLOUD - An extremely violent cloud of hot ash and gas expelled from a volcano.

PYROMETER - A very-high temperature thermometer. See also THERMOMETER.

QN-VECTOR - The measurement of convergence on either side of a line parallel to an average of the wind directions on either side of that line. Important for determining the development of squall lines and convergence based storm events.

RADAR - Radio Acquisition Designation And Ranging. A device using radio waves to display information on distant targets reflecting the waves back to the antenna.

RADAR INDICATED - A term used to describe a certain weather anomaly that was indicated by using a weather radar. Radar indicated tornadoes are sometimes acronymed RITOR (Radar Indicated TORnado).

RADIO FREQUENCY - Electomagnetic waves below the frequencies of infrared and visible light. This includes microwaves and most radio waves. Also called RF.

RADIATION - Energy or particles given off by a paticular source. The term can be used to describe the radiation, such as light, given off by the sun, or the radiation of ocean waves from a storm system. This term is very general.

RADIOSONDE - An instrumentation package using radio communications for data aqcuisition. Commonly used as an instrument payload for weather balloons to gather upper-air information.

RADOME - A globelike shell or protective covering for a radar dish or antennae. Often made of fiberglass or plastic.

RAIN - Liquid water falling from the sky.

RAIN SHADOW - A dry region downwind of high mountains where very little moisture is left in the air after passing over the mountains. Usually causes desert regions, especially in places like the Southwest US.

RAINBOW - An arc or semicircle containing all the component colors of visible light caused by the refraction of sunlight in raindrops (or ice crystals). Actually, a rainbow is a halo where one half of the circle is not visible due to the effects of the ground.

RAPID DEEPENING - Sudden pressure drop associated with a large scale cyclone such as a tropical system or extratropical storm. Tropical systems with a pressure drop of about 1.7 Mb per hour are said to be RAPIDLY DEEPENING. Not to be confused with RAPID DEVELOPMENT (Or EXPLOSIVE), as with thunderstorms.

RAPID UPDATE CYCLE - A rapidly updated numerical model ran every 3 hours at NCEP (National Center for Environmental Prediction). This model emphasizes mesoscale weather, such as thunderstorms on a short term basis up to 12 hours. Abbreviated as RUC.

RAT RACE - A chase where many thunderstorms develop and the chase track jumps from one storm to another searching for a favorable cell. Common when chasing multicell (or multiple supercell cluster) type storms.

RELATIVE HUMIDITY - Percentage scale based on the difference between temperature and dewpoint. The humidity at 100 percent means the air is saturated as in a cloud or rain. Is often high in tropical regions and very low in deserts. Also called RH.

RELATIVE VELOCITY - The apparent speed between two objects, considering any speeds of those objects. For example, a person is driving at 55 MPH to a storm. The storm is coming at him at 30 MPH. The relative velocity experienced by the person in the car closing in on the storm will be 85 MPH. Also called RELATIVE CLOSURE.

RELATIVE WIND - Apparent wind experienced by an object moving through the atmosphere, considering the prevailing winds and speed of that object. For example, a car drives at 20 MPH into a 10 MPH headwind. The relative wind measured by an anemometer on top of that car will be 30 MPH. Also called APPARENT WIND.

REX BLOCK - An upper level high directly north of an upper level low, inhibiting the upper level low's advancement. Incidentally, the upper-air low may also block or deflect other weather systems.

RF - Acronym for Radio Frequency.

RFD - Rear Flank Downdraft. A strong downdraft of air on the upwind side of a thunderstorm, usually a supercell, which is separated from the forward flank (outflow) regions of the storm. The RFD also may wrap around a rotating supercell storm, especially is the storm is tornadic.

RI SCHEME - A method used by the National Hurricane Center to consider intensification of tropical cyclones based on information from the SHIPS MODEL.

RIGHT MOVING STORM - See RIGHT TURNING STORM.

RIGHT TURNING STORM - A thunderstorm )or supercell) which makes a turn to the right relative to its movement axis. This storm apparently "moves against" its wind environment or backbuilds. Such storms often exhibit higher severe / tornadic incidences due to the increased storm-relative shear encountered by such a storm's movement. Sometimes the "RIGHT SPLIT" of a splitting storm becomes the RIGHT TURNING STORM. The right-turning storm also usually decreases speed in its movement. Also called RIGHT MOVING STORM.

ROAD NETWORK - Mapping of available roadways and paths in a given area. Common with storm chasing.

ROLL - A cycle of air in the atmosphere in a rolling or tubelike motion. Mountains or convection due to solar heating can cause rolls. Mountain rolls are sometimes called ROTORS. A convective roll lining up with prevailing winds may be called a HORIZONTAL CONVECTIVE ROLL (HCR) or ASPECT ROLL. Sometimes rolls develop between two shearing layers in the atmosphere due to the vertical shear between the layers. This can cause turbulence for aircraft and aid in the development of supercell thunderstorms in unstable sheared environments.

ROPE - A very narrow tornado or funnel cloud, with a structure resembling a rope. Common during the dissipation of some tornadoes but still very dangerous in terms of destructive potential.

ROSSBY WAVE - A wave or undulation in a fluid medium characterized by a change in the flow of that medium. Large ocean currents such as the Gulf Stream often meander like a river without a bank. Similar phenomina occurs with the jet stream creating a long wave trough or "dip" in the wind flow. These are all examples of rossby waves.

ROTATING HEAD - A thunderstorm cell just above a break in a storm line, cluster, or bow echo. This cell can sometimes evolve into a supercell.

ROTOR - A rotating region of air, commonly found on the lee-side of mountains in strong winds. Sometimes, a rotor can form along a shear region, such as along a front or boundary.

ROTOR CLOUD - A horizontal roll-cloud forming along or near a rotor region. See also ROTOR.

RUC MODEL - See RAPID UPDATE CYCLE.

RUNUP - Term commonly used in oceanography where large surf runs onto the beach. In extreme cases, runup can form a storm surge like tide that can flood coastal areas. For example, during the "Perfect Storm" around Halloween 1991, Florida had tides above normal due to high surf created from swells eminating from the storm off New England. The runup from the breaking waves caused a high surge even though the local winds were light.

SAE - Society of Automotive Engineers. Basis for non-metric / English measurement standards, such as an inch, foot, pound, etc. This system is discouraging for scientific use because it is not simple 10-based as the metric system. Since it is used widely in the US, it is still commonplace for many measuring units.

SAINT ELMOS FIRE - A glow or corona effect caused by ionization of air in an electric field. Common atop tall poles or objects prior to a lightning strike. Appears as a bluish or greenish glow around the top of the pole or object.

SAFFIR SIMPSON SCALE - An F1 to F5 scale for hurricane wind speeds and inflicted damages / storm surge. Developed by former director of the NHC Dr Saffir and structural engineer Dr Simpson.

SAL - Saharan Air Layer. Dry and dusty air from Africa advecting across the tropical Atlantic during the summer months. Commonly causes dust into Florida and the Caribbean. This air can also affect tropical cyclone development (such as weakening by dry air entrainment).

SARGASSO SEA - A pool of warm ocean water in the North Atlantic between Bermuda and the east coast of the United States. It has very weak currents because major ocean currents flow around it, with the Gulf Stream to its west. Its water is unusually warm and salty and often is covered with patches of seaweed called sargassum (sea grapes in Portuguese). This area is a very important region for tropical cyclone and oceanic extratropical storm formation.

SATELLITE - An instrument placed in earth orbit for communications and / or reconaissance. They provide invaluable images of earth from space showing weather patterns and cloud features.

SCALAR - A quantity that is expressed as a value without any directional consideration. Temperature is an example of a SCALAR quantity.

SCUD - Low broken clouds usually found beneath thunderstorm cloud bases, near fronts, or near cooler rain outflow regions. Common acronym for Scattered Cumulus Under Deck.

SDS - Storm Deprivation Syndrome. A slang but often used expression storm chasers experience when there is a period of no storms, especially outside of the chase "season". It is technically a form of obcessive stress which is very similar to a person being deprived of something he or she wants to do or experience and can't. Originated by Steve Miller of the Texas Tailchasers group as SUPERCELL DEPRIVATION SYNDROME. The more general term of "DEPRIVATION SYNDROME" is used by psychiatrists to denote symptoms a person experiences when he or she cannot fulfill desires or needs to an obcession.

SEA LEVEL - The average height of the earth's oceans. Often referred to as mean sea level, or MSL. Elevation and altitude is often measured relative to sea level. Normal mean sea-level pressure, or MSLP, is 1016 mb, or 30" of mercury (14.7 pounds per square inch).

SEABREEZE - A wind which blows off a large body of water due to the air over land heating up faster than that over the water. May also occur over large lakes as a LAKE BREEZE. Winds blowing towards the body of water because of the water being warmer is called a LAND BREEZE. Common over the tropics and during the hot summer months.

SEABREEZE FRONT - A leading edge of cooler marine air as the land heats up faster than the ocean in coastal areas. This forms a line of clouds, and could trigger deep convection in unstable environments. For large lakes, may be called a LAKE BREEZE FRONT.

SEVERE THUNDERSTORM - A thunderstorm which produces winds gusting at or above 58 MPH, hail at or exceeding 3/4", or where tornadoes develop. Either of these events, some, or all of them constitute a severe thunderstorm. Watches and warnings are posted for these type of storms. If the storm is over coastal waters, a SPECIAL MARINE warning may be required.

SEVERE WEATHER AWARENESS WEEK - A week dedicated to severe weather and tornado safety and awareness in the United States. Designation of this week varies by state.

SFC - Abbreviation for surface. May also be referred to as SURFACE OBS.

SFERICS - Radio frequency disturbances produced by electric discharges (usually lightning) in the atmosphere. Commonly causes crackling or static on the AM radio bands.

SHEAR - Changes of wind speed and / or direction along a horizontal or vertical path. Vertical wind shear often tilts the updraft in a thunderstorm causing it to continue to intensify because the precipitation and downdrafts fall away from the updraft column. The same kind of shear, however, can disrupt the structure of a tropical system leading to its demise. Horizontal shear may be important for the development of a cyclone (cyclogenesis).

SHEAR AXIS - A horizontal region, such as a front or boundary, where winds of different speed / direction meet.

SHEAR VECTOR - Measurement of the absolute vertical wind shear, incorporating both speed and directional shear, as a vector quantity (usually in knots with direction). For example, a 10 knot SE wind with a 30 knot NW wind above it should produce a SHEAR VECTOR of 20 knots from the NW.

SHELF CLOUD - A type of accessory cloud associated with outflow regions of a thunderstorm. Commonly associated with gust fronts. Do not confuse these with wall clouds. Sometimes called a ROLL CLOUD.

SHEET LIGHTNING - Simply is a flash of lightning, from a bolt of chain lightning from a distant storm or cloud which is not visible, that lights up the sky.

SHERIFFNADO - Slang term for a non-tornadic thunderstorm feature, such as scud clouds, that is reported as a funnel or tornado mistakingly by law enforcement.

SHIFOR MODEL - Statistical Hurricane Intensity Forecast model. A statistical-only moden from the National Hurricane Center for potential tracks of tropical cyclones based solely on linear regressions from a storm's track. Similar to the CLIPER MODEL.

SHIPS MODEL - Hurricane Intensity Prediction Scheme. A statistical synoptic model from the National Hurricane Center for potential tracks of tropical cyclones based on both environmental conditions as well as statistical data.

SHOCK WAVE - A rapid and transient change in air pressure in the same sense as an intense sound wave. May also travel through solids and liquids such as rock and water in the same sense. Shock waves are commonly generated in the atmosphere by explosions, lightning, and super sonic aircraft. They most commonly generated in rock by earthquakes. Thunder is the result of shock waves from the explosion of superheated air along a lightning discharge.

SHORT WAVE - A radio frequency spectrum near the upper end of the HF band. See also HF. In meteorology, refers to a small-scale synoptic low pressure trough. See also WAVE.

SIGMET - SIGnificant METeorological hazard or event. Aviation term for any hazardous or important meteorological concern to the aviation community. Often used with thunderstorms as a CONVECTIVE SIGMET.

SINGLE VORTEX - Term applied to a tornado containing only one main vortex.

SKYWARN - A spotter network for a particular area. This is a network of storm spotters and / or storm chasers who report severe thunderstorm and tornado observations to their local NWS office.

SLIPSTREAM - Aerodynamic term used for a fluid moving along the surface of an object. This term is commonly applied to objects in a strong wind field as well as along the surfaces of aircraft as they move through the air.

SLOSH - Computer based software for predicating and simulating storm surge and wave heights as a strong wind or tropical system affects a coastline.

SLOT - A region of air being drawn into a storm system. Usually used in medium to large scale storm systems as the famous DRY SLOT.

SMALL CRAFT ADVISORY - Non convective type advisory product of the National Weather Service warning of dangerous wind / wave conditions for mariners operating small vessels (less than 100 feet in length). Often applies to dangerous wave activity and / or winds less than 35 Knots (38 MPH). Sometimes a SMALL CRAFT EXCERCISE CAUTION may be issued for similar, but less dangerous conditions.

SNAIL - A prototype device designed by engineer Frank Tatom and deployed by storm researcher Tim Samaras and his colleagues for detecting vibrations produced by a tornado on the ground. Such seismic vibrations, at certain frequencies lower than the range of human hearing, can travel long distances. Detecting these waves can hint a tornado is nearby, and whether it is moving closer or farther away.

SNOW - A form of frozen precipitation in the form of flake like crystals or clumps. Normally falls from stratiform clouds where the temperature is below freezing at the ground to the cloud altitude.

SNOW SQUALL - A sudden and very heavy snow shower. Can be associated with snow thunderstorms or simply having strong winds prevailing off a relatively warm body of water in freezing, unstable conditions. These occur commonly along the Eastern Great Lakes in the winter months.

SONIC BOOM - A loud noise heard from the shock waves generated by an object traveling faster than sound. An object traveling faster than sound causes an accumulation of pressure waves on its leading edges as it passes through the air. These pressure waves expand and pile up forming a shock wave which contines outwards at the speed of sound, which is left behind the object. People on the ground hear a sonic boom as this shock wave reaches them.

SOUNDING - Data gathered by a weather balloon or similar instrument for the winds and conditions aloft. Very important for upper level wind and temperature or pressure profile determination.

SOUP - Slang name for multilayered and / or low clouds asso