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Severe Weather Glossary

 

Anvil - The top part of a thunderstorm that spreads out across the sky.  It usually spreads in all directions, but due to the upper level wind flow most of the anvil spreads in the direction of the upper level winds.  Anvils occur with severe and strong thunderstorms as the air rising in the updraft reaches the equilibrium level.

Air mass Thunderstorm - A thunderstorm not associated with a front.  Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to afternoon heating by the sun, and dissipate rather quickly after sunset.  Generally, they are less likely to be severe than other types of thunderstorms.

Arcus Cloud - A low, horizontal cloud formation associated with a thunderstorm's outflow.  The air from a thunderstorm can fan out in 360º.  So, a cloud formed from this rush air can be in the shape of a smooth curve.

Bow Echo- A radar signature formed by a line of thunderstorms where the line bulges outward into a bow shape.  Damaging winds often occur near the center of the bow echo but can occur anywhere along the bowing line.  Thunderstorms that form a bow echo are usually moving at a rapid pace of at least 40 mph.  It is rare, but bow echoes can produce brief tornadoes.  This can occur at either end of a bow echo, but most likely in the left (usually northern) end, where the circulation shows cyclonic rotation.

Box (Watch Box) - A severe thunderstorm watch or tornado watch. "Box"comes from the shape a watch usually takes.  It is that of a rectangle or a parallelogram when plotted on a map.  The Storm Prediction Center in Norman, Oklahoma issues such severe weather watches.

Cap - A layer of warm air, sometimes several thousand feet above the surface.  A cap will suppress or delay the development of thunderstorms by preventing unstable air near the surface from rising past the cap.  A cap, also called a "lid" sometimes, can be broken if the air near the surface becomes unstable enough to rise past the cap or "break the cap."  A cap can also be broken when cooler air aloft moves into the area of the cap.  When the cap is "broken" explosive thunderstorm development can result.

Cold Air Funnel - A funnel cloud that can develop from an elevated shower or thunderstorm when the air aloft is unusually cold (hence the reference to "cold air").  Since cold air funnels develop from an elevated storm they rarely touchdown.  If that happens the result will be a small, relatively weak tornado.

Convection - The vertical transport of heat and moisture, especially by updrafts and downdrafts in unstable air.  Showers and thunderstorms are forms of convection.  The more unstable the atmosphere the greater the convection will be and the more likely the resulting storms can become severe.

Cumulonimbus Cloud - A cloud characterized by strong vertical development in the form of huge cloud towers.  Sometimes it is topped wholly or partially by an anvil.  This type of cloud is more commonly called a thunderhead.  The root word "nimbus" denotes the cloud is producing rain.

Derecho - A widespread and usually fast-moving severe windstorm associated with a squall line usually containing a bow echo.  Derechos can produce damaging winds over areas hundreds of miles long and more than 100 miles across.  The length of time the severe winds last can be particularly damaging.  While a severe thunderstorm may produce severe wind gusts that last for several minutes at a point location, derecho wind can last 30 minutes or longer.

Downburst - A strong downdraft from a severe thunderstorm resulting in an outward burst of damaging winds on or near the ground.  Downburst winds can produce damage similar to that of a strong tornado.

Downdraft - The portion of a thundertorm that contains rain and hailstones and produces wind.  When downdrafts are strong enough they can produce downbursts.  If a thunderstorm is an engine, then the downdraft is the exhaust system.  It is opposite the updraft.

Dry Line - A boundary separating moist and dry air masses.  It is an important factor in initiating severe in the Great Plains.  Dry air is more dense than moist air, so when the dry line marches eastward, it lifts the moist air which will often birth a thunderstorm.  It typically lies north-south across the central and southern High Plains (the Panhandles) during the spring and early summer, where it separates moist air from the Gulf of Mexico and dry air from the Desert Southwest.  The dry line typically advances eastward during the afternoon and retreats westward at night.  However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even east, regardless of the time of day.

Dust Devil - A small atmospheric vortex not associated with a thunderstorm, which is made visible by a rotating cloud of dust or debris (dust whirl).  Dust devils form in response to surface heating during fair, hot weather.  They are most frequent in arid or semi-arid regions such as Texoma.  Very rarily do dust devils creat any type of damage.

Eye - A region in the center of a hurricane (tropical storm) where the winds are light and skies are clear to partly cloudy due to an area of sinking air.

Eye Wall - A wall of dense thunderstorms that surrounds the eye of a hurricane.  This is where the most intense winds of a hurricane occur.

Fujita Scale (or F-Scale) - A scale of wind damage intensity in which wind speeds are inferred from an analysis of the damage.  All tornadoes, and most other severe local wind storms, are assigned a single number from the scale according to the most intense damage caused by the storm. In February, 2007, the Enhanced Fujita Scale or EF Scale will be introduced.

F0(weak):	40-72 mph, light damage
F1(moderate):	73-112 mph, moderate damage
F2(strong):	113-157 mph, considerable damage
F3(strong):	158-206 mph, severe damage
F4(violent):	207-260 mph, devastating damage
F5(devestating):	261-318 mph, (rare) incredible damage

Flash Flood - A flood which is caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours.

Flood - The filling with water of a normally dry area of land caused by an increased water level in a stream, river or drainage ditch or by the ponding of rainwater.

Flood Warning - Flooding has been reported or is imminent. Take the necessary precaution if you are in a flood prone area.  This can be issued by the National Weather Service for either Flash Flooding, Area Flooding, or River Flooding

Flood Watch - Issued by the National Weather Service before an actual flooding event is expected to occur.  Flooding of streets, roads, the area in general, creeks, streams, and/or rivers will be possible within the watch area during the time frame of the Flood Watch. Sometimes this is called a Flash Flood Watch to indicate the possibility of rapidly rising water and flooding on streets, underpasses, in ditches, and around storm drains.

Funnel Cloud - A rotating funnel extending from the base of a severe thunerstorm, associated with a rotating column of air that is NOT in contact with the ground at the moment.  When the column is in contact with the ground and the cloud, then it is a tornado.  Until that moment, the terms funnel cloud and tornado are NOT interchangeable.

 

Gust Front - The leading edge of gusty surface winds from thunderstorm downdrafts.  It serves as the boundary between air flowing into a thunderstorm and the precipitation-cooled air flowing out of the storm.  An arcus or shelf cloud may be seen above its surface position.  There is a noticeable wind shift and temperature drop that occurs when the gust front passes (similar to a cold front).  The gust front often precedes the precipitation by several minutes.

Gustnado - Yes, it is a real term although its origin is slang.  It is a small, relatively weak spinning column of air that resembles a tornado and can occur with supercell thunderstorms that spawn tornadoes.  They are usually short-lived and occur along the gust front of a thunderstorm.  While they may be considered weak, gustnados can produce some damage.

Hail - Balls or chunks of ice larger than 1/4 inch in diameter which are produced due to strong updrafts in thunderstorms.  Hailstone sizes are typically estimated comparing the size of the stones to coins (dime, nickel, quarter, half-dollar, etc.) or sports balls (ping pong, golf, baseball, softball, etc.).  3/4 inch or penny-sized denotes severe hail.

High Wind Warning - A warning for sustained surface winds greater than 40 mph lasting more than an hour or winds over 58 mph over land that are either predicted or occurring for an unspecified period of time.

Hook Echo - A radar pattern characterized by a hook-shaped extension of a thunderstorm echo which is usually in the southwest part of the storm.  A hook is associated with a mesocyclone, and indicates favorable conditions for tornado development.  Tornado Warnings are often issued when the hook echo signature is detected.

Hurricane - A severe tropical cyclone having winds in excess of 64 knots (74 mph).

Hurricane Warning - Hurricane conditions are expected within 24 hours.  Complete all storm preparations and evacuate if directed by local officials.

Hurricane Watch - Hurricane conditions (heavy rain, tidal flooding, and winds above 75 mph) are possible within 36 hours.  Prepare to take immediate action in case a warning is issued.

Instability - The tendency for air parcels to accelerate when they are displaced from their original position.  The greater the instability, the greater the potential for severe thunderstorms.  One way instability is measured is with a value of CAPE (Convective Available Potential Energy) in Joules per kilogram (energy per unit mass).

Inversion - Usually used in reference to temperature.  It is an increase in temperature with height (which in the reverse of what usually occurs in the atmosphere).   An inversion can act like a Cap (see above) and either prohibit severe weather or enhance it.

Lapse Rate - The rate of change of an atmospheric variable, usually temperature, with height.  A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring.

Lifted Index (or LI) - A common measure of atmospheric instability. The value is obtained by computing the temperature that air near the ground would have if it were lifted to a higher level at 500 mb (usually around 18,000) and comparing that temperature to the actual temperature at that level.  Negative values indicate instability.  The more negative the value, the more unstable the air.

Lightning - A visible electrical discharge produced by thunderstorms caused by an imbalance of electrons between different storm cells or between the thunderstorm and the ground.

Mammatus Clouds - Rounded, sack-like protrusions hanging from the underside of a cloud (usually a thunderstorm anvil).  These clouds do not produce severe weather.  They often accompany severe thunderstorms, but may accompany non-severe thunderstorms as well.

Mesoscale Convective Complex (MCC) - A large complex of thunderstorms grouped in a cluster.  The cluster is generally round or oval-shaped and normally reaches peak intensity at night.  An MCC must meet certain criteria for size, duration, and shape.  MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which the potential for severe weather is greatest.  During peak intensity, the primary threat shifts toward heavy rain and flooding.

Mesoscale Convective System (MCS) - A term often used to describe a cluster of thunderstorms that does not meet the size, duration, or shape criteria of an MCC.  It is a complex of thunderstorms, which may be round-shaped or in line, and normally persists for several hours or more.  An MCS can still produce severe weather.

Mesocyclone - A region of rotation, typically 2 to 6 miles in diameter, often found in the southwest part of a supercell.  The circulation of a mesocyclone covers an area much larger than the tornado which may develop within it.  This is technically a radar term defining a signature of rotation on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration.  The Mesocyclone produces the hook echo (see above) signature.

Microburst - A small, concentrated downburst affecting an area less than about 2.5 miles across.  Most microbursts are rather short-lived (5 minutes or so), but on rare occasions have been known to last up to 30 minutes.

Moisture Advection - Transport of moisture by horizontal winds.  This can sustain severe and non-severe thunderstorms at night when the heat of the day is lost.

Outflow Boundary - A boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to that of a cold front, with the passage marked by a wind shift and usually a drop in temperature.  Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin.  New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary) or at the time of max heating the next day.

Pulse Storm - A thunderstorm that lasts for a brief period (pulse).  If the storm contains strong updrafts, the storm can produce a short episode of severe weather.  These storms generally are not tornado producers, but can produce large hail and/or damaging winds.

Roll Cloud - A low, horizontal, tube-shaped cloud associated with a thunderstorm gust front. Roll clouds are completely detached from the base of the thunderstorm and appear to be "rolling" about a horizontal axis. They are NOT horizontal funnel clouds.

Rope - A narrow, often contorted funnel which may appear as a rope dangling from the thunderstorm.  It is associated with the decaying stage of a tornado.

Scud - The common name for stratus fractus clouds.  They are small, ragged, low cloud fragments that are not attached to a larger cloud base (such as the base of a thunderstorm). They are often seen with and behind cold fronts and thunderstorm gust fronts.  These clouds do not produce severe weather.  When they are near, or attached to the base of the thunderstorm, they can be mistaken for funnel clouds.

Severe Thunderstorm - A thunderstorm with winds of 58 mph (50 knots) or more, hail 3/4 inch in diameter or larger, or a tornado. Structural damage may imply the occurrence of a severe thunderstorm. Severe hail, severe wind, and tornadoes are the only elements for classifying a thunderstorm as severe.  Occassional, frequent, or near constant lightning and flooding rains are elements of the thunderstorm itself and do not make the thunderstorm "severe."  However, more times than not they do accompany severe weather.

Severe Thunderstorm Warning - Issued by the National Weather Service when severe weather has been reported or is being indicated by Doppler radar.  Warnings indicate imminent danger and the appropriate action should be taken.  A warning is issued when a thunderstorm may produce wind gusts of or more than 58 mph and/or ¾" or larger hail.

Severe Thunderstorm Watch - An outlined area where severe thunderstorms are more likely to occur within a certain time frame.  Sometimes will be referred to as a "watch box".  During a watch you should stay informed and watch the weather situation closely by staying tuned to 7NEWS and your SkyWARN 7 Weather Team.   Another handy tool to have during a watch is a NOAA Weather Radio.

Squall Line - A solid or nearly solid line or band of active thunderstorms.

Straight Line Winds - Any wind that is not associated with rotation.  The term is mainly used to distinguish thunderstorm winds from tornadic winds.  Straight-Line winds originate as a downdraft of rain-cooled air, which reaches the ground and spreads out rapidly, producing a potentially damaging wind gusts. 

Suction Vortex - A small but very intense vortex within a tornado circulation.  Several suction vortices typically are present in a multiple-vortex tornado.  Much of the extreme damage associated with violent tornadoes is attributed to suction vortices which couple their damaging wind to that of the parent tornado.   It is the dancing of these vortices around the parent tornado which may create the phenomenon of the tornado missing one house completly while destroying the one next to it.

Supercell - A relatively long-lived thunderstorm with a persistent rotating updraft.  Supercells are rare, but are responsible for a remarkably high percentage of severe weather events-especially tornadoes, extremely large hail, and damaging wind storms.

Thunder - Thunder is produced as a sound wave.  Just as friction causes noise when you rub your hands together, massive friction between neighboring air molecules produces the loud noise.  The rapid expansion and subsequent contraction of air produces a sound wave that travels out from the source in all directions.  Thunder is produced along the entire length of the lightning channel where air expands rapidly.  This gives a prolonged thunder that lasts for several seconds.  Close lightning strikes don't sound as long because the thunder sound close to you is so loud that it overpowers the sound made when the lightning channel was further away.  Also, the sound waves from overhead thunder tend to curve away from a surface observer. Thunder that originates further away has a more rumbling sound to it due to reflection, scattering and damping of the noise as it moves away from the source region into low level terrain.  Sometimes, thunder is not heard altogether.  When more than 15 miles from the lightning discharge, the thunder will not make it to a surface observer since sound waves generally refract gradually away from the earth's surface due to the air density structure of the atmosphere.

Thunderstorm - A local storm produced by cumulonimbus clouds.  It is always accompanied by lightning and thunder.

Tornado - A violently rotating column of air in contact with the cloud and the ground.  The phrase "tornado on the ground" is redundant because by its very definition a tornado is on the ground.

Tornado Warning - A tornado has been reported or is being indicated as possible by Doppler radar.  Immediate action should be taken for you safety.  Follow the DUCK Rule.  Downstairs, Underneath Something, Center Part of House, Keep Away from Windows.

Tornado Watch - An outlined area where tornadoes and severe thunderstorms are more likely to occur within a certain time frame.  Sometimes will be referred to as a "watch box".  During a tornado watch you should stay informed and watch the weather situation closely by staying tuned to 7NEWS and your SkyWARN 7 Weather Team.   Another handy tool to have during a tornado watch is a NOAA Weather Radio.

Tropical Depression - A tropical cyclone in which the maximum 1-minute sustained surface wind is 38 mph or less.  They form from a tropical wave or tropical disturbance.

Tropical Disturbance - A discrete system of apparently organized convection originating in the tropics or subtropics, having a non-frontal migratory character and maintaining its identity for 24 hours or more.

Tropical Storm - A tropical cyclone in which the 1-minute sustained surface wind ranges 39-73 mph.  Tropical storms pose a threat to life and/or property in coastal areas.

Tropical Storm Warning - Tropical storm force winds are occurring or are expected within 24 hours.

Tropical Storm Watch - Tropical storm force winds between 37 and 74 mph are possible in the next 36 hours.

Updraft -A small-scale current of rising air.  If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or a cumulonimbus and can become the beginning of a thunderstorm.

 

Wall Cloud - A local, often abrupt lowering from the rain-free base of a thunderstorm.  Wall clouds can range from a fraction of a mile up to nearly 5 miles in diameter, and normally are found on the south or southwest side of the thunderstorm.  When seen from within several miles, many wall clouds exhibit rapid upward motion and counterclockwise rotation.  Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour.  Wall clouds must be monitored visually for signs of persistent, sustained rotation.

Waterspout - In general, a tornado occurring over water.  Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a cumulonimbus cloud or a towering cumulus cloud.  Waterspouts are most frequently observed in shallow waters off the Gulf Coasts of Texas, Mississippi, Alabama, and Florida, and off the Florida Atlantic and Caribbean Coasts.  They can occur in lance but over large lakes.