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Tornado Glossary

Don't be confused about terms commonly used in forecasting tornadoes.  Knowledge can mean safety.

Advisory
A statement issued by the National Weather Service when a weather event is expected to be an inconvenience to residents in the area, but does not meet warning criteria. During the spring months, watch for Significant Weather Advisories for strong storms that are just below severe thunderstorm limits.

Backing
A term used to describe a wind shift in a counterclockwise direction. Just ahead of a dryline, backing winds cause more convergence and increase the chance for developing thunderstorms.

Bow Echo
A line of thunderstorms that resemble a bow-shaped line on radar imagery. Bow echoes are often associated with damaging straight-line winds, and can sometimes produce weak tornadoes.

CAPE
Abbreviation for "Convective Available Potential Energy." A measure of the energy in the surrounding atmosphere available to cause convection. Higher values correspond to higher risks for severe weather.

Cold Air Funnel
Funnel clouds that develop from a small shower or thunderstorm when the air aloft is unusually cold. These funnels may touch down briefly, but are usually very short-lived and aren't associated with violent tornadoes.

Convergence
When winds at the surface run perpendicular or opposite each other, usually seen on either side of a moving dryline. When these winds collide at the surface, there is no way to go but up, which results in accelerated rising air and developing thunderstorms.

Cumulonimbus
A type of cloud, also called a "thunderhead." These clouds often resemble a blacksmith's anvil as the top of the cloud generally extends in a smooth plane outward from the center of the storm across the sky. These clouds are usually associated with lightning and strong winds.

Doppler Radar
Weather radar uses "The Doppler Effect" to measure the velocity of particles in the air, such as raindrops or hailstones. The Doppler Effect was named for Christian Doppler, an Austrian physicist who described the condition that causes a train whistle to sound differently depending on whether the train is traveling toward or away from an observer.

Downdraft
A powerful downward current of air, usually accompanied by precipitation within a thunderstorm.

Dryline
A frontal boundary between warm, moist air and warm, dry air. Drylines usually form near the Texas panhandle and move eastward through the day, causing severe thunderstorms in Oklahoma during the spring months.

Dust Devil
A rapidly rotating column of air that is made visible by the dust and debris that it picks up. Dust devils usually occur on hot and dry afternoons, when the ground warms very quickly.

EF-Scale
(Formerly known as the F-Scale) - A scale used to measure the strength of a tornado based on the damage it causes. The F-Scale, or "Fujita Scale" was introduced in 1971, created by Ted Fujita of the University of Chicago. The "EF-Scale" or "Enhanced Fujita Scale" was developed recently due to research that suggested wind speeds in the original scale were underestimated. The EF-Scale was put into operation early 2007. 

Eye
The calm, circular center of a tornadic storm. The eye of a tornado may be only a few feet or yards across.  

Funnel Cloud
A cloud, shaped like a funnel, that extends from the bottom of the cloud base. A rotating funnel cloud, associated with a wall cloud in a strong to severe storm, can develop into a tornado. A funnel cloud does not touch the ground. Once it makes contact with the ground, it is then considered a tornado.

Gustnado
A slang term used to describe a small, short-lived tornado that occurs along the gust front, or leading edge of a line of thunderstorms. Gustnadoes do not usually cause much damage.

Hail
Precipitation in the form of small pellets of ice, larger than 5 mm. Hail forms when water freezes at high altitudes within a thunderstorm and then falls to the ground. Strong thunderstorms, with powerful updrafts can produce larger pieces of hail when frozen water droplets stay within the cloud for longer periods of time and collect more ice, growing larger, until they are too heavy for the updraft to support and they fall to the ground. 

Hook Echo
A signature on radar, associated with a tornadic rotation that appears as a hook-like extension from a strong thunderstorm. Not all observed hook echoes accompany tornadoes, or vice-versa, but seeing a hook echo on radar is a good indication that a tornado may have formed, or may form quickly. 

Inflow
The inward flow of air moving toward a thunderstorm. Usually the air moving through the updraft is warm and moist air. 

Instability
The tendency for air parcels in an environment to rise very rapidly and possibly develop into thunderstorms. The greater the instability, the greater the chance for severe weather. 

Jet Stream
Very strong winds concentrated in a narrow stream in the upper atmosphere. Jet streams are the driving motion for weather systems across the continent. In the U.S., jet streams move from west to east.

Landspout
A whilrwind, similar in appearance to a tornado, but that does not occur from organized storm-level rotation. Landspouts are usually very short-lived, do not cause much damage and are seen as merely a dust whirl. 

Lightning
A sudden discharge of electricity produced when opposite electrical charges build up between clouds, between a cloud and the ground, or within one cloud. When the charges meet, a bright band of light occurs, accompanied by thunder, which is the sound of the super-heated, and quickly expanding air.

Low Level Jet
An area of relatively strong winds at the surface. Also abbreviated as LLJ, it usually refers to strong southerly winds that increase overnight, drawing in moisture from the Gulf of Mexico.

MCC/MCS
Abbreviation for Mesoscale Convective Complex/Mesoscale Convective System. These terms are usually used to define a large cluster of thunderstorms that persist for several hours. Sometimes this refers to a system that started as a line of thunderstorms and converged as the system weakened, especially in the evening, as it loses the heat from the sun. The main threats from these systems include high winds, heavy rainfall, and flooding. 

Mesocyclone
A cyclonic vortex of air, between approximately 2 and 10 km diameter within a convective storm.

Microburst
An area, usually defined as less than 2 ½ miles across, in which a concentrated downburst of sinking air can cause significant wind damage. Microbursts are usually short-lived and occur as a thunderstorm is decaying, and all of the air that had been lifted within the thunderstorm comes crashing to the surface. 

Multiple Vortex Tornado
A tornado during which, more than one condensation funnel is present at the same time, and are rotating around the same common center. Multi-vortex tornadoes can be among the most violent. 

NWS
Abbreviation for the National Weather Service. The NWS is a government agency that is the primary source of weather data for the United States. The NWS is responsible for issuing warnings during life-threatening weather situations. There are two NWS offices in Oklahoma; one located in Norman, the other right here in Tulsa. 

Outflow
The outward flow of air moving away from a thunderstorm. Usually air that is falling from a storm's downdraft and is usually cool and dry air.

Peak Gust
The highest instantaneous wind speed observed, usually within a thunderstorm. 

RFD
Abbreviation for Rear Flank Downdraft. An area of dry, sinking air on the back side of a thunderstorm. Usually associated with a clear area and cool winds that can be strong at times. 

Roll Cloud
A low, horizontal, tube-shaped cloud along the leading edge of a thunderstorm. Roll clouds are completely detached from the base of the thundertorm, unlike shelf clouds that are a part of the thunderstorm base. 

Rope Tornado
A narrow funnel, or tornado that resembles a rope and is usually seen in the decaying states of a tornado. 

Satellite Tornado
Refers to a single funnel within a multi-vortex tornado. Many times, a satellite tornado will be in the form of a smaller, rope-like tornado, rotating around a much larger, more violent tornado. 

Scud
A slang term referring to small, and ragged clouds that hang low along the horizon below the main cloud deck. These clouds are indicative of abundant surface moisture and upward motion close to the surface.

Severe Thunderstorm
A thunderstorm with winds of 58 mph or greater, and/or ¾" or larger hail. Heavy rainfall and frequent lightning are also threats from severe thunderstorms, and these storms have been known to produce tornadoes with little or no advanced warning. 

Shear
The change in wind speed or direction within a given area, either horizontally or vertically. 

Shelf Cloud
A low, horizontal, tube-shaped cloud along the leading edge of a thunderstorm, or sometimes along the leading edge of a cold front, even in the absence of thunderstorms. Unlike roll clouds, a shelf cloud is attached to the base of the thunderstorm, or the cloud deck associated with a cold front. 

SPC
Abbreviation for the Storm Prediction Center. A government agency that works closely with the National Weather Service Offices to monitor and forecast severe weather. The SPC is responsible for issuing all weather hazard watches across the U.S.

Squall Line
A line of thunderstorms, moving as one unit. Squall lines can move along a quickly advancing cold front or dryline. The main threats from a squall line are heavy rain, strong winds, hail. Tornadoes are also possible associated with bow echoes. 

Supercell
A dangerous convective thunderstorm with a persistant rotating updraft. Supercells are responsible for the majority of violent severe weather events, and are the most likely storms to produce tornadoes.

Tornado
A violently rotating column of air that is in contact with the ground. Sometimes a condensation funnel is not obvious for the entire length, cloud to ground, but even so, a debris cloud will be visible on the ground.

Tornado Alley
A geographic area in Mid-America that stretches from Texas to Nebraska and east into Iowa where tornadoes are most common. Tornadoes can form anywhere in the United States, but occur in greater numbers within Tornado Alley. 

Tornado Watch
Indicates that conditions are right within a certain area for tornadoes to form within developing thunderstorms. A tornado watch is issued for a large area that will include numerous counties. 

Tornado Warning
Indicates that a tornado has been detected, either by Doppler radar or by a storm chaser out in the field. A tornado warning is issued for a small area that will include one-two counties. When a tornado warning is issued, residents within the warned area should take cover immediately.

Triple Point
The intersection point between boundaries, such as a dryline and a cold front. Often used to describe the area of low pressure where a cold front, dryline, and warm front meet. This area can often be the focus for severe thunderstorm development. 

Updraft
A powerful upward current of air. Updrafts are composed of warm, moist air that feeds a thunderstorm. As it rises, the moisture condenses to form a cumulus cloud and this is often the first visual sign of a thunderstorm. 

Veering
A term used to describe a wind shift in a clockwise direction. As opposed to backing winds, veering winds cause divergence at the surface, and often aid in inhibiting severe weather. 

Virga
As seen in the sky, streaks or wisps of precipitation, either liquid or frozen, falling from a cloud, but evaporating before reaching the ground. Often virga can be detected on radar as very light rain, even though it is not reaching the ground.

Vortex
Is a spinning, often turnulent flow (or any spiral motion) with closed streamlines. 

Wall Cloud
A localized, and often abrupt lowering from the cloud base. Wall clouds form in the lower portion of a strong updraft, usually associated with a supercell. Wall clouds are normally found on the south or southwest wide of a thunderstorm, normally within the inflow region. Wall clouds that exhibit persistent, sustained rotation can often precede tornado formation. 

Waterspout
In the most general form, a non-supercell tornado over water. Waterspouts form in a different manner than tornadoes, and are not associated with a wall cloud, or a rotating thunderstorm. Waterspouts are often much weaker than a supercell tornado and are near the water equivalent of a landspout. Waterspouts are most prevalent during the summer months and more waterspouts are reported in the Florida Keys than any other place in the world. 

Wedge Tornado
A tornado that appears wider than it is tall, and has a wedge-like appearance. A tornado's destruction can not be defined by its size, but some of the most violent tornadoes have been reported as a wedge.

Glossary of Weather
Weather is its own language. There are many terms that are self explanatory, like wind, cloudy, sunny, but some terms can make you scratch your head, vortex, hook echo, gustnado. To answer all your weather term questions visit the National Weathers Services weather glossary

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