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Tornado/Severe Weather Definitions

Downburst:
Are created by a column of sinking air that, after hitting ground level, spreads out in all directions and is capable of producing damaging straight-line winds of over 150 mph (240 km/h), often producing damage similar to, but distinguishable from, that caused by tornadoes. This is because the physical properties of a downburst are completely different from those of a tornado. Downburst damage will radiate from a central point as the descending column spreads out when impacting the surface, whereas tornado damage tends towards convergent damage consistent with rotating winds. To differentiate between tornado damage and damage from a downburst, the term straight-line winds is applied to damage from microbursts.

Downbursts are particularly strong downdrafts from thunderstorms. Downbursts in air that is precipitation free or contains virga are known as dry downbursts; those accompanied with precipitation are known as wet downbursts. Most downbursts are less than 2.5 miles (4 km) in extent: these are called microbursts. Downbursts larger than 2.5 miles (4 km) in extent are sometimes called macrobursts. Sometimes downbursts are larger: in the extreme case, a derecho can cover a huge area more than 200 miles (320 km) wide and over 1000 miles (1600 km) long, lasting up to 12 hours or more, and is associated with some of the most intense straight-line winds, but the generative process is somewhat different from that of most downbursts.
Fujita Scale:
(F-Scale) standard scale for rating the severity of tornadoes as a measure of the damage they cause, devised in 1951 by the Japanese-American meteorologist Tetsuya (Ted) Fujita (1920–98). It classifies tornadoes on a hierarchy beginning with category F0, or "light" (winds of 40–72 mph; some damage to chimneys, TV antennas, roof shingles, trees, signs, and windows), which accounts for about 28% of all tornadoes. Category F1, or "moderate" (winds of 73–112 mph; automobiles overturned, carports destroyed, and trees uprooted), accounts for about 39% of all tornadoes. Category F2, or "significant" (winds of 113-157 mph; roofs blown off homes, sheds and outbuildings demolished, and mobile homes overturned), accounts for about 24% of all tornadoes. Category F3, or "severe" (winds of 158–206 mph; exterior walls and roofs blown off homes, metal buildings collapsed or severely damaged, and forests and farmland flattened), accounts for about 6% of all tornadoes. Category F4, or "devastating" (winds of 207–260 mph; few walls, if any, left standing in well-built homes and large steel and concrete missiles thrown great distances) accounts for about 2% of all tornadoes. Category F5, or "incredible" (winds of 261–318 mph; homes leveled or carried great distances and schools, motels, and other larger structures have considerable damage with exterior walls and roofs gone), accounts for less than 1% of all tornadoes. Category F6, or "inconceivable" (winds of 319–379 mph; automobiles become missiles), accounts for far less than 1% of all tornadoes.
Hail:
Precipitation in the form of pellets composed of ice or of ice and snow, occurring at any time of the year, usually during the passage of a cold front or during a thunderstorm. Small hailstones have a soft center and a single outer coat of ice. They are formed when the surfaces of snow clumps melt and refreeze or become coated with water droplets that subsequently freeze. Large hailstones usually have alternate hard and soft layers. There are various explanations of how these large stones form and grow. Some believe that they form in clouds when supercooled raindrops (i.e., drops chilled below the freezing temperature without solidifying) freeze on dust particles or snowflakes. These tiny hailstones are then blown repeatedly up and down by the winds in a cloud. Each time they are blown downward to a region whose temperature is above freezing, the stones collect more moisture, and each time they are blown upward to a region below freezing, the moisture solidifies into ice, and some snow may collect. The stones continue to grow, adding layer after layer, until they are too heavy to be supported by the winds and fall to the ground. In another explanation, it is suggested that hailstones continuously descend, gaining layers by passing through regions of the air that contain different amounts of water. Hailstones are spherical or irregularly spherical and usually vary in diameter up to 1-2 in. (1.3 cm); in rare cases hailstones having diameters up to 5 in. (12.7 cm) have been observed. Hail causes much damage and injury to crops, livestock, property, and airplanes.
Lightning:
Electrical discharge accompanied by thunder, commonly occurring during a thunderstorm. The discharge may take place between one part of a cloud and another part (intracloud), between one cloud and another (intercloud), between a cloud and the earth, or earth and cloud. Lightning may appear as a jagged streak (forked lightning), as a vast flash in the sky (sheet lightning), or, rarely, as a brilliant ball (ball lightning). Illumination from lightning flashes occurring near the horizon, often with clear skies and the accompanying thunder too distant to be audible, is referred to as heat lightning. There are numerous theories on why charges accumulate in the atmosphere. It is thought that temperature and water vapor pressure in thunderstorm clouds are associated with the positive and negative ions that cause lightning. Long-lasting lightning flashes with lower current are more damaging to nature and humans than shorter flashes with higher currents. Benjamin Franklin, in his kite experiment (1752), proved that lightning and electricity are identical.
Mesocyclone:
Is a vortex of air, approximately 2 to 10 km in diameter (the mesoscale of meteorology), within a convective storm. That is, it is air that rises and rotates around a vertical axis, usually in the same direction as low pressure systems in a given hemisphere. They are most often cyclonic, that is, associated with a localized low-pressure region within a severe thunderstorm. Such storms can feature strong surface winds and severe hail. Mesocyclones often occur together with updrafts in supercells, where tornadoes may form. Mesocyclones are normally relatively localized: they lie between the synoptic scale (hundreds of kilometers) and small scale (hundreds of meters).
Thunder:
Sound produced along a path of a lightning flash, caused by the rapid heating and expansion of the adjacent air. Rolling thunder occurs either as a result of the time difference between sounds from the far and near end of a flash, or when mountains, layers of air, or other obstructions cause reverberations. Since sound travels about 1 mi in 5 sec, the distance between a lightning flash and an observer may be determined by counting the seconds between the flash and the thunder. Thunder as far distant as 10 to 15 mi (15 to 25 km) from an observer is usually not heard, even though lightning is often seen.
Thunderstorm:
Also called an electrical storm or lightning storm, is a form of weather characterized by the presence of lightning and its attendant thunder produced from a cumulonimbus cloud. The lightning is caused by an electrical charge that builds up inside the storm due to the movement of water droplets or crystals carried by the wind. Thunderstorms are usually accompanied by heavy rainfall (heavy downpours), strong winds, hail, and sometimes tornadoes. During winter months, snowfall occasionally takes place in a thunderstorm, an occurrence sometimes termed thundersnow.
Tornado:
Dark, funnel-shaped cloud containing violently rotating air that develops below a heavy cumulonimbus cloud mass and extends toward the earth. The funnel twists about, rises and falls, and where it reaches the earth causes great destruction. The diameter of a tornado varies from a few feet to a mile; the rotating winds may attain velocities of 200 to 300 mi (320–480 km) per hr, and the updraft at the center may reach 200 mi per hr. The Fujita scale is the standard scale for rating the severity of a tornado as measured by the damage it causes. A tornado is usually accompanied by thunder, lightning, heavy rain, and a loud "freight train" noise.

In comparison with a cyclone or hurricane, a tornado covers a much smaller area but can be violent and destructive. The atmospheric conditions required for the formation of a tornado include great thermal instability, high humidity, and the convergence of warm, moist air at low levels with cooler, drier air aloft. Although tornadoes have occurred on every continent except Antarctica, they are most common in the continental United States, where tornadoes typically form over the central and southern plains, the Ohio valley, and the Gulf states. The area where the most violent storms commonly occur in the United States is known as Tornado Alley, which is usually understood to encompass the plains from N central Texas north to the Dakotas, with the peak frequency located in Oklahoma. A tornado typically travels in a northeasterly direction with a speed of 20 to 40 mi (32–64 km) per hr, but tornadoes have be reported to move in a variety of directions and as fast as 73 mi (117 km) per hr—or to hover in one place. The length of a tornado's path along the ground varies from less than one mile to several hundred. Tornadoes occurring over water are called waterspouts.
Tornado Watch:
Is issued when weather conditions are favorable for the development of severe thunderstorms that are capable of producing tornadoes. A tornado watch therefore implies that it is also a severe thunderstorm watch. A tornado watch must not be confused with a tornado warning. A watch does not mean that the severe weather is actually occurring, only that conditions have created a significant risk for it. If severe weather actually does occur, a tornado warning or severe thunderstorm warning will be issued.
Tornado Warning:
Means there is immediate danger for the warned and immediately surrounding area -- if not from the relatively narrow tornado itself, from the severe thunderstorm producing (or likely to produce) it. All in the path of such a storm are urged to take cover immediately, as it is a life-threatening situation. A warning should not be confused with a tornado watch (issued by a national guidance center, the Storm Prediction Center) which only indicates that conditions are favorable for the formation of tornadoes.
Straight-line Winds:
Are very strong winds that produce damage, demonstrating a lack of a rotational damage pattern. Such rotational damage patterns are associated with cyclonic storms including tornadoes and tropical cyclones. Straight-line winds are common with the gust front of a thunderstorm or originate with a downburst from a thunderstorm. Also known as thundergusts and hurricanes of the prairie.
 
Supercell:
Is a severe thunderstorm with a deep, persistently rotating updraft (a mesocyclone). Supercell thunderstorms are the largest, most severe class of thunderstorms. It has been argued that there are really only two types of thunderstorms: supercell and ordinary, though some have four classifications: single-cell, multi-cell, squall line, and supercell.

Supercells are usually found isolated from other thunderstorms, although they can sometimes be embedded in a squall line. Because they can last for hours, they are known as quasi-steady-state storms. Supercells have the capability to deviate from the mean wind. If they track to the right of the mean wind (relative to the vertical wind shear), they are said to be "right-movers." Alternatively, if they track to the left of the mean wind (relative to the shear), they are said to be "left-movers."

Supercells can be any size, large or small, low or high topped. They usually produce copious amounts of hail, torrential rainfall, strong winds, and substantial downbursts. Supercells are one of the few types of clouds that typically spawn tornadoes within the mesocyclone, although only 30% or less do so. Supercells can occur anywhere in the world under the right pre-existing weather conditions. According to some, the first storm to be identified as such was the Wokingham storm over England, which was studied by Keith Browning and Frank Ludlam in 1962. As with tornadoes in general, they are most frequent in the Great Plains of the United States.

Definitions obtained from http://encyclopedia.thefreedictionary.com

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