Chapter 11: Lightning, Thunder, and Tornadoes
Thunder
Caused by the explosive air expansion due to the tremendous increase in temp. during a lightning stroke. -A lag in lightning strike and thunder occurs due to sound traveling slower than light.
Multicell thunderstorms
Clusters of thunderstorms.
Charge separation
In order for lightning to initiate, electrical charges must separate within portions of a cloud. Positive charges accumulate at the top of a cloud, while negatively charged particles are in the lower part of a cloud. -Lightning only occurs in clouds that extend above the freezing level, and is also restricted to precipitating clouds. -Ice crystals in clouds influence the separation.
Supercells
Intensely powerful storms that contain a single updraft zone. -They are one powerful cell, as opposed to a cluster of cells. -Smaller than squall lines or MCCs but they are more violent and produce potential for very large tornadoes
Severe thunderstorm and tornado warning
Issued if a severe thunderstorm or tornado has already developed.
Mesocyclones
Large vortices deep within the cloud interior. The formation depends on the presence of vertical wind shear. -They often precede the formation of an actual tornado by some 30 minutes or so.
Sprites
Large, short duration electrical bursts from the tops of clouds producing lightning below. -Occur in only about 1% of all lightning events.
Suction vortices
Some of the most violent tornadoes have several small zones of intense rotations. -These small vortices are what cause one house to be totally destroyed while the next one remains unscathed.
Derechos
Spanish word meaning "straight ahead" Larger-scale horizontal winds caused by strong downdrafts from MCCs. Most prevalent in the U.S. between May and July. -They can last for hours at a time.
Runaway breakdown
The energy released when a large number of runaway electrons accumulate in a small volume.
Gust front
When downdrafts reach the ground and surge forward as a wedge of cold, dense air.
Stroke, return stroke
When the leader connects, electrons flow in an illuminated return stroke. Although the electrical current is from the cloud to the ground, the return stroke is in the opposite direction. -Usually more than one stroke is needed to neutralize all negative ions.
Severe thunderstorms
Wind shear, high water vapor, uplift, and instability are critical conditions for severe storm development. -Have wind speeds that exceed 93 km/hr (58mph), hailstones larger than 2.4cm (1in.) in diameter, or spawn tornadoes.
Air mass thunderstorm stages
*Cumulus stage: Unstable air rises, often by localized convection as some surfaces undergo more rapid heating than others, and cools adiabatically. *Mature stage: Marked by precipitation and the presence of both up and down drafts. -Downdrafts are initiated through frictional drag associated with falling precipitation. -This is also a time of lightning and thunder. -Updrafts dominate the interior portions of the storm while downdrafts occur toward the edges. *Dissipative stage: Occurs when downdrafts dominate airflow within the thunderstorm. -This suppresses updrafts and the addition of water vapor. -Precipitation then ceases and the cloud eventually evaporates.
Doppler effect
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Doppler radar
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Fair-weather electric field
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Mean electric field
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Severe storm and tornado watches
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Sweep
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Vault
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Volume sweep
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Cloud-to-ground lightning
20% of lightning It begins when negative charges build up in a cloud base and eventually discharge into the positively charged ground.
Cloud-to-cloud lightning
80% of all lightning. Occurs when discharges of electricity occur within a particular cloud or between clouds.
Flashes
A combination of all strokes.
Shelf cloud
A portion of a severe thunderstorm cloud that protrudes ahead of the main portion of the cloud and above a gust front.
Mesoscale convective complexes (MCCs)
Account for the greatest amount of severe weather in the U.S. and Canada. Circular clusters of thunderstorms which are self propagating, help create more thunderstorms. -The entire system typically propagates eastward
Ball lightning
An unusual type of lightning. Appears as a round, glowing mass of electrified air. -Small soil particulates bounce in the air as a result of chemical energy and burn brightly as they oxidize, releasing stored energy in the form of visible light.
Tornadoes
Areas of rapid, rotating, lifting winds beneath cumulonimbus clouds Strong counterclockwise winds originate in relation to large pressure gradients over small spatial scales -Most tornadoes occur in the springtime, when the contrast between warm and cold air in the atmosphere is the greatest. -Common to frontal boundaries, squall lines, MCCs, supercells and tropical cyclones. -Most violent tornadoes are associated with supercells -Usually only last a few minutes and travel northeast at comparable speeds of a car.
Enhanced Fujita scale
Scale which classifies tornadoes by the magnitude of damage they cause.
Blue jets
Similar to sprites in that upward electrical bursts occur from active thunderstorm regions.
Tornado outbreak
Describe an event in which a single weather system produces at least six tornadoes.
Microbursts
Downbursts with diameters of less than 4km (2.5mi) -Can produce dangerous situations at airports, as they impede air travel.
Stepped leader
For cloud-to-ground lightning to occur, this must emanate from the cloud base. The leader is an ionized particle which forks repeatedly until contact is made with an unlike charged area (ground).
Funnel clouds
Form when a narrow, rapidly rotating vortex emerges from the base of the wall cloud. -The only difference between funnel clouds and a tornado is that a funnel cloud has yet to touch the ground.
Wall clouds
Form where cool, humid air from zones of precipitation is drawn into the updraft feeding the main cloud. -Most significant tornadoes associated with supercells usually form within or near wall clouds.
Outflow boundary
Forms when the downdraft spreads outward and converges with the warmer surrounding air.
Downbursts
Gusts of wind that can reach speeds in excess of 270km/hr (165mph), and are potentially deadly.
Haboob
Sandstorms caused by strong horizontal winds over desert regions.
Heat lightning
Lightning that seems to occur without thunder. -Thunder is produced but the stroke is too far away to reach an observer.
Convective outlooks
Maps which display the probability of severe weather for the U.S. current day, next day, and the day after.
Lighting
Occurs when a voltage gradient in the cloud(s)/ground overcomes the electrical resistance of the air.
Hook or hook echo
One of the most noteworthy features of a supercell, which looks like a small appendage attached to the main body of the storm on the radar image.
Mesoscale convective systems (MCSs)
Organized groups of thunderstorms. The individual storm cells of an MCS form as part of a single system, and develop from a common origin or exist because some cells directly lead to the formation of others. -Many MCSs have life spans from up to 12 hrs to several days.
Roll cloud
Produced beneath the gust front
St. Elmo's fire
Rare occasion that occurs when ionization in the air- often just before the formation of cloud-to-ground lightning- can cause tall objects to glow as they emit sparks. -This often produces a blue-green tint to the air, accompanied by a hissing sound.
Dart leader
Repetitive process to remove remaining negative charge at cloud base
Air mass thunderstorms
The most common and least destructive thunderstorms. Relatively small, localized, short-lived (self-extinguished) thunderstorms that do not produce strong winds, large hail, or tornadoes.
Sheet lightning
The sky if typically uniformly lit while the stroke is buried within the cloud.
Squall lines
Thunderstorms that are arranged in a linear band. -Precipitation exists in the central and western areas of the squall line. -They usually form in the warm section of a midlatitude cyclone.
Waterspouts
Tornadoes that occur over warm-water bodies, instead of over land.