DEFINITION OF THUNDERSTORMS

Any storm that contains lightning and

thunder may be called a thunderstorm. However, the term

normally implies a gusty heavy rainfall event with numerous

lightning strikes and thunder, emanating from a cumulonimbus

cloud or cluster or line of cumulonimbus clouds. There is

a large range in the severity of thunderstorms from minor to

severe, with some causing extreme damage through high

winds, lightning, tornadoes, and flooding rains.

Thunderstorms are convective systems that form in

unstable rising warm and humid air currents. The air may

start rising as part of a converging air system, along a frontal

system, as a result of surface topography, or from unequal

surface heating. The warmer the rising air is than the surrounding

air, the greater the buoyancy forces acting on the

rising air. Scattered thunderstorms that typically form in summer

months are referred to as ordinary thunderstorms, and

these typically are short-lived, only produce minor to moderate

rainfall, and do not have severe winds. However, severe

thunderstorms associated with fronts or combinations of

unstable conditions may have heavy rain, hail, strong winds

or tornadoes, and drenching or flooding rains.

Ordinary thunderstorms are most likely to form in

regions where surface winds converge causing parcels of air to

rise, and where there is not significant wind shear or changes

in the wind speed and direction with height. These storms

evolve through several stages beginning with the cumulus or

growth stage, where the warm air rises and condenses into

cumulus clouds. As the water vapor condenses it releases large

amounts of latent heat that keeps the cloud warmer than the

air surrounding it and causes it to continue to rise and build

as long as it is fed from air below. Simple cumulus clouds may

quickly grow into towering cumulus congestus clouds in this

way. As the cloud builds above the freezing level in the atmosphere,

the particles in the cloud get larger and heavier and

eventually are too large to be kept entrained in the air currents,

and they fall as precipitation. As this precipitation is

falling drier air from around the storm is drawn into the

cloud, but as the rain falls through this dry air it may evaporate,

cooling the air. This cool air is then denser than the surrounding

air and it may fall as a sudden downdraft, in some

cases enhanced by air pulled downward by the falling rain.

The development of downdrafts marks the passage of

the thunderstorm into the mature stage in which the upward

and downward movement of air constitutes a convective cell.

In this stage the top of the storm typically bulges outward in

stable levels of the stratosphere, often around 40,000 feet (12

km), forming the anvil shape characteristic of mature thunderstorms.

Heavy rain, hail, lightning, and strong, turbulent

winds may come out of the base of the storms, which can be

several miles in diameter. Cold downwelling air often

expands out of the cloud base forming a gust front along its

leading edge, forcing warm air up into the storm. Most

mature storm cells begin to dissipate after half an hour or so,

as the gust front expands away from the storm and can no

longer enhance the updrafts that feed the storm. These storms

may quickly turn into gentle rains, and then evaporate, but

the moisture may be quickly incorporated into new, actively

forming thunderstorm cells.

Severe thunderstorms are more intense than ordinary

storms, producing large hail, wind gusts of greater than 50

knots (57.5 mi/hr, or 92.5 km/hr), more lightning, and heavy

rain. Like ordinary thunderstorms, severe storms form in

areas of upwelling unstable moist warm air, but severe storms

tend to develop in regions where there is also strong wind

shear. The high level winds have the effect of causing the rain

that falls out of the storm to fall away from the region of

upwelling air so that it does not have the effect of weakening

the upwelling. In this way the cell becomes much longer lived

and grows stronger and taller than ordinary thunderstorms,

often reaching heights of 60,000 feet (18 km). Hail may be

entrained for long times in the strong air currents and even

thrown out of the cloud system at height, falling several kilometers

from the base of the cloud. Downdrafts from severe

storms are marked by bulbous mammatus clouds.

Supercell thunderstorms form where strong wind shear

aloft is such that the cold downwelling air does not cut off the

upwelling air, and a giant rotating storm with balanced

updrafts and downdrafts may be maintained for hours. These

storms may produce severe tornadoes, strong downbursts of

wind, large (grapefruit-sized) hail, very heavy rains, and

strong winds exceeding 90 knots (103.5 mi/hr, or 167 km/hr).

Unusual winds are associated with some thunderstorms,

especially severe storms. Gust fronts maybe quite strong with

winds exceeding 60 miles per hour (97 km/hr), followed by

cold gusty and shifty winds. Gust fronts may be marked by

lines of dust kicked up by the strong winds, or ominous-looking

shelf clouds formed by warm moist air rising above the

cold descending air of the gust front. In severe cases, gust

fronts may force so much air upward that they generate new

multicelled thunderstorms with their own gust fronts that

merge, forming an intense gust front called an outflow

boundary. Intense downdrafts beneath some thunderstorms

spread laterally outward at speeds sometimes exceeding 90

miles per hour (145 km/hr) when they hit the ground and are

termed downbursts, microbursts, or macrobursts depending

on their size. Some clusters of thunderstorms produce another

type of unusual wind called a straight-line wind, or derecho.

These winds may exceed 90 miles per hour and extend

for tens or even hundreds of miles.

Thunderstorms often form in groups called mesoscale

convective systems, or as lines of storms called squall lines.

Squall lines typically form along or within a zone up to a

couple of hundred miles in front of the cold front where

warm air is compressed and forced upward. Squall lines may

form lines of thunderstorms hundreds or even a thousand

miles long, and many of the storms along the line may be

severe with associated heavy rain, winds, hail, and tornadoes.

Mesoscale convective complexes form when many individual

thunderstorm cells across a region start to act together, forming

an exceedingly large convective system that may cover

more than 50,000 square miles (130,000 km2). These systems

move slowly and may be associated with many hours of

flooding rains, hail, tornadoes, and wind.

Cumulonimbus clouds typically become electrically

charged during the development of thunderstorms, although

the processes that lead to the unequal charge distribution are

not well known. About 20 percent of the lightning generated

in thunderstorms strikes the ground, with most passing from

cloud to cloud. Lightning is an electrical discharge that heats

the surrounding air to 54,000°F (30,000°C) causing the air to

expand explosively, causing the sound waves we hear as thunder.

As the air expands along different parts of the lightning

stroke the sound is generated from several different places,

causing the thunder to have a rolling or echoing sound,

enhanced by the sound waves bouncing off hills, buildings,

and the ground. Cloud-to-ground lightning forms when negative

electrical charges build up in the base of the cloud, causing

positive charges to build in the ground. When the

electrical potential gradient reaches three million volts per

meter along several tens of meters, electrons rush to the cloud

base and form a series of stepped leaders that reach toward

the ground. At this stage, a strong current of positive charge

moves up, typically along an elevated object, from the ground

to the descending leader. As the two columns meet, huge numbers

of electrons rush to the ground, and a several-centimeterwide

column of positively charged ions shoots up along the

lightning stroke, all within a ten-thousandth of a second. The

process then may be repeated several or even dozens of times

along the same path, all within a fraction of a second.

See also PRECIPITATION; TORNADOES.

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