Severe thunderstorms
Thunderstorms
occur anywhere that warm moist air has absorbed enough
heat to make the air less dense than the surrounding
air. This commonly occurs along cold fronts, but can
occur in other places as well, particularly where daytime
heating forms hot air near the Earth's surface. As the
warm moist air rises it begins to cool and water begins
to condense into tiny droplets that form clouds. Condensation
of the water droplets in the clouds releases the latent
heat of evaporation, adding heat to the rising air, thus
decreasing its density and allowing it to rise to higher
levels in the atmosphere.
This rising air, called an updraft, starts to build clouds to heights of up to 6 km. Further rising and cooling within the clouds causes more condensation, as well as the formation of ice crystals which release further latent heat and build cloud heights up to 12 km. Eventually the water droplets and ice crystals in the clouds become so large that they can no longer be supported by the uprising air mass, and they begin to fall forming rapid downdrafts on the leading edge of the cloud. In the mature stage of thunderstorm development updrafts and downdrafts operate side by side within the cloud. This is the most dangerous stage of a thunderstorm because of the high winds accompanying the downdrafts, the heavy rain, as well as thunder, lightening, and possible hail and tornado development. Eventually the cloud reaches the dissipating stage as the downdrafts drag in so much cool dry air that it prevents further updrafts of warm moist air. With lack of updrafts of warm moist air, the cloud begins to dissipate and eventually it stops raining
Thunderstorms
can form as single cells, with only one cloud mass, or
as multiple cells, with several clouds moving along a
similar path.
Although thunderstorms can occur nearly everywhere, they
show an unequal distribution over Australia. Areas that
receive the highest number of thunderstorms are areas
where warm moist air moves southward from the tropics.
Hail
Hail is a rain of semi-spherical, concentrically layered ice balls th`t are dropped from some thunderstorms. Hail rarely kills people, but it does heavy damage to agriculture, roofs, and automobiles. The conditions necessary to form hail during a thunderstorm are:Large thunderstorms with high cloud tops formed from hot moist rising air. Upper level cold air with a large temperature contrast between the upper level air and the rising moist air. Strong updrafts within the thunderstorm to keep hailstones suspended in the cloud while layers of ice are added to the stones. When the stones become too large to be suspended by the cloud they fall to the surface as hail. Although thunderstorms are most common along the Gulf coast, thunderstorms that produce hail are more common in the mid-continent region where temperature contrasts between upper air masses and the rising hot air are greater. Hail can range from pea-size stones to grapefruit size stones.
Lightning
Lightning is the electrical discharge from clouds that causes thunder. Thus, lightning occurs from all thunderstorms. lightning is the major cause of forest fires and results in many deaths. Deaths from lightning have a similar distribution to the occurrence of thunderstorms, with Florida having both the most thunderstorms and the most lightning deaths.
Deaths from lightning usually occur outdoors as seen in the following table:
| Locations of Lightning Strikes (USA) | |
|---|---|
| Open fields, sport fields | 26% |
| Under trees | 15% |
| On boats & in water-related activity | 12% |
| On tractors & heavy road equipment | 6% |
| Golf courses | 5% |
| Via telephone | 1% |
| Unspecified | 35% |
Lightning is caused by an imbalance of electrical charge between and within clouds and the ground. Most of you have simulated lightning by walking across a carpet on a dry day and then touching a metal object like a door knob. Static electrical charge builds on your body and is discharged to the door knob as a bolt of electricity. During the buildup of a thundercloud, charged particles of water droplets and ice become separated in the cloud. Positively charged particles are moved to the top of the cloud and negatively charged particles are moved to the bottom of the cloud.
The thundercloud then begins to interact with the ground.
The negatively charged part of the cloud induces a build-up
of positive charges on the ground. Similarly beneath
the upper positively charged part of the cloud negative
charges are induced in the ground below. When the difference
in voltage between the oppositely charged parts of the
cloud and the ground become great enough, the electricity
is discharged as a bolt of lightning.
Note that lightning can travel from the cloud to the ground, from the ground to the cloud, and within the cloud itself. lightning travels at speeds of about 160,000 km/h (100,000 miles per hour), and usually includes several strokes that all occur within about 1/2 of a second. The discharge of electricity during a bolt of lightning heats up the air surrounding the bolt causing rapid expansion of the air. It is this rapid expansion of the air that causes the sound we call thunder.
