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Thunderstorms

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Thunderstorms

What lives fast and dies young?

That describes most thunderstorms. Thunderstorms can be very intense but may last for only a matter of minutes. They're fun (and dangerous) while they're active, though.

Thunderstorms

Thunderstorms are extremely common. Worldwide there are 14 million per year — that’s 40,000 per day! Most drop a lot of rain on a small area quickly, but some are severe and highly damaging.

Thunderstorm Formation

Thunderstorms form when ground temperatures are high, ordinarily in the late afternoon or early evening in spring and summer. The two figures below show two stages of thunderstorm buildup (Figure below).

(a) Cumulus and cumulonimbus clouds. (b) A thunderhead.

Growth

As temperatures increase, warm, moist air rises. These updrafts first form cumulus and then cumulonimbus clouds. Winds at the top of the stratosphere blow the cloud top sideways to make the anvil shape that characterizes a cloud as a thunderhead. As water vapor condenses to form a cloud, the latent heat makes the air in the cloud warmer than the air outside the cloud. Water droplets and ice fly up through the cloud in updrafts. When these droplets get heavy enough, they fall.

A mature thunderstorm with updrafts and downdrafts that reach the ground.

This starts a downdraft, and soon there is a convection cell within the cloud. The cloud grows into a cumulonimbus giant. Eventually, the drops become large enough to fall to the ground. At this time, the thunderstorm is mature, and it produces gusty winds, lightning, heavy precipitation, and hail (Figure above).

The End

The downdrafts cool the air at the base of the cloud, so the air is no longer warm enough to rise. As a result, convection shuts down. Without convection, water vapor does not condense, no latent heat is released, and the thunderhead runs out of energy. A thunderstorm usually ends only 15 to 30 minutes after it begins, but other thunderstorms may start in the same area.

Severe Thunderstorms

With severe thunderstorms, the downdrafts are so intense that when they hit the ground, warm air from the ground is sent upward into the storm. The warm air gives the convection cells more energy. Rain and hail grow huge before gravity pulls them to Earth. Severe thunderstorms can last for hours and can cause a lot of damage because of high winds, flooding, intense hail, and tornadoes.

Squall Lines

Thunderstorms can form individually or in squall lines along a cold front. In the United States, squall lines form in spring and early summer in the Midwest, where the maritime tropical (mT) air mass from the Gulf of Mexico meets the continental polar (cP) air mass from Canada (Figure below).

Cold air from the Rockies collided with warm, moist air from the Gulf of Mexico to form this squall line.

Lightning and Thunder

So much energy collects in cumulonimbus clouds that a huge release of electricity, called lightning, may result (Figure below). The electrical discharge may be between one part of the cloud and another, two clouds, or a cloud and the ground.

Lightning over Pentagon City in Arlington, Virginia.

Lightning heats the air so that it expands explosively. The loud clap is thunder. Light waves travel so rapidly that lightning is seen instantly. Sound waves travel much more slowly, so a thunderclap may come many seconds after the lightning is spotted.

Damage

Thunderstorms kill approximately 200 people in the United States and injure about 550 Americans per year, mostly from lightning strikes. Have you heard the common misconception that lightning doesn't strike the same place twice? In fact, lightning strikes the New York City's Empire State Building about 100 times per year (Figure below).

Lightning strikes some places many times a year, such as the Eiffel Tower in Paris.

An online guide to severe storms from the University of Illinois is found here: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/svr/home.rxml.

Summary

  • Thunderstorms grow where ground temperatures are extremely high.
  • Convection in the cloud causes raindrops or hailstones to grow. Downdrafts ultimately end convection.
  • Squall lines are long lines of thunderstorms that form along a cold front.

Practice

Use this resource to answer the questions that follow.

1. What causes thunderstorms to develop?

2. What determines the severity of the storm?

3. What causes the air to rise?

4. What causes the air mass to cool?

5. What causes lightning and thunder?

Review

1. Why are thunderstorms so common?

2. What is the energy source that feeds a thunderstorm?

3. What causes a thunderstorm to end?

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