Lesson 29

What are thunderstorms, hurricanes and tornadoes?

A thunderstorm is a storm with thunder, lightning, heavy rain, and strong winds.

Thunderstorms usually happen when a cold front and warm front meet.

They start when huge cumulus clouds form giant storm clouds.

Lightning is caused when giant storm clouds give off electricity.

The electricity causes the air to warm and expand quickly.

Thunder is caused by the expanding and contracting air.

Did you ever notice that you see lightning before you hear thunder?

That is because light moves faster than sound.

Sometimes, a very low pressure area forms over the ocean near the equator.

A tropical air mass picks up a lot of moisture from the warm waters.

Such low pressure causes very strong winds and heavy rain.

When such a powerful storm forms over an ocean, it is called a hurricane.

A hurricane moves in a wide circular path.

From an airplane, it looks like bands of spinning clouds.

The winds spiral toward the center of the storm, which is called the eye of the storm.

Does it surprise you that the eye of the storm is calm and clear?

Sometimes hurricanes reach land and cause much destruction.

Hurricane winds can reach close to 325 kilometers per hour, or 200 mph.

Their rains cause much flooding.

In 1969, hurricane Camille killed almost 300 people in the southern United States. It caused about half a billion dollars worth of damage.

The most violent kind of storm is a tornado. It looks like a dark, curving funnel reaching down from a black cloud.

It is a small storm and only lasts a short time, but in its path it causes tremendous destruction.

Its low pressure causes winds up to 800 km per hour, or 500 miles per hour.

It blows down buildings and trees.

It tumbles automobiles and trucks.

It can pick up trees and carry them hundreds of feet.

One of the worst tornados in history happened on March 18, 1925. This “twister” was much larger and faster than most.

It killed 689 people as it roared across parts of Missouri, Illinois, and Indiana.