In this topic you will learn about how violent storms form.

A violent, or severe, storm forms when special conditions exist in the atmosphere. The most common kind of severe storm, formed in cumulonimbus clouds, is a thunderstorm. Heavy rain, strong winds, and sometimes hail accompany these storms.

Thunderstorms develop in three stages. In the first stage, heat causes air to rise quickly, producing an updraft. The updrafts keep water and ice crystals in the cloud. The particles grow in size until the updrafts can no longer support them and they fall to the ground.

In the second stage, the falling rain causes downdrafts. The air moving up rubs against the air moving down, and static electricity builds up. A huge electrical spark, called lightning, quickly heats the air. The heated air expands and slams into the air around it, causing a noise known as thunder.

In the third stage, the downdraft becomes stronger than the updraft and the storm dies down. The most violent thunderstorms often spin off even more dangerous storms—tornadoes.

A tornado is a violent whirling wind that moves across the ground in a narrow path. When Earth's surface becomes very warm, convection currents and updrafts can get very strong. Air rushes into the convection cell from all directions. The air begins to spin. This lowers the air pressure even more, causing more air to rush in. The spinning becomes faster and faster, forming a funnel that reaches the ground. Winds in a tornado can destroy anything in the path of the tornado.

A hurricane is a very large swirling storm with very low pressure at the center. Hurricanes form in the tropics as a series of thunderstorms. The hurricane grows and the air pressure drops as the temperature increases and water evaporates from the ocean. Wind speeds increase. To be classified as a hurricane, the storm must have winds that blow at least 75 miles per hour. In the center of a hurricane is an area that is almost clear of clouds. This area is called the eye of the hurricane.

High winds and heavy rains accompany hurricanes. Hurricanes also cause large ocean waves. These waves produce a storm surge. A storm surge is like a bulge in the water. A storm surge can cause great damage when it reaches the shore.

The more you know about severe storms, the more you can avoid danger by being aware and by following simple safety rules.