Why do typhoons form

This movement and warming causes pressures at the surface to drop. Then air at the surface moves toward the lower pressure area, rises, and creates more thunderstorms. Winds in the storm cloud column spin faster and faster, whipping around in a circular motion.

When the winds reach between 25 and 38 mph, the storm is called a tropical depression. When the wind speeds reach 39 mph, the tropical depression becomes a tropical storm. This is also when the storm gets a name. The winds blow faster and begin twisting and turning around the eye, or calm center, of the storm. Wind direction is counterclockwise west to east in the northern hemisphere and clockwise east to west in the southern hemisphere. This phenomenon is known as the Coriolis effect. As a storm grows, it goes through a series of stages.

It starts as a tropical disturbance. Then, with cyclonic circulation and faster wind speeds, it becomes a tropical depression. If the wind keeps getting faster, it becomes a tropical storm and then a hurricane if winds are more than 74 miles per hour mph.

The classifications are based on the wind speeds in the storm, not the size of the storm. Hurricanes that look small on radar can have very high wind speeds. And large storms can have low wind speeds.

Wind speeds in hurricanes are often measured in knots. This difference is because of Earth's rotation on its axis. As the storm system rotates faster and faster, an eye forms in the center. It is very calm and clear in the eye, with very low air pressure.

Higher pressure air from above flows down into the eye. If you could slice into a tropical cyclone, it would look something like this. The small red arrows show warm, moist air rising from the ocean's surface, and forming clouds in bands around the eye. The blue arrows show how cool, dry air sinks in the eye and between the bands of clouds. The large red arrows show the rotation of the rising bands of clouds. When the winds in the rotating storm reach 39 mph, the storm is called a "tropical storm.

Tropical cyclones usually weaken when they hit land, because they are no longer being "fed" by the energy from the warm ocean waters. In different parts of the world, these storms have various names: for example, typhoons or hurricanes. But the scientific term for all of them is a tropical cyclone.

Tropical cyclones are "fed" with warm and humid air. They form over the ocean due to hot temperatures and water that evaporates from the surface of the ocean. Warm humid air rises, creating a "deficit" of air near the surface. In its place new air comes, which also heats up and rises.