Oceans play a critical role in the development of some of the earth's wildest storms. Hurricanes always form over ocean water that has a temperature of at least 80° F. Hurricanes usually start when a cluster of thunderstorms move across the ocean. Sometimes the low atmospheric pressure that occurs because of the warm water temperatures combines with the dense, humid air of the thunderstorms, which causes the air in the atmosphere to begin spinning. When this happens, it is called a tropical storm. About 5 percent of tropical storms become so powerful that they are classified as tropical cyclones. Storms like this which affect the US usually originate in the Atlantic Ocean and are called hurricanes. However, when these types of storms occur in the western Pacific Ocean, they are called typhoons. When they occur in the Indian and Southern Pacific oceans, they are called cyclones. Tropical cyclones always have an “eye” of calm winds and low pressure surrounded by high winds and high pressure.