Hurricanes are large, rotating storm systems with a low-pressure center that forms over warm tropical or subtropical ocean waters. When sustained winds near the surface reach 74 mph or more, the storm is classified as a hurricane in the Atlantic and eastern North Pacific oceans, while it is called a typhoon in the western North Pacific.
Hurricanes typically weaken as they move over cooler water or hit land, but dangerous winds can still cause significant damage and dumping many inches of rain may trigger flooding in coastal areas. Scientists study evidence of past hurricanes buried in sandy seafloor sediment and organic debris flung into nearby coastal areas, then extract cores from coastal ponds or lagoons, salt marshes, and blue holes (deep ocean basins). By studying these cores, scientists can learn about the frequency and severity of long-ago hurricanes, understand what causes them, and predict how hurricanes will affect our world.
When sunlight heats the ocean, it evaporates into the atmosphere, drawing in air from surrounding regions to create a rotating area of low pressure. If atmospheric conditions are right, this may become a tropical depression or disturbance. As the depression spins, it pulls in warm air from the ocean to fuel its growth. Winds circle around the center of the storm in a counter-clockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere, which helps form a defined eye.
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