Hurricanes, tropical storms and tropical depressions: The life cycle of a tropical system


We've heard of tropical storms and tropical depressions, but what do these names mean? And when does a storm become a hurricane?

We start with a tropical depression, a low-pressure area in the center of a group of thunderstorms. From a satellite, it doesn't look very organized, but it does have some rotation. Depressions are numbered by the National Hurricane Center. For example, TD8 was the eighth tropical depression to form this season.
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Once the depression sustains wind over 39 MPH, we call it a tropical storm. At this stage we are already seeing problematic weather, usually heavy rains. If this storm develops strong rotation, and the wind speeds exceed 74 MPH, we have a hurricane. These are much easier to recognize because of their distinctive shape and the dark eye in the center. Hurricanes are classified as Category 1 to 5, based on intensity. They can last for weeks, but they tend to lose power once they hit land, getting downgraded to a tropical storm again and then back to a tropical depression.

Instead of numbers, hurricanes get names, which are selected from a pre-determined list that gets recycled every six years.
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