As a storm grows, it goes through a series of stages before being classified as a hurricane. It might start as a collection of thunderstorms known as a tropical disturbance. Then, with cyclonic circulation, wind speeds get faster and the storm becomes a tropical depression. If the wind keeps getting faster, the storm 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. Compared to a mile per hour, knots are a little bit faster. Thirty miles per hour are about the same as 26 knots.
|Tropical Disturbance||Thunderstorms with light cyclonic circulation|
|Tropical Depression||Wind speeds between 20 and 34 knots (23-39 mph)|
|Tropical Storm||Wind speeds between 35 and 64 knots (40-73 mph)|
|Hurricane||Wind speed greater than 64 knots (74 mph)|