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Ask NOAA

Question

What are the differences between tropical depressions, tropical storms, and hurricanes?

Answer

Tropical depressions, tropical storms, and hurricanes are all types of tropical cyclones or storm systems characterized by a low pressure center with a closed surface wind circulation and numerous thunderstorms that can produce strong winds and flooding rain.  These storms originate almost exclusively in tropical regions of the globe, and have a counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. They are differentiated from each other based on maximum sustained winds:

• A tropical depression is a tropical cyclone with maximum sustained surface winds below 39 miles per hour. 
  
  
• A stronger tropical cyclone with winds between 39 and 73 miles per hour is considered a tropical storm and is given a name. 
  
  
• A hurricane is the strongest type of tropical cyclone with maximum sustained winds of at least 74 miles per hour. 

Once a cyclone has been classified as a hurricane, it is categorized on the Saffir-Simpson scale from one to five, with five being most severe.  The scale is used to give an estimate of the potential property damage expected along the coast from a hurricane landfall.  Cyclones categorized as three, four, or five on the scale are considered major hurricanes with maximum sustained surface winds of at least 111 miles per hour. 
The Atlantic hurricane season begins June 1 and ends November 30. The East Pacific hurricane season runs from May 15 through November 30.

For more information on hurricanes, please visit NOAA’s National Hurricane Center Web site: http://www.nhc.noaa.gov/.

This month’s expert: Dennis Feltgen, NOAA National Hurricane Center.

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