Tropical Storm Rafael

What is a tropical storm? a tropical depression?

A "tropical storm" usually refers to a tropical low pressure area whose winds have increased to at least 35 knots (about 40 mph) but less than 65 knots (about 75 mph, which then becomes "hurricane" strength).

A tropical storm goes through a "tropical depression" stage first, which has winds less than 35 knots, and a closed cyclonic circulation. Some tropical waves can be very strong, with peak winds of at least 35 knots, but if they do not have a closed cyclonic (circular) circulation, they are classified as a wave rather than a depression.

Even weaker systems are sometimes called a "tropical disturbance". These may be no more than a persistent area of showers and thunderstorms.
(page last updated 11/2/2012)
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