Tropical Storm Classifications

What is a Hurricane?

A hurricane is a type of tropical cyclone, the general term for all circulating weather systems (counterclockwise in the Northern Hemisphere) over tropical waters.

Hurricanes are products of the tropical ocean and the atmosphere. Powered by heat from the sea, hurricanes are steered by the easterly trade winds and the temperate westerlies as well as by their own ferocious energy. Around their core, winds grow with great velocity, generating violent seas. Moving ashore, hurricanes sweep the ocean inward, spawning tornadoes and producing torrential rains and floods.

Average Numbers

Each year on average, 10 tropical storms (of which six become hurricanes) develop over the Atlantic Ocean, Caribbean Sea, or Gulf of Mexico. Many of these remain over the ocean. However, about five hurricanes strike the United States coastline every three years. Of these five, two will usually be major hurricanes (category 3 or greater on the Saffir-Simpson Hurricane Scale).

Tropical Storm Classifications

Tropical Disturbance

A moving area of thunderstorms in the Tropics that maintains its identity for 24 hours or more; a common phenomenon in the tropics

Tropical Depression

An organized system of clouds and thunderstorms with a defined circulation and maximum sustained winds of 38 miles per hour (mph) (33 knots) or less.

Tropical Storm

An organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 39 - 73 mph (34-63 knots)


An intense tropical weather system with a well-defined circulation and maximum sustained winds of 74 mph (64 knots) or higher. In the western Pacific, hurricanes are called "typhoons"; similar storms in the Indian Ocean are called "cyclones."