Basic Definition

A tropical cyclone is a warm-core, low-pressure system producing high winds that spiral counter-clockwise (in the northern hemisphere) and inward, with the highest winds near the center of circulation. The large counter-clockwise and inward flow is characteristic of the nearly symmetric structure of tropical cyclones as they are comprised of rain bands spiraling toward the center. These warm-core storms typically form over the tropical and subtropical oceans and extract their energy from the heat content of the oceans.

Three Categories of Organization

Tropical cyclones are organized into three main states, depending on their overall structure and maximum wind speed. The three main categories are:

• Tropical Depression
  • Closed low-pressure system
  • Winds must rotate fully around the closed low-pressure center
  • Maximum sustained winds of up to 39 mph
• Tropical Storm
  • Appear more circular than a Tropical Depression, indicating more organization
  • Clearly recognizable rotation
  • Maximum sustained winds from 39 to 73 mph
• Hurricane
  • Well-organized, often with a distinct eye
  • Pronounced, strong rotation
  • Maximum sustained winds greater than 73 mph
  • Large range in intensity is described by the Saffir-Simpson Scale

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