What is a tropical cyclone?

A tropical cyclone is a type of large storm system having a circular or spiral system of violent winds, typically hundreds of kilometers or miles in diameter. The winds spiral around a region of low atmospheric pressure. "Tropical Cyclone" is the name given to these storms when they occur in the Indian Ocean. "Hurricane" is the name given to these storms in the Atlantic Ocean and Eastern Pacific. Similar storms that occur in the Western Pacific are called "Typhoons". For stronger cyclones, a characteristic structure called the "eye" forms when the maximum wind speeds exceed about 85 miles per hour, or 140 kilometers per hour. The eye is a region of clear air with no clouds, and is a few tens of kilometers in diameter. The energy that powers cyclones comes from the evaporation of warm ocean water. The water vapor rises to the top of the cyclone along the sides of the eye, then condenses into clouds. Warmer ocean water produces more powerful cyclones.

The physics of tropical cyclones depends on a balance between the low pressure at the center of the storm and the Coriolis force that comes from the rotation of the earth. Since the Coriolis force is zero at the Earth's equator, cyclones can only form at latitudes that are more than about 10 degrees north or south of the Earth's equator. Due to the Coriolis force, the winds in a tropical cyclone spiral in the counter-clockwise direction in the northern hemisphere when observed from above. The winds spiral in the clockwise direction in the southern hemisphere.

Tropical cyclones are measured on the Saffir-Simpson scale. Category 1 storms have the lowest wind speeds. Category 5 storms are the most powerful and have the highest wind speeds.

Contributing source: NOAA

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