What is a hurricane? A hurricane, the strongest storm on Earth, is a storm that derives its energy from cloud formation and rainfall. This is unlike mid-latitude (baroclinic) storms that derive their power from a temperature gradient (cold front). A hurricane begins as a tropical depression. A tropical depression has a sustained wind speed of less than 39 mph (63 km/hr). Next comes a tropical storm with winds from 39-73 mph (63-118 km/hr). Tropical storms are named in the Atlantic, East, Central and Northwest Pacific, and in the South Indian Ocean. Finally, when the winds are sustained (one minute average) at 74 mph (119 km/hr), the storm becomes: In the Atlantic Ocean, East Pacific, Central Pacific (east of the International Dateline) and Southeast Pacific (east of 160°E) a Hurricane; in the Northwest Pacific (west of the International Dateline) a Typhoon; in the Southwest Pacific (west of 160°E) and Southeast Indian Ocean (east of 90°E) a Severe Tropical Cyclone; in the North Indian Ocean a Severe Cyclonic Storm; and in the Southwest Indian Ocean (west of 90°E) a Tropical Cyclone.