Learn About Hurricanes, Typhoons, and Cyclones

A tropical storm is a tropical cyclone with maximum sustained winds of at least 34 knots (39 mph or 63 kph). Tropical storms are given official names once they reach these wind speeds. Beyond 64 knots (74 mph or 119 kph), a tropical storm is called a hurricane, typhoon, or cyclone based on the storm location.   Tropical Cyclones A  tropical cyclone  is a fast-spinning storm system  that has a  low-pressure  center, a closed low-level atmospheric circulation,  strong winds, and a spiral arrangement of  thunderstorms  that produce heavy rain.   Tropical cyclones tend to form over large bodies of fairly warm water, typically oceans or gulfs. They get their energy from the evaporation of  water  from the  ocean  surface, which ultimately  recondenses  into  clouds  and rain when moist air rises and cools to  saturation. Tropical cyclones are typically between 100 and 2,000  kilometers in diameter. Tropical  refers to the geographical origin of these systems, which form almost exclusively over  tropical  seas.  Cyclone  refers to their cyclonic nature, with wind blowing  counterclockwise  in the  Northern Hemisphere  and clockwise in the  Southern Hemisphere. In addition to strong winds and rain, tropical cyclones can create high waves, damaging  storm surge, and  tornadoes. They typically weaken rapidly over land where they are cut off from their primary energy source. For this reason, coastal regions are particularly vulnerable to damage from a tropical cyclone as compared to inland regions. Heavy rains, however, can cause significant flooding inland, and storm surges can produce extensive coastal  flooding  up to 40 kilometers from the coastline.   When They Form Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active month. November is the only month in which all the  tropical cyclone basins  are active. Warnings and Watches A tropical storm warning is an announcement that sustained winds of 34 to 63 knots (39 to 73 mph or 63 to 118 km/hr) are  expected  somewhere within the specified area within 36 hours in association with a  tropical,  subtropical, or  post-tropical  cyclone. A tropical storm watch is an announcement that sustained winds of 34 to 63 knots (39 to 73 mph or 63 to 118 km/hr) are  possible  within the specified area within 48 hours in association with a  tropical,  subtropical, or  post-tropical  cyclone. Naming of Storms Using names to identify tropical storms goes back many years, with systems named after places or things they hit before the formal start of naming. The credit for the first use of personal names for weather systems is generally given to the  Queensland Government  Meteorologist  Clement Wragge  who named systems between 1887-1907. People stopped naming storms after Wragge retired, but it was revived in the latter part of  World War II  for the Western Pacific. Formal naming schemes have subsequently been introduced for the  North and South Atlantic, Eastern, Central, Western and Southern Pacific basins  as well as the  Australian region  and the  Indian Ocean.