Science, Tech, Math › Science All About Tropical Storms Share Flipboard Email Print Science Weather & Climate Storms & Other Phenomena Understanding Your Forecast Chemistry Biology Physics Geology Astronomy By Rachelle Oblack Rachelle Oblack is a K-12 science educator and Holt McDougal science textbook writer. She specializes in climate and weather. our editorial process Rachelle Oblack Updated September 07, 2018 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.