Tropical cyclones include any large low-pressure system with circular surface winds and thunderstorm activity over tropical or sub-tropical waters, according to the Atlantic Oceanographic and Meteorologic Laboratory. Depending on the specific wind speed and location on the globe, a tropical cyclone can be classified as a hurricane, typhoon, cyclonic storm, tropical storm or tropical depression.
Tropical storms are a type of tropical cyclone that has a recorded wind speed of 17 meters per second, or 39 miles per hour, sustained at 10 meters above the water surface for at least one minute. In Australia, tropical storms are called a Category 1 cyclone. When a tropical cyclone reaches tropical storm wind speeds, the National Hurricane Center gives it a name. Once the cyclone's wind speed reaches 33 meters per second, or 74 miles per hour, it moves out of the tropical storm designation and becomes a hurricane if it is in the North Atlantic Ocean or Northeast Pacific Ocean or a typhoon if in the Northwest Pacific Ocean. After reaching land, a hurricane soon becomes downgraded into a tropical storm once again as the wind speeds drop.
Tropical cyclones begin when waves of low pressure move westerly across the surface of the Atlantic Ocean, Pacific Ocean, Gulf of Mexico and Caribbean Sea. Thunderstorms form in these low-pressure areas and pull moist air up from the water surface. The rotation of the Earth causes the winds to start turning counterclockwise if the storm is in the Northern Hemisphere and clockwise if the storm is in the Southern Hemisphere. In most cases, this tropical disturbance simply dissipates over time. However, if the winds continue to increase in intensity, it can become a tropical depression, the weakest form of tropical cyclone, and later develop into a full-fledged tropical storm or hurricane.
In the U.S., the National Hurricane Center in Miami, Florida, is responsible for monitoring the Atlantic Ocean, most of the Northeast Pacific Ocean, Gulf of Mexico and Caribbean Sea for tropical cyclones. The western portions of the Northeast Pacific Ocean fall under the jurisdiction of the Central Pacific Hurricane Center in Honolulu. These agencies forecast and announce tropical cyclone formation, issue warnings and advisories and give these storms a name when they reach tropical storm level.