Tropical cyclones: a working definition

A tropical cyclone is a rapidly rotating storm system characterised by a low-pressure centre, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. References describe these systems as drawing energy from the latent heat released as warm, moist ocean air rises and condenses, which is why they form and intensify only over warm tropical or subtropical seas typically exceeding 26 to 27 degrees Celsius (about 80 degrees Fahrenheit) at the surface.

Depending on its location and strength, a tropical cyclone is referred to by different names. In the North Atlantic and the eastern North Pacific the term hurricane is used; in the western North Pacific the same kind of storm is called a typhoon; and in the South Pacific and Indian Ocean the term tropical cyclone or severe tropical cyclone applies. The underlying meteorological phenomenon is the same. Sustained surface winds of 33 metres per second (74 mph or 119 km/h) or greater are the threshold above which the storm is classified as a hurricane, typhoon, or severe tropical cyclone, depending on basin.

In addition to wind, tropical cyclones carry several distinct hazards: storm surge along coastlines, freshwater flooding well inland, tornadoes spawned by outer rainbands, and prolonged power and communications outages. Common practice in preparedness references is to plan for each hazard separately rather than treating the storm as a single event, because a household well-prepared for wind can still be devastated by surge or flooding. This pack is general reference; in any actual storm, follow instructions from local emergency services, the National Hurricane Center, and your state and county emergency management.