Hurricanes, tornadoes and tropical storms

Hurricanes, tornadoes and tropical storms are among the most powerful and destructive forms of weather. Although all involve strongly rotating air, they differ greatly in size, origin and duration. A hurricane is a large tropical cyclone — a vast, organised system of thunderstorms that rotates around a low-pressure centre, or eye. The same kind of storm is called a typhoon in the western Pacific and simply a cyclone in some other regions. These systems form over warm ocean water, which supplies the heat and moisture that fuel them.

A tropical storm is an earlier or weaker stage of the same kind of system; when its sustained winds reach a defined threshold it is reclassified as a hurricane. Tornadoes, by contrast, are much smaller and shorter-lived. A tornado is a narrow, violently rotating column of air reaching from the base of a thunderstorm to the ground, often only a few hundred metres across, yet capable of extreme local wind speeds. Tornadoes can form within the rain bands of a hurricane but also occur on their own, especially where warm, moist air meets cooler, drier air.

Meteorologists monitor these systems using satellites, radar and weather aircraft, and they issue forecasts and warnings to give communities time to prepare or evacuate. Predicting the track and intensity of a hurricane has improved considerably, although tornadoes, because they are small and develop quickly, remain harder to forecast far in advance. Storm-protection measures and early-warning systems are central to reducing the loss of life from these events.

Frequently asked questions

What is the difference between a hurricane and a tornado?

A hurricane is a very large tropical cyclone forming over warm oceans, while a tornado is a small, short-lived rotating column of air beneath a thunderstorm over land.

When does a tropical storm become a hurricane?

A tropical storm is upgraded to a hurricane once its sustained wind speeds rise above a defined threshold set by meteorological agencies.

Why do hurricanes form over warm oceans?

Warm ocean water provides the heat and moisture that power the storm; without that energy source, a hurricane weakens, which is why it fades after moving over land.