These tropical storms are called hurricanes over the North Atlantic Ocean, cyclones over the Indian Ocean and typhoons over the north-west Pacific. Sea surface temperatures must be above 26.5 degrees C for hurricanes to form. A hurricane is one of the most powerful of all weather systems powered by the heat energy released by the condensation of water vapour. High speed winds (occasionally reaching 350 km/h (200 mph) ) spiral around a calm low pressure "eye", often clearly seen in satellite photos.

As well as causing damage with its violent winds, they also cause flooding on low lying coastlines with an associated storm surge. This is caused by the intense low pressure at the eye of a hurricane combining with the effect of strong winds to raise the ocean surface by 3-4 metres. The sea rises 1 cm for every millibar so if the pressure is 930 millibars, the sea surge will be about 80 cm. The hurricane winds will push the surge along in front of its path. On the lee side of a hurricane, the sea level may actually fall as the winds blow back round the hurricane pushing the water away. Coastal regions can also get battered by huge waves on top of the surge effect. In addition there can be flooding overland as a result of torrential rainfall that falls out of the storm's cumulonimbus clouds.