Tropical cyclone

Tropical cyclones attaining hurricane force (Force 12 on the Beaufort Scale, i.e. 118 km/h) in the Atlantic and Northeast Pacific are referred to as hurricanes; they are called cyclones in the Indian Ocean, the sea area around Australia, and the South Pacific, and typhoons in the Northwest Pacific. Below hurricane force, i.e. in the 62–117km/h range (8–11 on the Beaufort Scale), they are referred to as tropical storms.

They can extend over large areas – with wind speeds exceeding 250 km/h and in individual cases even 300 km/h. Coastal regions and islands between latitudes 10° and 40° north and south of the equator are particularly affected. The wind field is usually 100–500 km in diameter.

Tropical cyclones quickly get weaker inland, which is primarily due to friction with the earth‘s surface and the reduced energy input from water vapour. Nevertheless, as the huge masses of water taken up over the warm sea usually fall as rain on the windward side of mountains, this may result in extreme floods and landslides even far inland.

The catastrophe potential of tropical cyclones is exceptionally large in many coastal regions due to the high concentration of values in such areas, their high recreational value, and the associated influx of people. This had a major impact on the insurance industry again in 2005, when Hurricane Katrina caused original insured losses of around US$ 62bn.

Cross-section of a tropical cyclone (hurricane)

Hurricanes get their energy from the evaporation of warm surface water. This schematic drawing shows how warm air rises in the central eyewall of the hurricane (1). This is where the strongest condensation of water vapour occurs, consequently producing extreme precipitation. Outside the eyewall (2) and in the eye of the storm (3) – a windless, dry zone in the centre of the hurricane – the air cools and streams back downwards. Over the sea surface (4) it takes on heat and moisture again – providing additional fuel for the atmospheric thermal engine. Over land areas, however, the system loses energy fast when the addition of water vapour stops and friction with the ground sets in.