Tropical cyclones

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Tropical cyclones

These are generic terms given to tropical cyclones depending on the area they effect, as outlined on the map below:

Tropical cyclones

They are extremely powerful low-pressure systems, and are believed to be responsible for a greater number of deaths than any other natural hazard. They are generated over the Atlantic and Pacific oceans and are most common in autumn when ocean temperatures are at there highest. There is some evidence to suggest that their frequency and severity is increasing as a result of global warming.

Formation of hurricanes

Several basic conditions are necessary for hurricane formation:

A Warm oceans with surface temperatures in excess of 27 degrees, and a deep layer of water to 60m
B A location between 5degrees north and south of the equator. (Without this the corriolis force is not sufficient to create the spinning motion characteristics of hurricanes.)
C Relatively stable and uniform atmospheric conditions of temperature, humidity and pressure. In the upper troposphere air drawn in at lower altitudes must be able to escape.
D Relative humidity of over 60% to provide sufficient energy to power the hurricane.
E Little change of horizontal wind with height.
F Existing cyclonic spinning of winds in the lower troposphere.

Structure of hurricanes

Hurricanes have several distinctive features which are outlined in the diagram below:

Tropical cyclones

A The eye is 20-60km across, existing in centre of the hurricane where conditions are calm and clear.
B Intense rainfall due to rapidly cooling and rising air. Amount can be up to 25cm a day.
C Strong winds. In order to be classified as a hurricane they must average 119km per hour.

Impact of hurricanes

This can be huge although MEDC's are usually more equipped to cope.

Hurricane Andrew - Florida August 1992

  • Deaths = twelve
  • Homes destroyed = fifty thousand
  • Insurance claims = approximately $20 billion
  • Homes without electricity = two million.

Most tornado activity (70%) is found in the Great Plains of the USA. They form as a result of intensive convective rainfall systems and are highly unpredictable.

Tropical cyclones


Different air types meet and due to their different properties, do not mix. In the USA warm moist air with its origins in the Caribbean meets colder air that has originated in the northern Rockies. There is a vast contrast between the two, and massive instability results. A tornado can form if:

A The two meeting air masses are conditionally unstable.
B Low level air has a relative humidity of over 65%.
C A low level southerly jet stream exists in the humid air.


They are unpredictable, have extreme low pressure at their centre, winds of over 50m per sec.


These are considerable. The drop in pressure van lead to explosions, trains are de-railed roofs removed, and flying debris leads to further damage. Entire communities can be flattened.