The Formation, Distribution and Frequency of Tropical Storms

• • A tropical storm is a localised, very intense low-pressure wind system, forming over tropical oceans and with winds of hurricane force - by definition, tropical storms must have an average wind speed in excess of 120 km/h (75mph).
• • Cyclone (India)
  • Hurricane (North Atlantic)
  • Typhoon (south-east Asia)
• What factors affect the distribution of a tropical storm?

  • Oceans
  • High temperatures
  • Atmospheric instability
• • Oceans – tropical storms derive their moisture (through the process of evaporation) and energy (in the form of latent heat) from the oceans. Tropical storms form and continue to develop over ocean areas and then peter out then they reach land. Ocean depth must be at least 70m to provide moisture and latent heat.
• • High temperatures – a sea-surface temperature of 27°C or higher is required for tropical storm formation, which is why they are formed in low latitudes during the summer, when temperatures are at their highest. 
• Atmospheric instability – tropical storms are most likely to form in regions of intense atmospheric instability, where warm air is being forced to rise. The Inter Tropical Convergence Zone (ITCZ), where two limbs of the Hadley Cell converge to form low pressure on the ground, is a perfect spawning ground for tropical storms.
• What factors affect the distribution of a tropical storm?

  • Rotation of the Earth
  • Uniform wind direction at all levels .
• Uniform wind direction at all levels – winds from different directions at altitude prevent a tropical storm from attaining height and intensity. The vertical development is effectively ‘sheared off’ by the multidirectional winds.
• • Rotation of the Earth – a certain amount of ‘spin’ is needed to initiate the characteristic rotating motion of a tropical storm. The influence of the Earth’s rotation on surface phenomena is called the Coriolis effect. This increases with distance away from the Equator and explains why tropical storms do not usually form in the regions between 5°N and 5°S