Tropical cyclone is the generic term used by the World Meteorological Organisation to define weather systems developing over tropical or subtropical waters in which winds exceed 34 knots or 63 km/h. Tropical cyclones also have a definite organised surface circulation.

A hurricane is a tropical cyclone in which the maximum average wind speed near the centre (the “eye”) exceeds 74mph or 119Km/h. They are natural weather systems formed over warm seas when hot air from the surface is transferred upwards and condenses. This condensation forms clouds which spiral as the earth rotates. The winds rotate in an anti-clockwise direction around a region of low pressure. In the Atlantic and Eastern Pacific areas, these weather systems are called Hurricanes, in the Western Pacific they are referred to as Typhoons, and in the Indian Ocean – Cyclones.

Four conditions are necessary for the development of a tropical cyclone:

1. A warm sea temperature in excess of 26 degrees Celsius or 79 degrees Fahrenheit.
    
2. High relative humidity (degree to which air is saturated by vapour).
    
3. Atmospheric instability.
    
4. A location of at least 4 - 5 latitude degrees from the Equator.

PHASES

Tropical weather systems develop in phases before they become a hurricane:

• Tropical Disturbance
  A weather system which gives rise to a specific area of cloudiness with embedded showers and thunderstorms.
   
• Tropical Depression
  A tropical cyclone system with definite counter-clockwise wind circulation with maximum sustained winds of less than 38 mph (61 km/h). At this stage the system is given a number.
   
• Tropical Storm
  When a tropical depression has intensified to the point where its maximum sustained winds are between 63-117 km/h (39-73 mph), it becomes a tropical storm. At this stage it is given a name. During this time the storm tends to become more organized and begins to adopt a more circular shape resembling a hurricane.
   
• Hurricane
  A tropical cyclone with wind speed greater than 74 mph (119 km/h). The weather system is now well organized. A clear rotation develops around the central core and the system develops an “eye”. Inside the “eye” the weather is calm; however, located just outside the “eye” is an area called the “eye wall”. This area of the hurricane causes the most damaging winds and intense rainfall. There are five categories of hurricanes, the last 3 stages are all considered to be intense.