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Types of Tropical Cyclones

There are three types of tropical cyclones:

1) Tropical depression - An organized system of clouds and thunderstorms with a well-defined circulation and maximum sustained wind of 37 to 62 kilometres per hour (20 to 33 knots).
Tropical depression

2) Tropical storm - An organized system of strong thunderstorms with a well- defined circulation and maximum sustained winds of 63 to 117 kilometres per hour (34 to 63 knots.) It is at this point that the storm is given a name.
Tropical Storm

3) Hurricane - An intense tropical weather system with a well-defined circulation and maximum sustained winds of 118 kilometres per hour (64 knots) or higher. In the western Pacific, hurricanes are called "typhoons," and similar storms in the Indian Ocean are called "cyclones." At this stage, the storm has an "eye."

(Images from University of Illinois)

Life of a Tropical Cyclone

Andrew A Tropical Cyclone will progress through a series of stages from birth to dissipation. First, they begin as a tropical disturbance: a large area of organized thunderstorms that maintain their identity for more than 24 hours. If the area of thunderstorms organizes so that a definite rotation develops and winds become strong, the system is upgraded to a tropical depression. At this point, a low pressure centre exists (there is at least one closed isobar) and it is given a number.

If winds continue to increase to 63 kilometres per hour (34 knots), the system becomes a tropical storm and is given a name. The system now has several closed isobars at the surface. The storm becomes more organized and the circulation around the centre of the storm intensifies. As surface pressures continue to drop, the storm becomes a hurricane when wind speed reaches 118 kilometres per hour (64 knots). An eye develops corresponding to the lowest atmospheric pressure near the centre of the storm, with spiral rain bands rotating around it.

A hurricane will begin to dissipate when the conditions for tropical cyclone formation (see below) are taken away. When a storm passes over cooler waters it will begin to dissipate, as its main energy source, the warm ocean, is no longer there. Likewise, when a hurricane makes landfall, it will begin to dissipate because its energy source (the warm water) is missing. Since land is a rough surface, friction will slow down the movement of the storm, disrupt the low level inflow into the hurricane, and weaken the deep convection.

Tropical Cyclone Formation

Tropical Cyclones (known as hurricanes and tropical storms in the Atlantic Ocean, typhoons in the Pacific Ocean and cyclones in the Indian Ocean) can be a very powerful and destructive type of storm. This type of storm develops over oceans 8° to 15° north and south of the equator.

In order for tropical cyclones to form, there are several environmental conditions that must be present:

  1. The cyclone must originate over ocean water that is least 26.5 °C. Hurricanes draw their energy from the warm water of the tropics and latent heat of condensation.
  2. There must be an atmosphere that cools quickly with height so that it is potentially unstable. If the air is unstable, it will continue rising and the disturbance will grow. The disturbance will only grow when winds at all levels of the atmosphere from the ocean up to 30,000 feet or higher are blowing at the same speed and from the same direction. In other words, there must be low values of vertical wind shear.
  3. The cyclone will not form closer than approximately 500 kilometres to the equator because the Coriolis Force is too weak. It is the Coriolis Force (an effect that results from the turning of the earth) that initially makes the cyclone spiral and maintains the low pressure of the disturbance.
  4. An upper atmosphere high pressure area above the growing storm should be present. The air in such high pressure areas is flowing outward. This pushes away the air that is rising in the storm, which encourages even more air to rise from the low levels.

Hurricanes will not always form if these conditions are present. However, a hurricane will only form if these conditions are present.

Parts of a Tropical Cyclone

An intense tropical cyclone is an almost circular storm of extremely low pressure and high winds. Winds spiral inward at high speed, accompanied by heavy rainfall. Tropical cyclones can range in size from only a few hundred kilometres across for a small storm to over 1000 kilometres across for a monster hurricane. Tropical cyclones have three distinctive parts: the eye, the eye wall and spiral rain bands.

Eye One of the most familiar parts of a tropical cyclone is the eye. The eye is located in the centre of the tropical cyclone and is produced by the intense spiraling of the storm. It is composed of air that is slowly sinking. As the eye passes over a site, the sky clears and calm prevails. Then, the storm strikes again with winds from the opposite direction. The eye is the region of lowest surface pressure and warmest temperature aloft. It has been found that the eye may be 10 °C warmer than the surrounding air at an altitude of 12 kilometres.
© Environment Canada, 2004

Eyewall The eye of a tropical cyclone is surrounded by an eyewall. The eyewall is the area of highest surface winds in the tropical cyclone. It is composed of many strong updrafts and downdrafts. The mechanisms by which the eye and the eyewall are formed are not well understood, but it is generally thought that the eye feature is a fundamental component to all rotating fluids. Thus, it is similar to water that is going down the drain!
© Environment Canada, 2004

Hurricanes are surrounded by spiral rain bands. These are bands of heavy convective showers that spiral inward toward the storm's centre. Cumulus and cumulonimbus (thunderstorm) clouds ascend and lightning develops.
© Environment Canada, 2004

Spiral Rain Bands