O Level Notes : Geography - Weather and climate - Tropical Cyclones

In  the  northern hemisphere the wind circulates in an anticlockwise direction but in the southern hemisphere it blows in a clockwise direction. Tropical cyclones are also known as typhoons or hurricanes. They are windy and violent with the wind speed reaching up to 240km/hour.

Conditions leading to the formation of tropical cyclones

The conditions necessary for the formation of tropical cyclones include:

• High temperatures of up to 27 degrees Celsius
• Cooling of the atmosphere to allow formation of convection clouds
• Plenty of water, for example, in oceans
• High humidity
• Inward flowing and rapidly rising air
• An outward flow of air at the upper levels
• Convergence of different air masses, for example, at the Intertropical Convergence Zone(ITCZ)

Development and distribution of tropical cyclones

Tropical cyclones occur in oceans where temperatures exceed 26°c to the depth of 60meters. This provides abundant water vapour in the air by evaporation. The structure of a tropical cyclone is shown in fig 1.10 below

The eye

Structure  of a tropical cyclone

It is the centre of a tropical cyclone. It is located at the area of lowest pressure. The air at this part is sinking at the centre of circulation. This area is made up of clear skies and less wind. At mature stage the eye appears as a circular hole in the central cloud mass. This eye may cover up to 40km in diameter.

The eye wall

It is the area surrounding the eye. It is a wall of dense convective cloud rising about 15km into the atmosphere. This part contains the most violent winds and heaviest rainfall occur.

Cyclone Eline at Landfall in Mozambique (February 2000)

The eye is shown by the satellite image as a small hole at the center.

TROPICAL  CYCLONES  AND   WEATHER HAZARDS

Tropical cyclones bring violent weather conditions. They are associated with very strong winds travelling at an approximate speed of 240km/hr (NASA; 2017). These include winds and thunderstorms as well as heavy rains cause severe destruction. Some of the hazards caused by tropical cyclones include:

• Death of people from lightning, strong winds and flooding.
• Destruction of homes
• Destruction of infrastructure, for example, power lines, bridges and roads
• Destruction of crops
• They lead to flooding which causes severe soil erosion
• Death of livestock
• Leaching of fertilizers
• Diseases outbreak becomes more pronounced.
• Flooding

 Cyclone Dineo destroyed bridges in Mberengwa

Case Study: Tropical cyclones in Zimbabwe

Cyclones affecting Zimbabwe most of the time will have passed through Mozambique. Most of these cyclones are from the Indian ocean. The eastern parts of Zimbabwe are usually the most affected due to proximity to Mozambique though other parts also suffer the damage. A good example of cyclones which affected the entire Zimbabwe are Eline and Emilie. Cyclones usually hit between December and March.

Monthly frequency of cyclones in Zimbabwe

Effects of cyclones in Zimbabwe include excessive rainfall amounts which exceeds 200mm, for example, in March 2003 due to cyclone Japhet in Eastern parts of Zimbabwe (Magumbate; 2003). These intensive amounts of rainfall lead to flooding. Low lying areas suffer destruction of homes, crops and infrastructure from floods. The tropical cyclones are so violent that gusty winds destroy huts, houses and schools. Excessive rain leads to mudslides, rock falls and roads are undercut by flooding water.

Mitigation strategies in the case of cyclones in Zimbabwe includes weather forecasts by the metrological department, evacuation of of people from low lying areas, provision of first aid, food donations and provision of medication to the affected people. The affected people should also know what to do in case of tropical cyclones, for example, they should listen for information from the radio and television. Creation of information centres also help in information determination.  Other measures include: locking up pets, filling water containers and fuelling cars, checking emergency kit, education, awareness campaigns and early warning systems.