The formation of a cyclone. Image via Imagarcade.

The formation of a cyclone. Image via Imagarcade.

ZIMSEC O Level Geography Notes: Cyclones

Features of a cyclone.

  • These are systems of intense low pressure.
  • There are sometimes known as hurricanes (the U.S and the American continents) and typhoons( in Asian countries).
  • On a map the isobars used to show a cyclone form a circular shape.
  • They have the lowest pressure at the center.
  • Strong winds blow into the center of the cyclone.
  • These are deflected into an anti-clockwise direction in the northern hemisphere and clockwise direction in the southern hemisphere.
  • They rush upwards with a great force creating a vortex which is the area that surrounds the eye of the cyclone.
  • The rapidly rising air gives rise to torrential rains and the strong winds cause considerable damage to infrastructure,crops and vegetation.
  • They occur in the tropics latitudes 5° and 20° South and North of the equator.
  • They occur in autumn when sea temperatures are at their highest.
  • Thus they occur around February and March in the southern hemisphere and August and September in the northern hemisphere.
  • They form over warm oceans where sea temperatures exceed 26°C and where there is a considerable depth of warm water.
  • Examples of tropical cyclones that have affected Zimbabwe over recent years are Cyclone Eline, Cyclone Japhet and Cyclone Gamede.
Pictures showing wind direction in an anti-cylcone (right) in both the southern and nothern hemisphere. Image by Phyical Geography

Pictures showing wind direction in an cyclone (left) and anti-cylcone (right) in both the southern and nothern hemisphere. Image by Phyical Geography

The structure of a cyclone

A cyclone has 3 sections namely:

  1. The front vortex (violent wind, dense clouds, thunderstorms).
  2. The eye (calm and clear).
  3. The rear vortex(violent winds, dense clouds, thunderstorms)

The development of cyclones

  • Tropical cyclones tend to develop:
  1. Over warm tropical oceans, where sea temperatures exceed 26°C and where there is a considerable depth of warm water.
  2. In autumn when sea temperatures are at their highest.
  3. In the trade wind belt, where the surface winds warm as they blow towards the Equator.
  4. Between latitudes 5°-20° North and South of the equator since the Coriolis force is insufficient to enable the “spin” needed for their formation nearer the equator.
  • Tropical cyclones develop along the inter-tropical front where the air masses brought by the northerly and southerly trade winds meet.
  • They form over oceans because air masses that which have traveled over oceans have warm and moist lower layers while the upper layers are cooler and drier.
  • When such two air masses meet one is lifted above the other.
  • The rising air cools and its moisture condenses to produce heavy rainfall.
  • Latent heat is set free by the condensation and it is this energy that allows the cyclone to rotate.
  • Tropical cyclones move in a general western direction.
  • They follow erratic courses that are hard to predict.
  • On reaching land their energy quickly dissipates and
  • They die out gradually as their supply of warm moist air is cut off.
  • For a cyclone to form there must also be an abundant source of warm, moist air of temperatures around 27°C near to the sea/ocean surface.
  • The air must be blowing inwards and rising rapidly to great heights to give cloud of great vertical extent (e.g. cumulonimbus) capable of producing torrential rainfall.
  • There must be an outward flow of air at the upper levels.
  • A cyclone is funnel shaped as shown in the first diagram above.

Why cyclones originate over seas

  • There is a constant supply of moist air of about 27°C.
  • Seas have minimal friction, friction kills cyclones.

The weather associated with a cyclone.

  • Prior to its arrival the air becomes very still, temperature and humidity are high.
  • As the front of the vortex arrives, gusty winds develop and thick clouds appear.
  • When the vortex arrives, the winds become violent due to upward surges and can reach very high speeds of around 250 Km/hr. Dense clouds and torrential rainfall occurs with visibility reduced to only a few metres.
  • Calm conditions return as the eye arrives and passes.
  • The arrival of the rear vortex  brings violent winds, dense clouds and heavy rain. The wind now blows in a direction opposite to that of the front of the front vortex.

Effects and Impact of cyclones.

In the examination students are typically asked to provide the effects of cyclones. You should know that the effects of all cyclones are generally the same. Often times in a bid to mess with your head examiners might ask you to provide the effects of a given cyclone by name. You should never really worry about the name of the cyclone as we have already pointed out the effects are invariably the same even though the severity of the damage is usually proportional to the size and course of the cyclone.

  • Heavy rainfall and landslides, rockfalls and mudslides.
  • Destruction of infrastructure such as houses, dams, oil rigs and coastal installations.
  • Destruction of civic infrastructure for example small bridges might be washed away.
  • Blockage of roads and destruction of power transmission lines due to falling trees.
  • Crop damage due to excessive rains.
  • Destruction of communication lines and cellphone towers.
  • Death’s due to lightning strikes.

Mitigation of damage.

  • Cyclones are a natural hazards and therefore cannot be stopped from occurring or be simply wished away. People and authorities can however work on mitigating/lessening the impact/damage of cyclones.
  1. Accurate weather forecast and satellite imaging to see developing cyclones in advance.
  2. Create an early warning system that can quickly disseminate information to the areas that are likely going to be affected.
  3. Evacuating people from the affected areas to high lying areas, areas that are less likely to be affected by landslides and away from flood plains.
  4. Construction of stronger houses that can withstand the impact of heavy rains and winds.
  5. Planting trees such as eucalyptus to create wind breaks.
  6. Well organised rescue and relief efforts to help those in affected areas this might involve the use of vertical take off planes such as helicopters.
  7. Providing tents for the homeless.
  8. Providing safe drinking water to those affected.
  9. Policing of affected areas to prevent looting and lawlessness.
  10. Agriculture and building insurance should be used to lessen the economical impact of the cyclones.

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