Global Distribution of Cyclones


Quick Summary 

What are cyclones?

Cyclones are extreme versions of depressions. The pressure is so low that dangers caused by this weather system are deadly compared to a depression. Cyclones have other names e.g. the are called ‘hurricanes’ in the West, ‘Typhoons‘  in oriental Asia and ‘Willy willy’ in Oceania.

Why do they occur?

If we are looking at hurricanes in America then they tend to start off in places like the sub-Saharan African continent. Geographers  are not sure the exact cause but small disturbances in atmosphere over land and start of a low pressure system of thunderstorms. As this weather system travels west over the warm pacific ocean it gains energy and all the thunderstorms start swirling around the eye – the area with the lowest pressure. As this system moves on land it releases its energy through precipitation and strong winds up to crazy speeds like 176mph and even higher. However, it doesn’t last too long as it looses its energy on land as there is no warm water from the ocean evaporating.

What conditions are required for them to take place?

  1. Warm tropical oceans where sea temperatures are at least 27 degrees 
  2. Oceans where the depth is at least 60m.
  3. Late summer and early autumn where sea temperatures are at their highest
  4. The area of the trade wind belt between latitudes 5 degrees and 20 degrees on either side of the equator
  5. Wind speeds need to be constant between ground level and 12km above ground level.

How are they measured?

The Saffir-Simpson scale is used to measure hurricanes. It gives each hurricane a rating between 1 and 5. It basis the rating according to the wind speed, storm surge and damage. The rating each hurricane is given is called ‘category’. Hurricane Katrina was so intense that it had a rating of category 5 on this scale. Below I have put the rating table:


Sustained winds (mph)

Storm surge




4-5 feet




6-8 feet




9-12 feet




13-18 feet




>18 feet


Where do they happen – the global distribution?

So it is quite clear the global distribution is limited because of the large criteria required for hurricanes to form. Hurricanes occur in equatorial areas in the latitudes of 5 degrees to 20 degrees to either side of the equator. This includes regions such as the Gulf of Mexico, the Bay of Bengal and the Indian Ocean. Any coastal area in this latitude which is surround by warm water is at risk of experiences cyclones.

When do they occur most often?

They occur in the months of late summer and early autumn. This usually in the months of July to November in the Northern Hemisphere and December to May in the Southern Hemisphere.

Can a depression be classed as an extreme weather event?

Describe how the depression shown on this chart might produce extreme weather hazards.
What is an extreme weather event?
It is severe or unusual weather conditions e.g. hurricanes which cause severe impacts on the environment, people and economy.
What are the weather conditions shown in this map?

Firstly there is a depression. The pressure is below 1000mb (low pressure) and winds are blowing in an anti-clockwise direction. There is also quite a small gap between isobars showing winds are strong which fits in the model of a depression. There will be a large cloud cover as a depression is when warm air meets could air and this causes the warm air to cool resulting in precipitation. The depression has built up through cold and wet conditions in the Arctic Ocean. The Polar Maritime air mass is the main one associated with this weather system.
There is also a cold front passing through South East of England. Traditionally, in low pressure conditions we see occluded fronts forming on the actual land but even if it is close there will still be torrential rain on land. In this front the cold air advances and force the warm air to rise sharply. The result of a cold front is a rapid period of torrential rain.
So what are the risks with this condition?
    1. Firstly, temperatures can become extremely low bringing many health risks like hypothermia especially to children and the elderly.
    2. Torrential rainfall can lead to river flooding which especially with cold fronts lead to flash floods. The can cause many adverse effects  for example the Boscastle 2004 flash floods caused injuries, £2 million damage to buildings and roads and coastal pollution as debris and fuel from cars flowed out into the sea.
    3. In extreme situations could lead to a snowstorm or blizzard which as we saw in February 2009 causes disruptions and deaths.
    4. It could lead to high waves and surge may be destructive to the coast.
    5. In the case of flooding if sewage systems failed then they would be a risk of cholera and other disasters.