Hurricanes – also know as typhoons or cyclones – form in specific conditions. Different countries have different ways of preparing and responding to their devastating impact.
The strongest tropical storms are called hurricanes, typhoons or tropical cyclones. The different names all mean the same thing, but are used in different parts of the world. If these huge storms start in the Atlantic off the west coast of Africa, they are called hurricanes.
In an average year over a dozen hurricanes form over the Atlantic Ocean and head westwards towards the Caribbean, the east coast of Central America and the southern USA (Florida in particular). Hurricanes may last as long as a month and although they travel very slowly – usually at about 24 km/h (15 mph) – wind speeds can reach over 120 km/h (75 mph).
Map showing route of Hurricane Mitch, 1998
How hurricanes form
Hurricanes need a lot of heat to form and a sea surface temperature of at least 26°C, which is why they usually occur over tropical seas. They also need to be between 5 and 20° north or south of the equator. It works like this:
- When this warm and wet air rises, it condenses to form towering clouds, heavy rainfall. It also creates a low pressure zone near the surface of the water.
- Rising warm air causes the pressure to decrease at higher altitudes. Warm air is under a higher pressure than cold air, so moves towards the ‘space’ occupied by the colder, lower pressure, air. So the low pressure ‘sucks in’ air from the warm surroundings, which then also rises. A continuous upflow of warm and wet air continues to create clouds and rain.
- Air that surrounds the low pressure zone at the centre flows in a spiral at very high speeds – anti-clockwise in the northern hemisphere – at speeds of around 120 km/h (75 mph).
- Air is ejected at the top of the storm – which can be 15km high – and falls to the outside of the storm, out and over the top, away from the eye of the storm. As this happens, it reduces the mass of air over the ‘eye of the storm’ – causing the wind speed to increase further. Some ejected air also cools and dries, and sinks through the eye of the storm, adding to the low pressure at the centre.
- The faster the winds blow, the lower the air pressure in the centre, and so the cycle continues. The hurricane grows stronger and stronger.
- Seen from above, hurricanes are huge circular bodies of thick cloud around 450 km (300 miles) wide. The cloud brings heavy rain, thunder and lightning.
- In the centre is the eye of the hurricane, about 45 km across (30 miles) across. Often there will be no clouds in the eye. Seen from below it will seem calmer, with a circle of blue sky above. The eye is formed because this is the only part of the hurricane where cold air is descending.
- In the northern hemisphere, the prevailing easterly tropical winds tend to steer hurricanes toward land – although their course is unpredictable. As hurricanes move inshore, their power gradually reduces because their energy comes from sucking up moist sea air.
Cross section diagram of a hurricane
Preparation and prediction
Preparation and prediction techniques can be very different in MEDCs and LEDCs.
Bangladeshi villager affected by cyclone Sidr
MEDCs have the resources and technology to predict and monitor the occurrence of storms, eg using satellites and specially equipped aircraft. They are also equipped to train the emergency services appropriately and to educate people about necessary precautions.
Storm warnings can be issued to enable the population to evacuate or prepare themselves for the storm. People can prepare by storing food and water or boarding up their windows.
LEDCs are often less prepared. They may rely on aid (sometimes reluctantly) from MEDCs for the rescue and recovery process, as was the case with Cyclone Sidr in Bangladesh, November 2007.
Effects of tropical storms
The intense winds of tropical storms can destroy whole communities, buildings and communication networks. As well as their own destructive energy, the winds generate abnormally high waves and tidal surges. Sometimes the most destructive elements of a storm are the subsequent high seas and flooding.
MEDCs are better placed to reduce the effects of tropical storms because they have more financial, educational and technological resources to help deal with them. They better able to observe and predict storm behaviour and can invest in infrastructure to withstand storms – as well as spending more money on repairing the damage caused.
Case study: Hurricane Katrina, 2005
The path of Hurricane Katrina
- Katrina was a category 4 storm.
- Storm surges reached over 6 metres in height.
- New Orleans was one of the worst affected areas because it lies below sea level and is protected by levees. These protect it from the Mississippi River and Lake Ponchartrain. The levee defences were unable to cope with the strength of Katrina, and water flooded into the city.
- Despite an evacuation order, many of the poorest people remained in the city.
- People sought refuge in the Superdome stadium. Conditions were unhygienic, and there was a shortage of food and water. Looting was commonplace throughout the city. Tension was high and many felt vulnerable and unsafe.
- 1 million people were made homeless and about 1,200 people drowned in the floods.
- Oil facilities were damaged and as a result petrol prices rose in the UK and USA.
Flooding caused by Hurricane Katrina in New Orleans 2005
There was much criticism of the authorities for their handling of the disaster. Although many people were evacuated, it was a slow process and the poorest and most vulnerable were left behind.
$50 billion in aid was given by the government.
The UK government sent food aid during the early stages of the recovery process.
The National Guard was mobilised to restore and maintain law and order in what became a hostile and unsafe living environment.