Hurricanes are large rotating storms centred around low pressure areas. They usually develop between 5 and 30 degrees of latitude, starting life as a body of warm moist air over a tropical ocean that has reached the critical temperature of 26°C. Driven by the rotating earth’s Coriolis Force, a flow of air develops around a central eye whenever prevailing winds near the ocean surface are able to form inward spirals of air.
The ocean’s heat drives this process, causing evaporation and sending moisture-laden clouds high up into the atmosphere. Hurricanes play an important role in transferring heat and energy between the equator and the poles. There are five levels of hurricane strength, according to the Saffir-Simpson scale (below which lie an additional two weaker levels, labeled “tropical storms”).
Storms are not confined to the tropics of course, and the UK experiences them too – less extreme manifestations known as depressions. Depressions are cousins of hurricanes – cyclonic systems with a low pressure centre, formed when tropical and polar air masses converge at the polar front (between 40N and 60N). Under conditions of divergent flow in the jet stream (a meandering belt of air moving 12 km above ground level), cyclogenesis (depression formation) takes place and a wave form is produced with a warm front leading a cold front.
In the film "An Inconvenient Truth", Al Gore suggests that the US is facing increased risk of hurricane damage. The 2004 hurricane season certainly brought unusually high costs to the southern states of the US as well as to the Caribbean. Four major hurricanes – Charley, Frances, Ivan and Jeanne - brought around two thousand deaths to the region, as well as property damage amounting to many billions of dollars.
Then the 2005 season brought Hurricane Katrina to New Orleans, devastating an entire city.
Can these events be attributed to climate change? Hurricanes are fed by water vapour over oceans and the evidence does suggest that unusually warm waters over the Gulf of Mexico helped elevate Katrina from a Category 1 hurricane to Category 5. It was the third strongest hurricane ever to make landfall in the US.
Meanwhile, the UK has been reported as becoming twice as stormy in the past 50 years. According to new Hadley Centre research, climate change has modified the North Atlantic oscillation (NAO) and forced deep depressions that used to hit Ireland further south, causing more of them to pass over the UK. Once again, this shows that climate change is not simply bringing about simple uniform warming of the atmosphere. It is apparently modifying the structure and functioning of circulatory systems, further compounding the challenges ahead.
For a full account of the origins of hurricanes visit the BBC site: http://www.bbc.co.uk/weather/features/understanding/hurricane_cycle.shtml
Part of your answer may explain why low-pressure hurricanes and storms occur in certain regions and at certain times of the year. You might also consider whether long-term changes are now taking place in the frequency and severity of extremely low-pressure cyclonic events as a result of climate change.