ESL Progress Newsletter 200706

What's in the Wind?
Hurricanes and Tornadoes

What exactly are hurricanes and tornadoes?
Tornadoes occur when warm and cool air streams collide resulting in the formation of low pressure storm clouds. These clouds rotate and form a funnel of spiralling wind which can, in a small minority of cases, reach speeds of 300 mph. Tornadoes develop over land whereas hurricanes only form at sea. Both have an "eye" in the middle with winds swirling around this centre.

Hurricanes are basically large, intense, tropical storms rotating around a centre of very low pressure. They are the most powerful weather phenomenon on the planet but they need very specific conditions to develop and can form only when the sea temperature is higher than 26.5 degrees C and there is some sort of weather disturbance already in progress. Only when the wind speed of a tropical storm exceeds 73 mph can it become classified as a hurricane. In 1998 Hurricane Gilbert developed wind speeds of 160 mph and caused devastation in Jamaica. However, most of the tropical storms which develop each year stay out at sea and never make landfall.

Where do they happen?
As far as tornadoes are concerned America seems the obvious answer but according to some figures it's Britain! This is because there are an average of 33 tornadoes annually in Britain and bearing in mind that Britain is nearly 40 times smaller than the USA this means that the British are actually twice as likely to see a tornado as the Americans!

Hurricanes seem to occur near America, regularly devastating parts of the East Coast and major cities such as New Orleans. However, hurricanes don't only crop up in the Atlantic Ocean and the Gulf of Mexico. They also affect South-East Asia and India. So why do we never see newspaper headlines informing us of a hurricane in the Indian Ocean? Simply because hurricanes in the Indian Ocean have a different name - they are called cyclones. In South-East Asia they are referred to as typhoons.

Why do hurricanes have names?
Unlike tornadoes, hurricanes and tropical storms are given names. Since 1979 there has been a six year name list operating on a rota basis for naming tropical storms whose wind speed reaches 39 mph. Both male and female names are used. They were brought in partly to reduce confusion when there was more than one storm happening at any given time and partly for legal reasons (insurance claims, etc.) as well as to make it easier for people to refer to them directly. The names of especially destructive and deadly hurricanes are removed from the list or "retired". The name list for the 2007 tropical storms is as follows: Andrea, Barry, Chantal, Dean, Erin, Felix, Gabrielle, Humberto, Ingrid, Jerry, Karen, Lorenzo, Melissa, Noel, Olga, Pablo, Rebekah, Sebastien, Tanya, Van and Wendy.