Thursday, May 15, 2008

How a hurricane forms: an explanation by Fred Cuny

I'm reading Disasters and Development by Fred Cuny. It's an excellent work by one of the most important people in the Disaster Relief field. Fred Cuny engineered the repatriation of 400,000 Kurds after Gulf War I, arguable saving the lives of tens of thousands of people. Cuny disappeared in Chechnya in 1995.

Disasters and Development has one of the clearest explanations of how a hurricane forms that I have ever seen:

Much is known about how a cyclone forms. In order to develop, a cyclone must have a warm sea and calm warm air. The warm air rises -- heavy, humid, and full of water vapour. Its place is taken by air rushing in from the sides and, because of the earth's rotation, this moving air is given a twist, so that the entire system begins to revolve. The warm rising air meets cooler air and releases its water vapour in the form of rain. It takes a tremendous amount of energy for the air to lift the water in the first place, and now this energy is released in the form of heat. This increases the rate of ascent of the air and a continuous cycle begins to develop. More water is released and also more heat : more the water and heat released, faster the cycle moves. This cycle becomes the engine that drives the beast, and gradually it goes faster and faster and the air mass becomes much larger.

Because the wind system is revolving, centrifugal force tends to throw the air outward so that the pressure in the center becomes very low, thus forming the eye of the storm. The pressure on the outside is very high, so the wind moves faster in an attempt to fill that low pressure area. The faster it moves the more the centrifugal force throws it outward. Soon there are very fast circular winds and, when they reach 120 kilometers per hour, the system becomes a cyclone or hurricane.

The system then begins to move forward like a spinning top. This brings it into contact with more warm sea and air, and the process becomes self-sustaining. Once a cyclone is formed, it will continue to move and expand until it passes over land or over an area where the sea is cooler.

In the northern hemisphere, cyclones generally move in a north-westerly direction; in the southern hemisphere, in a south-westerly direction. Little is known about what makes these storms move and change direction, other then that they are affected by the high altitude winds and rotation of the earth. So far, scientists have found it difficult to predict the movement of the cyclone, making this hazard one of the most dangerous.

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