Hurricanes begin over tropical oceans, starting with huge masses of cloud spiralling into a centre of low pressure. Generally water from the oceans evaporates into the air, increasing its humidity. As the damp air rises it cools, eventually reaching a temperature - the condensation point - at which it is saturated. Further cooling then causes water vapour to condense into small droplets, which gather in their billions to form clouds.
As condensation increases and tiny droplets coalesce into larger ones, they may start to fall as rain. To get an idea of the dimensions involved, the water droplets that form clouds are only about a thousandth of a millimetre in diameter. Drizzle comprises droplets of about a tenth of a millimetre; a fully fledged rainstorm hits us with drops a millimetre across.
So our tropical clouds ought, if they are behaving sensibly, to rise then dissipate their contents through rain. It is only in very special conditions, which are still not properly understood, that they grow huge and windy and turn into hurricanes.
What is known is that hurricanes occur only when the temperature of the sea is greater than 27C. High-level winds cause low pressure by blowing away more air than is flowing in to take its place. The low-level Coriolis effect near the equator gives the system a slight anticlockwise motion, but the energy of the whole dynamic system is considerably enhanced by the latent heat of the water vapour as it condenses into droplets. The result is a faster-moving spiral sucking in more and more cloud-forming air. "It's a kind of heat engine that burns up energy from the ocean," says Edwin Lai (pictured left).
When it does all blow up into a hurricane, the first thing that is done is to give it a name. Four lists of names are maintained by the US navy at the Joint Typhoon Warning Centre in Guam. On each list, names alternate between male and female, with initial letters running through the alphabet, omitting Q, U and X.
While we are still generally unable to tell when hurricanes will happen, super-computers are now used to predict their likely paths with great accuracy. Last month Hong Kong and China were hit by hurricane Victor, which killed at least 65 people and caused damage estimated at half a billion dollars. Soon after, Winnie flooded Manila, causing nearly 200 deaths in Taiwan and China and $2.7bn damage. It would have been worse if accurate predictions had not enabled the Chinese to evacuate 790,000 people from homes in its path. The alphabet ran out with hurricane Zita, but now hurricane Amber is hitting Taiwan.Reuse content