THE MAKING OF A HURRICANE
You need four ingredients to cook up a perfect storm: warm water, wet air, slow winds high in the atmosphere and a cluster of thunderclouds waiting to become something bigger. Hurricanes often start out as small storms off the coast of Africa. Known as ‘tropical depressions’, these embryonic cyclones move west, picking up strength as they cross the Atlantic. They can only grow when the temperature of the water exceeds 27 degrees Celsius, and this is only at certain times of year. In the Atlantic, hurricane season runs from 1 June to 30 November. In the eastern Pacific it starts about two weeks earlier.
When the water is warm, it starts to evaporate, making the air wet. The vapour rises into the atmosphere, and if the wind is calm it condenses to form clouds. If the air moves too quickly, the top of the developing storm simply blows away. But if the conditions are right it starts to draw more air upwards, just like a plug hole drawing water from a bath. Just like a plug hole, the air entering the storm starts to spin.
The spin of a hurricane is down to the Coriolis effect. Earth, as everyone knows, is a spinning ball. Every point on its surface takes 24 hours to complete one rotation. This means that the points on the equator move much faster than the points on each of the poles. And this means that winds travelling over Earth’s surface don’t go in a straight line. In the Northern Hemisphere they bend off to the right, and in the Southern Hemisphere they bend left. That’s why hurricanes always spin in the same direction.
But just because the winds twist in a predictable way doesn’t mean that the storms
themselves are easy to forecast. Of the hundreds of storms that roll towards the US each year, only around 29 gather enough strength to get names, 15 reach wind speeds high enough to become hurricanes and just seven become major hurricanes.
It’s the job of the National Hurricane Center (NHC) to keep track of them all and to work out when to sound the alarm. A team called the Hurricane Specialist Unit keeps a constant watch over the Atlantic and the Eastern North Pacific oceans. Satellites orbit the world from top to tail twice every day, taking pictures, measuring the temperature and tracking the amount of moisture in the air. If anything looks suspicious, the NHC sends in the hurricane hunters, This elite team of planes and pilots specialise in chasing storms. They fly right into the wind, gathering measurements as they go. It might sound dangerous, but the risk of damage to the aircraft is low. Meteorologists use the data from these flights, the satellites and other weather instruments to predict what a storm might do next. It’s not an exact science, so the NHC repeats the process every few hours, adjusting its forecasts as each storm evolves.