What causes hurricanes? The science behind storm Irma

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Hurricane Irma has ravaged across the Caribbean, destroying homes and causing extensive damage to a string of islands.

By midday UK time on Thursday, at least nine people had been confirmed dead in the category five storm – thought to be the most powerful to hit the Atlantic in a decade.

It reached wind speeds of up to 180 mph and was set to travel towards Turks and Caicos. 

The death toll is expected to rise.

Here is an explanation of what causes hurricanes and how they are formed:

Hurricanes, the strongest and most violent type of tropical storm on the Earth, are formed above swathes of warm ocean.

Hurricane Irma Kills Seven And Causes Total Devastation

Depending on where they start in the world, they can also be called typhoons or cyclones – but those that form over the Atlantic or eastern Pacific are referred to as hurricanes.

They start close to the equator where warm and wet air above the water rises.

When the humid air rises it condenses, leaving an area of lower air pressure above the ocean’s surface which is quickly filled by air in the surrounding areas. 

Hurricane: Irma (AFP/Getty Images)

The air that rises quickly cools off, condensing into thick clouds.

This pattern is repeated – with air constantly being sucked in and rising up – causing a swirling upflow pattern in surrounding clouds.

The clouds and wind continue to spin and grow, fed by the warm ocean and water evaporating from the surface.

The air around the central, low pressure, zone spirals at speeds of around 75 mph in an anti-clockwise direction (storms formed south of the equator spin clockwise).

As the speed increases, the centre of the storm forms an “eye” – where the very low air pressure renders it calm and clear. 

The eye is formed because it is the only part of the storm with cool air descending as high pressure air from above flows down into the low pressure centre.

The faster the winds blow, the lower the air pressure in the centre, and so the cycle continues. The hurricane grows stronger and stronger.

When the winds in the rotating storm reach 39 mph, the storm is called a “tropical storm.” And when the wind speeds reach 74 mph, the storm is officially a hurricane.

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