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Gale force

Hurricanes feed on heat! More heat equals more destructive hurricanes. They are known as ‘tropical’ cyclones because they usually occur in the region close to the equator, known as the tropics. If the planet heats up as a result of global warming, we may even find them happening in our back yards!

How hurricanes form

What is a hurricane?

A hurricane is a tropical cyclone. A cyclone is something that rotates. They are all basically the same thing and share the same science. They are given lots of different names:

  • Hurricanes (if they happen in the Atlantic)
  • Typhoons (if they happen in the Pacific)
  • Tropical storms
  • Cyclones

How do they form?

Hot air rises: this is basic convection theory. As air heats up it expands and becomes less dense. It is lighter than the cooler air around it so it moves up and ‘floats’ on the denser air below. This is what starts a storm forming, a column of rising warm air.

How does it become a cyclone?

If the Earth didn’t spin, hurricanes wouldn’t form. Imagine looking straight down on the Earth from above the North Pole. What would you see? The point directly below you, the pole, would hardly be moving. If you were standing here you would just spin gently once every twenty-four hours. But if you were standing on the equator you would actually be moving at just over a thousand miles an hour! If you walked from the pole to the equator you would go from being stationary to hurtling around at nearly twice the speed of sound.

Now, these rising columns of rising air are big. They can often be hundreds of miles across. This means that one side is closer the pole and one side is closer to the equator. Are you starting to get it?

One side must move faster than the other. This gives the air a twist. It starts it spinning. Once it starts spinning, the upward movement of air speeds it up.

  • Cyclones north of the equator always spin in the same direction. Can you work out which?
  • South of the equator they always spin in the opposite direction.

How fast do they spin?

The really big ones can produce wind speeds of over 150mph. These can tear up the strongest buildings.

Boats displaced by Hurricane Katrina's floods

Why so much damage?

It’s not just the huge wind speeds that do the damage. The big killer is storm surge.

What is a storm surge?

Hurricanes always form over the oceans. Underneath a hurricane is an area of low atmospheric pressure. This is because warm air is ‘lighter’ than cold air. If the air presses down less in the centre of a hurricane than around the edges, the sea can be pushed from the edges into the middle and bulge up wards. This bulge is just part of a storm surge.

The winds around the edge are also very strong which causes the waves to build up to much higher levels. It often rains very heavily during a hurricane so rivers become very swollen. All these factors caused most of the damage when Katrina hit New Orleans. The storm surge came right over the top of sea defences and swamped the city. The storm surge was over 9m (30ft), and this caused most of the death and destruction, not the high winds.

Hurricane devastation

What effect could all this have on us?

No one is really sure yet. Hurricanes that start in the Caribbean nearly always head off in a north easterly direction, towards Britain. Usually, by the time they get here, they have blown themselves out and we just get our usual storm. But, if there is more heat around, we may be in for something more like a proper hurricane!

The big questions:

What are hurricanes?

Why can they be so destructive?

Are there going to be more of them?

Could they start to affect the UK?

Could manmade global warming cause more hurricanes?