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Tropical storms
Cyclone in the Philippines

What they are
Where they are found
The Saffir-Simpson Scale
How they form
Further things to do/find out more

What they are
A tropical storm is an intense low pressure weather system, that can last for days to weeks within the Tropical regions of our planet. 

Where they occur

They are known by many names, including hurricanes (North America), cyclones (India) and typhoons (Japan and East Asia).  They all occur in a band that lies roughly between the tropics of Cancer and Capricorn and despite varying wind speeds are ferocious storms. Some storms can form just outside of the tropics, but in general the distribution (location) of these storms is controlled by the places where sea temperatures rise above 27C.

The highest number of storms does not occur in the Atlantic close to the USA, but in the North Pacific affecting countries such as the Philippines and Japan. This is despite the fact that in the UK we only really get to hear about tropical storms affecting the USA. The most affected area being South East Asia receives an average of 26 storms per year. The least affected area is India where there is an average of 2 tropical storms per year.
Tropical storm track map

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The Saffir-Simpson scale

Tropical storms are defined by their wind speeds and the potential damage they can cause, using what is known as the Saffir Simpson scale. Many tropical storms form between the tropics, some develop into tropical depressions but not many actually develop into full blown hurricanes/cyclones/typhoons.
The Saffir-Simpson scale for Hurricane classification

Wind speeds are used to decide what category of storm a tropical storm is, over 120Kph or 74 mile per hour is needed for a category 1 hurricane, Over 250Kph or 149 miles per hour is the worst hurricane, a category 5 which would cause extreme damage.  Watch an animation of the Saffir-Simpson scale in action.

The 2 cases studies on this site, Hurricane Floyd and the Orissa Cyclone, were the worst category of hurricane, category 5, with wind speeds over 155 miles per hour.
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How Tropical storms form;

Diagram showing the structure of a hurricane

Tropical storms form whenever sea temperatures rise above 27 C.  The suns heat passes through our atmosphere and warms the ocean water throughout the summer.  Because the sea is constantly moving and heat is redistributed to deeper parts of the ocean this takes quite some time (this is why hurricanes occur in late summer - when sea temperature is at its highest).
This causes the seas temperature to rise to 27C and above, which encourages evaporation and the rising of air and water vapour up through the atmosphere in thermals (find out more from USA
As these thermals rise the temperature drops (approximately 1C per 100m) causing the water vapour to condense into droplets.  This helps to form huge cumulonimbus clouds. Latent heat is released during condensation fuelling the storm further. Eventually these droplets will collide with one another, become bigger and fall as rain.
Because the air has risen in the centre of this storm, an area of low atmospheric pressure exists at the surface.  The Earth's atmosphere acts to balance this out as air rushes from surrounding high pressure areas to the centre of the storm.  This creates the high winds in the storm, and the lower the pressure gets in the centre of the storm relative to the pressure surrounding the storm, the stronger the winds will become.
The whole storm slowly migrates across oceans towards land, and because of the Earths rotation or spin (known as the Coriolis force or effect(click here to see an animation)), the whole storm starts to spiral around a central more calm point, known as the eye.
As tropical storms pass over land the lose their source of energy, and the die out. You can see an animated guide to this process at the BBC.
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Things to do/find out more

Case studies
  LEDC - the Orissa super cyclone Case studies
MEDC - Hurricane Floyd   

1. Describe the global distribution of tropical storms using the map below (from the Earth Observatory)
2. Why do Tropical storms occur in these locations?

hurricane_formation.jpg (26053 bytes)

3. Quizzes on Hurricanes

4. Watch Hurricane movies from NASA

5. Try a quiz from CBBC

6. A quiz from the Red Cross

7. Hurricanes key words match up

Internet Geography's pages on tropical storms

Track hurricanes from 2004 and 2005

The BBCs animated guide to Hurricane Katrina

YouTube Videos

The Saffir-Simpson scale explained, with fantastic graphics Fantastic animation on Hurricane tracks across the globe Sentimental footage of Hurricane Katrina


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