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Year 9 - Risky Earth - Hurricanes what

A tropical storm is an intense low pressure weather system, that can last for days to weeks within the Tropical regions of our planet.  Hurricanes occur when these tropical storms develop wind speeds of over 74mph (miles per hours).

They are known by many names, including hurricanes (North America), cyclones (India) and typhoons (Japan and East Asia). 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.

Saffir Simpson scale

Saffir Simpson scale

Wind speeds are used to decide what category of storm a tropical storm is, over 120Kph or 74 miles per hour is needed for a category 1 hurricane, Over 250Kph or 155 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.

How Tropical storms form;

Tropical storm diagram
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 27°C 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 1°C 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 they lose their source of energy, and the die out. You can see an animated guide to this process at the BBC.

Find out more

Watch Hurricane movies from NASA
Try a quiz from CBBC
Internet Geography's pages on tropical storms




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