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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.
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 27°C.
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 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.
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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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 Today.com).
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 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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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?
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 |