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 tropical revolving storms because
they are spun on their journey by the Coriolis force of the Earth’s spin. The Earth is 40,000 kilometers (24,900
miles) around at its widest part, the equator. Because it spins on its axis
once in 24 hours, a point on the Earth's equator is traveling about 1,700 km
per hour (1,000 miles per hour) relative to its axis. But the closer you get to
the poles, the smaller the track a point takes in its daily rotation. At 60°
North or South latitude, the track is only half the distance that it is at the
equator, and so a point travels only half as fast. Air (or water) moving from
high latitudes to low then tends to lag, and a person on the surface would feel
a wind blowing out of the east. On the other hand, air moving from low
latitudes to high is deflected westwards. This also means that moving air or
water is deflected to the right in the northern hemisphere, and to the left in
the southern hemisphere. It is this
deflection that causes tropical storms to rotate.
Where they occur:
Tropical storms 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 and is heated
to a sufficient depth.
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 - Eye
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. As the sea
temperature increases uplift of air increases and pressure decreases. The Saffir Simpson also accounts for the
height of the storm surge, the huge waves of water that are whipped up by the
storms.
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.
How Tropical storms
form:
Tropical storms form whenever sea temperatures rise above 27
°C and can be up to 650km across. They occur where the trade winds converge and
often when the ITCZ has migrated to its most Northerly extent allowing air to
converge or come together at low levels.
The suns heat passes through our atmosphere and warms the ocean water
throughout the summer. The sea is
constantly moving and heat is redistributed to deeper parts of the ocean so
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 vapor up through the atmosphere in thermals. As these
thermals rise the temperature drops (at 9.7°C per 1000m ascent or the
DALR). Progressively the relative
humidity rises as the air ascends (as cooler air can hold less water vapour
than warmer air), eventually this causes the water vapour to condense into tiny
droplets around dust and pollen (condensation nuclei). These droplets collide together to form bigger
droplets and thus helps to form huge cumulonimbus clouds. Latent heat is
released during condensation fueling the storm further. Eventually these
droplets will collide and coalesce with one another, become bigger and fall as
rain. As a result of condensation, latent heat is released and the air cools at
a slower rate, the SALR, this fuels the storm further.
Because the air has
risen in the center 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 along the pressure gradient. 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 as the pressure
gradient steepens.
The whole storm
slowly migrates across oceans towards land, and because of the Earth’s 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. The pressures and weather are more stable in the eye, as the updrafts of
air are balanced by descending cooled air.
As tropical storms
pass over land they lose their source of energy, and the die out.
