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What is a Cyclone?
Cyclones are huge revolving storms caused by
winds blowing around a central area of low
atmospheric pressure. In the northern
hemisphere, cyclones are called hurricanes or
typhoons and their winds blow in an anticlockwise circle. In the southern hemisphere,
these tropical storms are known as cyclones,
whose winds blow in a clockwise circle.
How do Cyclones occur?
Cyclones develop over warm seas near the Equator. Air
heated by the sun rises very swiftly, which creates areas
of very low pressure. As the warm air rises, it becomes
loaded with moisture which condenses into massive
thunderclouds. Cool air rushes in to fill the void that is
left, but because of the constant turning of the Earth on
its axis, the air is bent inwards and then spirals upwards
with great force. The swirling winds rotate faster and
faster, forming a huge circle which can be up to 2,000 km
across. At the centre of the storm is a calm, cloudless area
called the eye, where there is no rain, and the winds are
fairly light.
• As the cyclone builds up it begins to move. It is
sustained by a steady flow of warm, moist air. The
strongest winds and heaviest rains are found in
the towering clouds which merge into a wall
about 20-30 km from the storm's centre. Winds
around the eye can reach speeds of up to 200
km/h, and a fully developed cyclone pumps out
about two million tonnes of air per second. This
results in more rain being released in a day than
falls in a year in a city like London.
Three separate systems
When and where do Cyclones occur?
• Cyclones begin in tropical regions, such as
northern Australia, South-East Asia and many
Pacific islands. They sometimes drift into the
temperate coastal areas, threatening more
heavily populated regions to the South.
Northern Australia has about four or five
tropical cyclones every year during the
summertime wet season. For a cyclone to
develop, the sea surface must have a
temperature of at least 26ºC.
Cyclone track
Why do Cyclones occur?
• When warm air rises from the seas and condenses into clouds,
massive amounts of heat are released. The result of this mixture of
heat and moisture is often a collection of thunderstorms, from
which a tropical storm can develop.
• The trigger for most Atlantic hurricanes is an easterly wave, a band
of low pressure moving westwards, which may have begun as an
African thunderstorm. Vigorous thunderstorms and high winds
combine to create a cluster of thunderstorms which can become
the seedling for a tropical storm.
• Typhoons in the Far East and Cyclones in the Indian Ocean often
develop from a thunderstorm in the equatorial trough. During the
hurricane season, the Coriolis effect of the Earth's rotation starts
the winds in the thunderstorm spinning in a circular motion.
Cyclone Danger
• Cyclones create several dangers for people living around
tropical areas. The most destructive force of a cyclone
comes from the fierce winds. These winds are strong
enough to easily topple fences, sheds, trees, power poles
and caravans, while hurling helpless people through the air.
Many people are killed when the cyclone's winds cause
buildings to collapse and houses to completely blow away.
• A cyclone typically churns up the sea, causing giant waves
and surges of water known as storm surges. The water of a
storm surge rushes inland with deadly power, flooding lowlying coastal areas. The rains from cyclones are also heavy
enough to cause serious flooding, especially along river
areas.
Katrina damage
• Long after a cyclone has passed, road and rail
transport can still be blocked by floodwaters.
Safe lighting of homes and proper
refrigeration of food may be impossible
because of failing power supplies. Water often
becomes contaminated from dead animals or
rotting food, and people are threatened with
diseases like gastroenteritis.
Typhoon saomi
Tornadoes
The enormous tornado that struck in Moore,
Oklahoma, on Monday has added a chilling entry into
the list of the deadliest tornadoes on record. 22 May
2013
• The event has many recalling a recordbreaking tornado that struck in precisely the
same region in 1999, during which the fastest
winds ever seen on the Earth's surface were
recorded: over 500km/h (310mph).
• In simplest terms, warm, wet air blowing in
from the Gulf of Mexico meets cold, dry air
coming from the massive Rocky Mountain
range, hemmed in by air masses on the
eastern part of the country. That frequently
creates the conditions for grand
thunderstorms.
• There are more tornadoes in total being
recorded in recent years, mainly due to better
reporting and fewer truly unpopulated areas
where they would go unseen.
• Yet there is no indication that the frequency of
large tornadoes is increasing. While 2011 saw
the largest number of storms among records
dating back to 1954, 2012 was among the
lowest
• And the average number of fatalities caused
by tornadoes has been steadily declining since
1925 - before Monday's storm, only one of the
25 deadliest tornadoes occurred in the last 58
years, and most of that list stretches back
further than a century.
• Much of this can be attributed to better
building codes and increasingly advanced
warning systems in affected areas - the
National Weather Service in Oklahoma issued
a warning 16 minutes before the storm hit.
• But such warning systems can only give a
rough indication of an area in which
conditions are almost certain to spark
tornadoes; what they cannot provide is a
precise prediction of where a tornado will
touch down.
Multi-vortex
Sequence of formation
texas
waterspout
Dust devil
Levelled house