In
meteorology, a 'cyclone' is an area of
low atmospheric pressure characterized by inward
spiraling
winds that rotate
counter clockwise in the
northern hemisphere and
clockwise in the
southern hemisphere of the Earth.
[1][2] Since the generic term covers a wide variety of
meteorological phenomena, such as
tropical cyclones,
extratropical cyclones, and
tornadoes, meteorologists rarely use it without additional qualification.
Etymology
The word cyclone appears to have been coined by a Captain
Henry Piddington, who used it to refer to the storm that blew a freighter in circles in
Mauritius in February of 1845.
[3] Tropical cyclones are then circular wind storms that form in the tropics. It may have been derived from the Greek word ''kyklon'', which means moving in a circle.
[4][5] Some say that Piddington derived the word from the Greek word ''cyclos'', meaning "coils of a snake", in comparison with the tropical storms in the Bay of Bengal and in the Arabian Sea.
[6] Others argue that the word was derived from ''
Cyclops'', a creature in Greek mythology with one eye, as it is 'turning wind with one eye'.
[7]
Structure
There are a number of structural characteristics common to all cyclones. Their center is the area of lowest atmospheric pressure, often known in mature tropical and subtropical cyclones as the ''eye''. Near the center, the
pressure gradient force (from the pressure in the center of the cyclone compared to the pressure outside the cyclone) and the
Coriolis force must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure. The wind flow around a large cyclone is
counterclockwise in the northern hemisphere and clockwise in the southern hemisphere as a result of the
Coriolis effect.
Formation
Main articles: Cyclogenesis
Cold-core cyclones (most cyclone varieties) form due to the nearby presence of an upper level trough, which increases divergence aloft over an area that induces upward motion and surface low pressure. Warm-core cyclones (such as tropical cyclones and many mesocyclones) can have their initial start due to a nearby upper trough, but after formation of the initial disturbance, depend upon a storm-relative upper level high to maintain or increase their strength.
Categorisation
Each of the six main types of cyclone has further characteristics which define it as either a
Polar cyclone,
Polar low,
Extratropical,
Subtropical,
Tropical, or
Mesoscale.
Polar cyclone
Main articles: Polar cyclone
'Polar' or 'Arctic cyclones' are vast areas of low pressure. They should not be confused with what are commonly referred to as polar lows, which behave like hurricanes. A polar cyclone is a low pressure
weather system, usually spanning 1,000–2,000 kilometers, in which the air circulates in a counterclockwise fashion in the northern hemisphere.
Polar low
Main articles: Polar low
A 'polar low' is a small-scale, short-lived atmospheric
low pressure system (depression) that is found over the ocean areas poleward of the main
polar front in both the Northern and Southern Hemispheres. The systems usually have a horizontal length scale of less than 1,000 km and exist for less than a couple of days. They are part of the larger class of
mesoscale weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms.
Polar lows have been referred to by many other terms, such as comma cloud, mesocyclone, polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air depression. Today the term is usually reserved for the more-vigorous systems that have near-surface winds of at least gale force (17 m/s).
Extratropical
A ficticious synoptic chart of an extratropical cyclone affecting the UK. The blue arrows between
isobars indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm
frontal boundaries.
An 'extratropical cyclone', sometimes inaccurately called a 'cyclone', is a
synoptic scale low pressure weather system that has neither
tropical nor
polar characteristics, being connected with
fronts and horizontal
gradients in
temperature and
dew point otherwise known as "baroclinic zones".
[8]
The descriptor "extratropical" refers to the fact that this type of cyclone generally occurs outside of the tropics, in the middle latitudes of the planet. These systems may also be described as "mid-latitude cyclones" due to their area of formation, or "post-tropical cyclones" where
extratropical transition has occurred,
[9][10] and are often described as "depressions" or "lows" by weather forecasters and the general public. These are the everyday phenomena which along with
anti-cyclones, drive the weather over much of the Earth.
Although extratropical cyclones are almost always classified as
baroclinic since they form along zones of temperature and dewpoint gradient, they can sometimes become
barotropic late in their life cycle when the temperature distribution around the cyclone becomes fairly uniform with radius.
Subtropical
Main articles: Subtropical cyclone
A 'subtropical cyclone' is a weather system that has some characteristics of a
tropical cyclone and some characteristics of an
extratropical cyclone. It can form in a wide band of
latitude, from the
equator to 50°.
Tropical
Main articles: Tropical cyclone
A 'tropical cyclone' is a storm system fueled by the heat released when moist air rises and the water vapor in it
condenses. The term describes the storm's origin in the
tropics and its cyclonic nature, which means that its circulation is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Tropical cyclones are distinguished from other cyclonic windstorms such as
nor'easters,
European windstorms, and
polar lows by the heat mechanism that fuels them, which makes them "warm core" storm systems.
Depending on their location and strength, there are various terms by which tropical cyclones are known, such as 'hurricane', 'typhoon', 'tropical storm', 'cyclonic storm' and 'tropical depression'.
Tropical cyclones can produce extremely strong winds,
tornadoes, torrential rain, high waves, and
storm surges. The heavy rains and storm surges can produce extensive
flooding. Although their effects on human populations can be devastating, tropical cyclones also can have beneficial effects by relieving
drought conditions. They carry heat away from the tropics, an important mechanism of the global
atmospheric circulation that maintains equilibrium in the Earth's
troposphere.
Mesoscale
Main articles: Mesocyclone
A 'mesocyclone' is a cyclonic vortex of air, between approximately 2 and 10 km diameter within a convective storm. They can often be found in association with updrafts in
supercells, where
tornadoes may form. The term refers only to mesoscale cyclones found within convective storms, and does not apply to other cyclones on the
mesoscale.
[11] Storms with mesocyclones can feature strong surface winds and severe
hail.
Extraterrestrial cyclones
Main articles: Extraterrestrial cyclones
Cyclones are not unique to Earth. Perhaps the most famous extraterrestrial cyclonic storm is
Jupiter's
Great Red Spot.
Mars has also exhibited cyclonic storms.
See also
★
Anticyclone
★
Cyclogenesis
★
Eye
★
Iowa State Cyclones
★
List of environment topics
★
Meteorology
★
Subtropical cyclone
★
Tropical cyclone
★
Tropical cyclogenesis
References
1. BBC Weather Gloassary - Cyclone
2. UCAR Glossary - Cyclone
3. Storm, Whipple, Addison, , , Time Life Books, 1982, ISBN 0-8094-4312-0
4. The Weather Doctor: Cyclone Origin Keith C. Heidorn, PhD
5. Banglapedia: Piddington, Henry
6. Cyclone Related Disasters - What Are Cyclones?
7. ClearlyExplained.com - Cyclones Richard Conan-Davies BSc Dip Ed
8. ESCI 241 – Meteorology; Lesson 16 – Extratropical Cyclones Dr. DeCaria
9. ExtraLessonMillUni
10. Synoptic Composites of the Extratropical Transition Lifecycle of North Atlantic TCs as Defined Within Cyclone Phase Space Robert Hart and Jenni Evans
11. American Meteorological Society Glossary - Mesocyclone
External links
★
Fundamental of Physical Geography: The Mid-Latitude Cyclone - Dr. Michael Pidwirny, University of British Columbia, Okanagan
★
Glossary Definition: Cyclogenesis - The National Snow and Ice Data Center
★
Glossary Definition: Cyclolysis - The National Snow and Ice Data Center
★
Weather Facts: The Polar Low - Weather Online UK
★
NOAA FAQ
★
Cyclones 'ClearlyExplained'
★
Kent Cyclone - Listen to 'Cyclone What?' here
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