A cyclone (of the Greek kuklos , circle) is a weather term which indicates simply a depressionary system in rotation. Even if any depression can be called a cyclone, this term is generally reserved for certain particular types of systems which are formed above warm water of the tropical seas, the tropical cyclones. By extension, the cyclonic circulation is the direction which the flow of air around a depression or a barometric Creux will take, that is to say anti-clockwise in the northern hemisphere and time in that of the south.
One also applies the Suffixe cyclone to certain phenomena of very small scales where a rotation occurs.
StructureThe heart of the cyclone is a basic area pressure. The Gradient of pressure between the system and the zones moreover high pressure, generates a air volume displacement. Under the effect of the Force of Coriolis, these winds are deviated towards the line in the northern hemisphere (left in that of the south) what gives a circulation whose trajectory becomes circular around the basic center pressure. The more important the difference in pressure is, the more the Vent S are strong.
On both sides of the equator, the cyclones have different directions of rotation. In the northern Hemisphere, a cyclone turns in the opposite direction of the needles of a watch whereas in the Southern hemisphere, it turns in the direction of the needles since the force of Coriolis acts conversely from one hemisphere to another.
Types of cyclonesThere exist several types of cyclones according to the place where they are formed.
See also: tropical Cyclone
The tropical cyclones (also named hurricanes in the Atlantic northern, the Gulf of Mexico and is northern Pacifique or typhoon in the west of the northern Pacific and the southernmost China Sea) are formed with the top of warm water of the tropical seas and draw their energy in the latent heat of condensation of water.
Several conditions are necessary to the formation of such a cyclone:
the temperature of the sea must be higher than 26 °C, on a depth of at least 50m, at the place of the formation of the depression which will become cyclone.
To be sufficiently far away from the equator so that the Force of Coriolis can act (5 with 10° of latitude).
the winds on the various levels of the atmosphere must be of homogeneous direction and force in the zone of formation of the cyclone. If the winds of high-altitude blow in a way very different from the basic winds altitude, the formation of the cyclone will be opposed.
The intensity of the cyclone is determined by the force of the maximum wind which it generates, because it is the parameter easiest to estimate and which characterizes the potential destruction well. In the North Atlantic, one uses as criterion the moderate wind over one minute. If the wind is lower than 34 nodes (63km/h), it is a tropical depression. If the wind lies between 34 nodes and 63 nodes (117km/h), it is a tropical storm, and the cyclone receives a name then. If the constant wind exceeds 64 nodes (118km/h), it is a hurricane. Variations of this classification are used in the Pacifique and the Indian Ocean. The scale used for the tropical cyclones, including the hurricanes, is the scale of Saffir-Simpson. It takes again the force of the winds where the scale of Beaufort stops, that is to say Ouragan which is Force 12 out of 12 in the scale of Beaufort is equal to Category 1 of 5 on the scale of Saffir-Simpson.
See also: Cyclone extratropical
A cyclone extratropical, sometimes named cyclone of the average latitudes , is a weather system basic pressure, of synoptic scale, which is formed between the line of the Tropique S and the Polar circle. It is associated with face S, that is to say horizontal zones of Gradient S of the Température and Dewpoint, which one names also " zones Barocline s".
The extratropicaux cyclones have characteristics different from the tropical cyclones, supplied with the Convection, and from the polar cyclones more in north. They are in fact the weather depression S which pass daily on the majority of the sphere. With the Anticyclone S, they govern the time on the Earth, producing Nuage S, Pluie, Vent S and Orage S.
See also: subtropical Cyclone
The subtropical cyclones are extratropicaux cyclones which present some of the characteristics of the tropical cyclones, such as for example a heart becoming hot. They are generally formed beyond the Tropique S, until a Latitude of 50° (northern and southern). Indeed, one finds there a stormy activity around his center which tends to form a hot heart to him but one finds it in a frontal zone weak. With time, the subtropical storm can become tropical.
See also: polar Cyclone
A polar cyclone is a depressionary system of broad scale passing in the artic regions and antarctic.
See also: polar Depression
A phenomenon similar to the tropical cyclones exists on the Arctic Ocean, which one calls polar depression.
These depressions can be more violent than the tropical cyclones but of size more reduced. They have from 100 to 400 km in diameter with wind forces of hurricane S, developing like bombs and lasting a pair of days only. These depressionary systems occur in the zones of important thermal contrasts as to the edge of the zone of the ices with the open sea whereas very cold air passes in altitude. They can give conditions of Poudrerie and very localized blizzard.
On the other hand, they have much less impact since in the polar regions, the human and animal population density is very low. On the satellite images, the clouds are rolled up around the center as for a hurricane or a typhoon. Probes released by planes of research show a hot heart as in the latter.
Extrapolations of the term
See also: Mésocyclone
The mésocyclones are not depressionary systems but rather a rotation imbricated in a Orage (Cumulonimbus).
Indeed, the change of the winds between surface and the top of the boundary layer of friction of the atmosphere (less than 2 km thickness) give a horizontal rotation of the winds. Let us think of gigantic a wind which would undergo more winds of west at its top than at its base, its blades thus put themselves to turn because those the top undergo a greater force than those of bottom.
The ascending Courant under a storm will change the axis of this rotation to make it vertical. When that occurs, one can visually observe, or on the Doppler data of a weather Radar, that certain parts of the cloud are in rotation.
A mésocyclone is not a tornado. The tightening of its rotation, by particular conditions of circulation of the winds around the storm, can however lead to the formation of a tornado under the storm. That is identical to the acceleration of the rotation of a skater when it brings back his arms towards his body.
See also: Tornado
A Tornade is not a cyclone because it is not a depressionary system. It is in fact a Vortex (Tourbillon) of Vent S extremely violent one, usually occurring at the base of the Cumulonimbus strongly stormy which are the only ones has to produce some. weather Phenomenon with the destroying capacity higher than that of a tropical Cyclone, but fortunately limited in time and space, the tornadoes generates the strongest winds which exist on the surface of the Globe, bursting sporadically and with fury, killing each year more people than any other phenomenon of the kind. The tornadoes are classified according to the damage which they cause and winds that they generate. The scale to classify them was the scale of Fujita (of F0 with 65 98km/h in F5 of more than 516 km/h) but lately the scale was reorganized and is called the INCREASED SCALE OF FUJITA (with F5 = 358 km/h or more). Their diameters can vary from 20 meters to more than 2 kilometers and leave for some of very large visible traces since space. As a cyclone is associated with a cloudy mass which can contain storms, tornadoes can thus be overlapping in a cyclonic system.
Gouvernementaux and universitaires:
- Cyclones on Weather France
- National Hurricane Center
- Tropical Site Cyclones ([[Co-operative Institute for Satellite Meteorological Studies] or CIMSS)]
- Cyclone, hurricane, typhoon: who are they? by Futura sciences
- Site Cyclone Xtrème
- Followed tropical cyclones by Allmetsat
- Cyclones of the Indian Ocean - Island of the Meeting
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