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A cyclone is an atmospheric perturbation in the aspect of a large cloud mass spiral bands, associated with a very intense depression with swirling winds more than 74.mph (120.km/h). In the southern hemisphere, the winds rotate in the direction of clockwise in the cyclone. And the direction of rotation is opposite in the northern hemisphere. The direction of rotation of the wind around the depression is due to the rotation of the Earth called CORIOLIS force .

A cyclone is made with a "eye" of about average diameter 20 to 50 km, but can reach 200 km according to the cyclone. Closer one gets to the eye of the cyclone more the temperature rises 50°F (10°C) higher than the surrounding air in high altitude and from 32 to 35,6°F (from 0 to 2°C) higher than surface air), more the pression is lower, the precipitation is greater and become violent winds up to 155 mph (250 km/h) for the most intense cyclones, while when we are in the eye winds are very low.

Tropical cyclones generally have a diameter of a few hundred kilometers, but they can reach 1000 or even 1500 km and cloud structures can reach 20 km height.




Warm, moist air rises high altitude turning around the
eye of the cyclone and dry air down into the eye.


A video giving information on cyclones


there are several types and main mechanisms of formations of cyclones :

Tropical cyclones form over warm water in tropical seas and draw their energy from the latent heat of condensation of water.

To develop a tropical cyclone favourable conditions are necessary : a depression must cross over ocean heated more than 78,8°F (26°C) at least 60 meters depth and distant for more than 550 km from equator. This is from 5° latitude that the Coriolis force can act on the trigger initial vortex as it is null at the equator. The force generated by the earth's rotation causes wind deflection to the east in the northern hemisphere. This allows warm moist air to rise to 9.000 m. Then the atmospheric pressure decreases. With the Coriolis force, the hot and humid air begins to swirl. Water vapor forms clounds. Gerater is the difference between the air temperature low and high altitude, the more air is unstable, creating violent thunderstorms. Spiral, cold air descends and it warms and rises again.

When a tropical cyclone reaches land or in cold water the winds weaken as it is no longer powered by the vapor of warm water.

A depression passes over an area of water to more than
78,8°F (26°C). Warm and humid air rises up then is combined with
winds lower altitudes. The pressure drops to the sea surface
and increases at the highest clouds to the tropopause, the
top of the troposphere. The cold air descends it warms and
turns in a spiral around the depression forming in central
the eye of the tropical cyclone. So a cyclone is created.


Extra-tropical cyclones (winter storms) occur in the middle latitudes of the Earth and draw their energy from areas with heavy difference of temperatures between the tropics and the poles. The majority of their power is due to baroclinic processes. Their field of temperature and humidity is asymmetric.

EPSILON, VINCE, DELTA of 2005 are called extra-tropical cyclones according to the season and thus as the temperature of the ocean was lower at 78,8°F (26°C).

Différences entre les cyclones extra-tropicaux et les cyclones tropicaux

extra-tropical cyclones
tropical cyclones
Energy source
temperature difference
between North-South
water condensation
Storm season
(northern hemisphere)
october to march
summer / autumn
Location of storms
tropical and subtropical areas
Diameter storms
1000-2000 km
500-1000 km
Peak wind gusts
44-112 mph (20-50 m/s)
74-201 mph (33-90 m/s)
Life at a given
point in time
3-24 hours
2-6 hours
Concomitant phenomena
tidal wave
tidal waves, tornadoes
Extent of damage
many small damages
small and large damages


Subtropical cyclone are cyclones which both the characteristics of tropical and extratropical cyclones. Their characteristics of tropical cyclones is that they can have a warm heart. But some have a cold heart. The strong winds in subtropical cyclones is wider and far from the nucleus. These cyclones can have a radius of maximum diameter 420.km.

Sometimes these subtropical systems are transformed into storms or tropical cyclones as was the case with Hurricane Florence in November 1994 in the Atlantic. They are formed at subtropical latitudes so beyond the tropics, up to a latitude of 50°.


Polar cyclones are low pressure systems from 1000 to 2.000.km that originate in high latitudes, areas where the thermal contrast is significant as the border of the ice zone with the open sea while that the very cold air goes at altitude. They form there where the water is not too cold and without ice in the northern seas: the Labrador Sea, the Gulf of Alaska and the sea of Japan. But they are also common on polar waters surrounding Antarctica. More polar cyclones formed in the northern hemisphere than in the southern hemisphere. About 15 polar cyclones per winter occur in the Eurasian Arctic, Greenland and the Canadian Arctic zone.

As tropical cyclones they turn in the opposite direction to clockwise in the Northern Hemisphere. Polar cyclones can give very localized conditions of blizzards.



Polar cyclone off the coast of the Iceland


Cyclones can move thousands of kilometres at an average speed of 12 to 15 mph (20 to 25 km/h). This speed is quite low in the phase of genesis, but it gradually increases that they stray tropical waters and head to the North (for the northern hemisphere). Then their displacement can be faster (700 to 1000 km per day).


This map shows the areas where hurricanes hit more often and
their directions. As you can see it is all along the tropics
(to 10° of latitude) they develop as the water is hot enough.

Click here to see the track of tropical cyclone activity

In the northern Indian Ocean is from January to March they develop. In the North Atlantic and North Pacific, it is between June and September that tropical cyclones form. In the South Pacific and the South of the Indian ocean is from May to June and from October to December that they form.

However, in the South Atlantic there is logically not tropical cyclones that form as water is not warm enough, and then because of the permanent presence of a strong vertical wind shear in the troposphere and the intertropical convergence zone which disrupts cyclone formation.

But the 03/28/2004 a tropical cyclone was observed for the first time in the South Atlantic and hit the coast of southeastern Brazil with winds up to 93 mph (150 km/h) and an average of 74 mph (120 km/h). This cyclone known as Catarina was born as an extratropical stage then is passed to the hybrid stage (warm air at the centre in lower layers, cold air aloft) and became a tropical cyclone at warm core (warm air at the centre at all levels) where there is exchange of energy between ocean-atmosphere.

Cyclone Catarina has hit Brazil, this is a rare occurence


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