1st PART












The lowest layer of the atmosphere called the troposphere. It rises between 4,97 miles (8 km) at the poles and 9,94 miles (16 km) above the equator. The boundary between the troposphere and the stratosphere is the tropopause, defined by stabilized temperatures. The temperature decreases with increasing altitude of 33,08°F (0,60°C) every 100 m, on average, as a result of the rarefaction of the air and the progressive removal from the substratum.

The troposphere is the densest of the four layers of the atmosphere and contains up to 75% of the mass of the atmosphere. It consists mainly of nitrogen (78%) and oxygen (21%) with only small concentrations of other gases. Almost all atmospheric water vapor or humidity is in the troposphere.

The troposphere is overcast by the tropopause, a region where the temperature is stable. The air temperature begins to rise into the stratosphere. Such a temperature increasing prevents much air convection beyond the tropopause, and therefore most weather phenomena, including bearing clouds of thunderstorms, cumulonimbus, are confined to the troposphere. This is the most troubled layer, constantly shook by vertical and horizontal movements. The vertical turbulence is due to the vicinity of the Earth's surface, which determines by on the one hand mechanical ascents (rubbing) and on the other hand thermic ascents (by thermoconvection and instability).

The circulation of the atmosphere depends on cosmic factors (solar radiation), global (state of the atmosphere, Earth's rotation around its axis, ocean salinity and temperature), geographical (distribution of continents and seas, vegetal covers, ice cover). It results in longitudinal, latitudinal, rising and decreasing movements.

This chart is produced and update with data of the
NOAA National Climatic Data Center (NCDC)


Air is a gas which has weight. The atmospheric pressure is the weight of a column of air extending from a given altitude to the top of the atmosphere and this weight is applied to all objects on the surface of the Earth.

It measures with a barometer by counterbalancing the weight of air with mercury. This method became so widespread that often the height of a column of mercury expresses the pressure. So the pressure can be measured in millimeters or inches of mercury, or in kilopascals (kPa) or hectopascal (hPa) or millibars (mb).

At sea level, the pressure is 101,32 kPa or 1013,20 hPa or mb. When the pressure exceeds 1013 hPa this corresponds to an anticyclone but when the pressure is less than 1013 hPa is a depression and the lower it is more is windy. An increase of air pressure generally favors the good weather while falling pressure is often associated with bad weather and if it goes down very quickly or 4 hPa or more in the last 6 hours, a storm and even thunderstorms approaches.

In France :
The highest measured pressure : 1050 hPa in Paris on February 6, 1821.
The lowest measured pressure : 947 hPa at Boulogne-sur-Mer, on December 25, 1821.

In the world :
The highest measured pressure: 1086,8 hPa in Tosontsengel, Khöusgöl (Mongolia) on January 20, 2010.
The lowest measured pressure: 870 hPa at the center of Typhoon Joan in the Philippines on October 14, 1970 and in the heart of Typhoon Tip in the Pacific on October 12, 1979 : 870 hPa.

The atmospheric pressure is an essential element for weather forcast, even if atmospherical pressure forcasts weather at 80%, it remains 20% spent on other elements of meteorology.

A video explanation of atmospheric pressure


According the Buys Ballots rule surface winds do not blow exactly from anticyclones to depressions. The Coriolis force deflects the wind from their theoretical trajectory to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deviation is null at the equator and maximum at the poles.


The air leaves the anticyclone (A) by turning in the direction
of clockwise and penetrates into the depressions (D) by
turning counter clockwise in the northern hemisphere.

The rising air cools, indeed, by adiabatic expansion, and the opposite characterizes the descending air. The more air hot the more air is light and vice versa.

The heated tropical air becomes less dense and
therefore lighter and rises in altitude. Then
low pressures take form at the equatorial sea.


In polar regions it is quite the opposite. Air masses
cool and become denser and therefore heavier air go
down. Then high pressures take form at the sea level.

Click here to see an animation of a cold front into a warm front


Animation of atmospheric circulation in 3D:
In blue anticyclone and in red depression

Patricia Régnier helped me correct mistakes, please you to visit her blog
I’m not english speaker, some improprieties can appear to english masters.
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