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.
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
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
In France :
The highest measured pressure : 1050 hPa in Paris on February
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.
Animation of atmospheric circulation
blue anticyclone and
in red depression
Régnier helped me correct mistakes, please you to visit
Im not english speaker, some improprieties can appear
to english masters.
Could you help me reporting by mail any fault you read. Thank
you for all.