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The atmosphere is a gaseous envelope surrounding the Earth. Huge amounts of methane, ammonia, water vapor and carbon dioxide that form our atmosphere come from the center of the earth !

At first Earth had no atmosphere. It must have escaped into space the same way than the vapor of a boiling liquid. Indeed, the primitive atmospheric gases, helium and hydrogen, were light enough to escape the attractive force of the Earth under effect of intense solar radiation, and most part of these gas is lost in space. Some gases have been expelled by volcanoes at the beginning of the existence of the Earth and the first living beings.

The atmosphere has several roles : it provides us the air we breathe, its gases restrain heat the Earth get, and its protective d'ozone layer serves us as a screen against UV radiation emitted by the Sun.
It also serves as a reservoir for natural substances and so emissions resulting from human activity. In this "warehouse", physical and chemical actions and reactions occur, most of which can change our climatic or weather systems.

The atmosphere is mainly made of a gas mixture : air. It is composed of :

  • Carbon dioxide (CO2) : 0,035 %
  • Nitrogen (N2) 78 %
  • Oxygen (O2) 21 %
  • Argon (A) 0,93 %
  • Water vapor (H2O) 0 - 4 %
  • Neon (Ne) 0,0018 %
  • Krypton (Kr) 0,000114 %
  • Hydrogen (H) 0,00005 %
  • Nitrous oxide (N2O) 0,00005 %
  • Xenon (Xe) 0,0000087 %
  • Ozone (O3) 0 - 0,000001 %
  • Quantities proportionally very small of, helium, methane, carbon monoxide, organic compounds,...


The most important components whose amount is variable over time are: water steam, carbon dioxide, ozone and some particles suspended in air (for example pollutants).

Water exists in three states (or phases) in nature: gaseous (water vapor in the atmosphere), liquid (groundwater, rivers, lakes), solid (glaciers, ice caps). It depends on the temperature.

We evaluate the quantity of molecules in the atmosphere 1044 (44 zeros after the 1!!!). All these molecules are subjected to two forces :

- the molecules themselves have a speed of about 500 m/s that propel them into the vacuum of space ;
- the weight of the molecules which tends to fall upon our globe (consequence of gravity).

The result of these two forces made sure that half the mass of the atmosphere is in the first 3,1 miles (5 km) altitude. It is up to 12,4 miles (20 km) to reach 90 % of the total mass of the atmosphere.

The atmosphere is thicker at the equator (8-11,18 miles (13-16 km)) than at the poles (4,3-4,9 miles (7-8 km)).

The ICAO (International Civil Aviation Organization) defined a law of variation of atmospheric pressure and temperature that characterizes the standard atmosphere. This feature allows, among other things, the flight instrument calibration and certification of records.

HEIGHT IN
TEMPERATURE IN
ATMOSPHERIC PRESSURE
MILES

METERS

FAHRENHEIT
(°F)
CENTIGRADES DEGREE (°C)
PRESSURE
IN
HECTOPASCALS (HPa)
DECREASE OF
PRESSURE FOR 1000 METERS OF HEIGHT IN ( HPa)
0
0
59
15.00
1013
114
0,62
1 000
47.3
08.50
899
104
1,24
2 000
35.6
02.00
795
94
1,86
3 000
23.9
-4.50
701
85
2,48
4 000
12.2
-11.00
616
76
3,10
5 000
0.5
-17.50
540
68
3,73
6 000
-11.2
-24.00
472
62
4,35
7 000
-22.9
-30.50
410
53
4,97
8 000
-34.6
-37.00
357
50
5,59
9 000
-46.3
-43.50
307
43
6,21
10 000
-58
-50.00
264
37
9,32
15 000
-69.7
-56.50
120
13
12,43
20 000
-69.7
-56.50
55
4

 

Although air is well mixed throughout the atmosphere which is not itself physically uniform but has significant variations in temperature and pressure with altitude, which define a number of atmospheric layers. It consists of four layers where the temperature is alternately decreasing or increasing.

LA TROPOSPHERE LA STRATOSPHERE LA TROPOSPHERE L'IONOSPHERE LA TROPOSPHERE LA MESOSPHERE LA STRATOPHERE LE JET STREAM
Click here to enlarge this diagram of the atmosphere


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