The atmosphere is a mixture of invisible gases, completely surrounding the Earth. The most important of these gases are:

• Nitrogen:                78,09%
• Oxygen:                 20,95 %
• Water Vapeur:       0 to 3 %
• Carbon dioxine:      0,03 %
• Ozone:                   0,000003 %

Carbon dioxide has a strong impact on climate, but water vapour determines weather conditions.

Water vapour causes clouds, fog and precipitation, simply because of its inherent instability. Depending on temperature and pressure, it varies between a vapour, liquid and solid.

Obviously, the atmosphere is more than just water vapour. Indeed, it also contains billions of tonnes of gas and various particles from human activities and natural phenomena.

In the lower levels, especially in the troposphere, which is the atmospheric layer in which we live and move about, there are considerable quantities of liquid or solid particles in suspension, known as aerosols.

Water vapour condenses on aerosols of liquid particles to form droplets, clouds and fog.

Ice crystals form mainly around aerosols of solid particles, consisting mostly of dust, pollen, ash, smoke, sea salt and sand, to form clouds or ice fog.

Aerosols are also responsible for haze and smog, and are the main triggers of changes in state of water vapour.

The farther we travel from the Earth's surface, the thinner the air becomes, until it almost disappears. In fact, 99% of the total mass of the atmosphere lies within 30 km (100,000 feet) of the ground, while fully half of it is within the first 5.5 km (18,000 feet).

But the trails left by some satellites tell us that it extends up to some 1600 km above the Earth. Indeed, the aurora borealis can often be seen at an altitude of over 1000 km.

Although data collected at very high altitudes are not used in the preparation of day-to-day forecasts, their study is important for an overall understanding of weather.

Air is inert, fluid, viscous, expansible and compressible, and as such is subject to the laws of thermodynamics.

Inertia Since air is inert and has mass, some force is necessary to set it in motion, accelerate or stop it.

The density of air varies considerably from one point to another. At sea level, it is about 1.125 grams per litre, whereas at 10 km (33,000 feet), it is only about 0.414 grams per litre.

Fluidity The atmosphere is also highly fluid; it tends to take up all available space and to exert pressure on all the bodies it surrounds.

Viscosity The air's viscosity is an important property. Although it allows the air to move and to influence the movements of neighbouring layers, this same property prevents excessive wind shear.

The movement caused by this viscosity is always slowed by a much more inert body, however: the obstacles making up the Earth's surface and the friction they exert.

Expansibility and compressibility As soon as the pressure of an air mass at some point increases in relation to the surrounding pressure, the air in question expands and tends to take up more room.

If the surrounding pressure increases, however, the air mass will decrease in volume as it is compressed.

Thermodynamics Air is as thermodynamic as all other gases, under the effects of pressure and temperature.

In absolute terms, we know that as soon as a small quantity of air rises above the surface, its pressure and temperature fall and its volume increases.

On the other hand, air descending toward the surface obviously undergoes the opposite changes: it is compressed, its volume shrinks and its temperature rises.

This phenomenon is not the main cause of atmospheric warming or cooling, but does play a crucial role in cloud formation due to vertical air movements.

Figure 1 On the left, rising air expands and cools as its pressure falls. On the right, descending air is compressed and warms, which increases its pressure.

Scientists agree on major divisions of the atmosphere, but the exact dividing lines and characteristics vary with different disciplines and purposes.

We will discuss two classifications: those of the International Union of Geodesy and Geophysics (IUGG) and that of Goody, based on temperature and ionization.

In meteorology, the IUGG system is used, because it divides the atmosphere into layers according to their thermal structures.

The IUGG system consists of the following layers, by distance from the surface:

• troposphere
• stratosphere
• mesosphere
• thermosphere

To identify each of the transition zones, the suffix sphere is simply replaced with pause, to give:

• tropopause
• stratopause
• mesopause
• thermopause

Figure 2 Systems for dividing the atmosphere into layers.

Troposphere It is in this first layer that the atmosphere, in contact with the Earth, is the warmest. The Earth's surface captures the Sun's rays and becomes a radiating body that warms the air, setting it in motion by simple thermodynamic action.

This produces large-scale rising air currents that warm the upper levels, while moving gigantic volumes of air horizontally. These movements are what we call weather systems.

As a result, most weather activity occurs in this lowermost layer of the atmosphere, the great variety of phenomena that affect us from day to day all around the world.

The intense concentration of water vapour, associated with strong rising air currents, leads to clouds and precipitation, thunderstorms, hurricanes and tornadoes.

Just below the tropopause, the shear caused by the strong contrast between the troposphere and stratosphere generate the very powerful winds called jet streams, and gives them their sometimes complex structures.

These narrow and powerful streams of air, which originate only in the north of the Northern Hemisphere, are so influential that they are part of what is called the general circulation, that is the trajectory of the Earth's air masses.

Tropopause The first thermal boundary. The tropopause marks the end of the biosphere and is the coldest part of the lower atmosphere.

The average altitude of this layer is about 11 km (36,000 feet). Above the poles, it is about 8 km (26,000 feet) from the surface, while at the equator is at about 18 km (59,000 feet).

The tropopause shifts according to temperature and is highest in summer. It changes altitude abruptly near the jet streams.

Stratosphere The stratosphere is a 15 km, 50 000 feet layer where the temperature increases gradually to 0° C or even as much as 10° C.

In this layer there is almost no water vapour, and no major vertical air currents. Occasionally some mother-of-pearl clouds formed from the rare ice crystals at this level can be seen.

The main characteristic of the stratosphere is the ultraviolet shield it contains—the ozone layer. Stratospheric warming occurs as this layer absorbs part of the ultraviolet rays from the Sun.

Stratopause At the top of the stratosphere lies the stratopause. Past this level, the temperature begins falling again as the ozone becomes increasingly thinner with height.

Mesosphere The only significant characteristic of this layer is the rarity of ozone and consequently its absolute minimum temperature of about -80° C, recorded at a distance of 80 to 90 km from the Earth's surface. This is the lowest temperature ever recorded in the atmosphere.

It is also in this layer that meteorites burn up as they come into contact with the atmosphere.

Mesopause The mesopause is another thermal transition layer; after this point the atmosphere again begins warming with altitude.

Thermosphere Air molecules are rare in this layer, which is why the Sun's rays strike with such force and temperatures can theoretically rise to enormous levels.

Exosphere According to the GOODY classification, this upper part of the atmosphere is where particles begin escaping from the Earth's pull, and are subject to atomic and molecular exchanges with solar and cosmic particles. The exosphere is where the aurora borealis occur.

The bottom of the exosphere is not very clearly defined, but is estimated to lie 500 to 800 km from the Earth's surface.

The wealth of direct observational data available for the first 20 km of the atmosphere make it possible to prepare detailed descriptions of average conditions within this layer.

These data are organized into standard atmospheres, defined on the basis of average altitude, pressure and temperature conditions at 40° north latitude.

Figure 3 ICAO standard atmosphere. Pressure falls with height, particularly in the lower altitudes.

Some of the values of particular significance are:

• mean sea level (MSL) pressure: 1013.25 hectopascals (hPa or mb)
• mean sea level temperature: 15 ° C
• rate of decrease of temperature with height (lapse rate) in the troposphere: 6.5° C per kilometre (1.98° C per 1000 feet)
• height of the tropopause: 11 kilometres (36,000 feet) above mean sea level
• temperature of the tropopause: -56.5° C
• temperature is constant from the tropopause to the top of the standard atmosphere.