The air is the mixture of Gaz constituting the terrestrial atmosphere .

Because of reduction in the Pressure of the air with altitude, it is necessary to pressurize the cabins of the Avion S and others Aéronef S. In practice, the pressure imposed in the cabins is higher than the external pressure, although less than the pressure on the level of the ground.

The compressed air is often used in the Plongée underwater.

Composition of the air

The dry air is composed of approximately

• 78% of Diazote in volume,
• 21% of Dioxygène in volume,
• 1% of other gases of which:

- the rare Gas S (the Argon mainly),

- and other gases Carbon dioxide…)

At the temperature of 40°C, the air can contain from 0 to 7% of vapor of Eau. The proportion of steam depends on the rate of Hygrométrie of the air and its temperature. It is limited by the Steam pressure saturating with the Eau. The meteorologists are interested closely in the variations of this component in the atmosphere.

The rate of Carbon dioxide varies with time. On the one hand, it undergoes an annual variation from approximately 6,5 PMV of amplitude. In addition, annual average rate increases by 1,2 to 1,4 PMV per annum. About 378 PMV (0,0378%) at the end of 2004, it was of 278 PMV at the time preindustrial, 315 PMV into 1958,330 PMV in 1974 and 353 PMV in 1990. This Gaz with greenhouse effect plays a big role in the Climate warming of planet.

The Méthane is another Gaz with major greenhouse effect whose rate increases with time: 800 mm³/m³ (0,8 PMV) at the time preindustrial, 1585 mm³/m³ in 1985,1663 mm³/m³ in 1992 and 1676 mm³/m³ in 1996.

The composition of the air varies with altitude. It also changes at the time of the Respiration: the exhaled air is richer in water and carbon dioxide that the inhaled air.

1 ppm (left per million) = 0,0001%

The mass proportions can be evaluated roughly by multiplying the voluminal proportions by the report/ratio of the molar mass of gas considered divided by the molar mass of the air is approximately 28,95 G, for example in the case of CO2 this report/ratio is not negligible since it is worth 44/28,95 = 1,52 of or the mass content CO2 in the air equal to 382 * 1,52 = 580 PMV

Density

The air being a compressible gas, its Density (in kg/m³) is function of the pressure, the temperature and the water content.

For dry air under normal atmospheric pressure:

One generally takes 1,293 with 0°C and 1,204 with 20°C.

This is generalized in: $\backslash \; rho=1,293\; \backslash \; cdot\; \backslash \; frac\; \{273\}\; \{273+T\}$ with T in °C.

Potential of total warming

The potential of total warming (PRG, Total GWP Warming Potential in English) or CO₂ equivalent makes it possible to measure the harmfulness of each Gaz to greenhouse effect. For the Carbon dioxide, it is worth 1 (reference), it is of 23 for the Méthane, 310 for nitrogen peroxide (N₂O), from 6200 to 7100 for the Dichlorodifluorométhane (CFC), of 1300 to 1400 for chlorodifluorométhane (HCFC), 6500 for the carbon 2 tetrafluoride (CF₄), 6500 for sulfur hexafluoride (SF₆).

The index of refraction of the air

The expression for the index of refraction of air “in the standard conditions” is:

$n\_s\; =\; 1\; +\; 643,28\; \backslash ;\; 10^\; \{-\; 7\}\; +\; \backslash \; frac\; \{294981\; \backslash ;\; 10^\; \{-\; 7\}\}\; \{(146\; -\; \backslash \; sigma^2)\}\; +\; \backslash \; frac\; \{2554\; \backslash ;\; 10^\; \{-\; 7\}\}\; \{(41\; -\; \backslash \; sigma^2)\}$

$\backslash \; sigma\; =\; \backslash \; frac\; \{1000\}\; \{\backslash \; lambda\}$ where $\backslash \; lambda$ is the wavelength expressed out of nanometers (nm).

where $\backslash \; sigma$ is reciprocal wavelength out of micrometers.

It is for the dry air with carbon dioxide 0,03%, with a pressure of 101325 Pa (760 millimetres of mercury) and of a temperature of 288.15 Kelvin (15°C).

To modify the index “n_s” for a different temperature or pressure, by using one or the other of the following expressions:

$n\; =\; 1\; +\; (n\_s\; -\; 1)\; \backslash \; times\; \{(\backslash \; frac\; \{p\}\; \{p\_s\})\; \backslash \; times\; \{(\backslash \; frac\; \{T\_s\}\; \{T\})\}\}$

with:

• T, temperature in Kelvin
• p, pressure in Pascals
• T_s, 288,15 K
• p_s, 101325 Pa
• n_s, index of refraction of air given above

or:

$n\; =\; 1\; +\; (n\_s-1)\; \backslash \; times\; p\; \backslash \; times\; 1\; +\; p\; \backslash \; times\; \backslash \; beta\_\; \{(T)\}\; \backslash \; times\; (1+\; 15\; \backslash \; times\; \backslash \; alpha)/\{760\; \backslash \; times\; (1\; +\; 760\; \backslash \; times\; \backslash \; beta\_\; \{15\})\; \backslash \; times\; (1\; +\; T\; \backslash \; times\; \backslash \; alpha)\}$

with:

• T, temperature in degrees c
• T_s, 15 degrees c
• p, pressure in mm of mercury
• p_s, 760 mm
• $\backslash \; alpha~$, 0,00366
• $\backslash \; beta\_\; \{(T)\}~$, (1,049 - 0,015*T) *1.e-6
• $\backslash \; beta\_\; \{15\}\; ~$, 8,13e-7
• n_s, index of refraction of air given above

image of the curve of index (N)

Thermophysical properties

According to the tables published by Frank Mr. White, " Heat and Farmhouse transfer" , Addison-Wesley, 1988.

with:

• T, temperature in Kelvin;
• ρ, Density;
• μ, dynamic Viscosity;
• ν, kinematic Viscosity;
• C_p, specific heat with constant pressure;
• λ, thermal Conductivity;
• has, thermal Diffusivité;
• Pr, Number of Prandtl.

Liquefaction of the air

The air is formed various gases and those, if they sufficiently are cooled, end up passing in the liquid state, then at the solid state. For example, oxygen becomes solid at the temperature of -218°C, the nitrogen with -195°C and helium becomes solid with - 270°C.This temperature, all the gases are then solid and one obtains from “the frozen air”… The air could not be liquefied before are not known the critical pressures and temperatures which mark the theoretical limits beyond whose a compound can exist only in a gas state. The air being a mixture, these values do not have a strict direction; but, in fact, with an higher temperature with - 140 °C, the air is not liquefiable any more.

The first drops of air Liquide were obtained almost simultaneously by Louis Paul Cailletet and Raoul Pierre Pictet in 1877, by brutal relaxation between 300 and 1 atmosphere. In 1894, the Dutch physicist Heike Kamerlingh Onnes developed the first installation of liquid air. During the forty years which followed, of the researchers in France, Great Britain, Germany and Russia many improvements brought to the process. Sir James Dewar, liquefied initially the Hydrogène, in 1898, and Kamerlingh Onnes the Hélium, the gas most difficult to liquefy, in 1908. Independently of Carl von Linde, Georges Claude develops since 1902 an industrial process of liquefaction of the air (the patents which it takes on this occasion are at the origin of the company the Liquid air) and recommends since 1910, but in vain, the use of liquid oxygen in iron and steel industry. Claude discovers in 1913, with Arsonval the explosive properties of the liquid air, which will be used during the First World War (mines with the liquid air and the lampblack).

Others

• In a nonscientific field, the air is one of the Four elements (with the fire, the Eau and the Ground) which one considered formerly (and that one still considers in certain cultures) as the substances on which would be based all the life. The air is also often associated with different other concepts such as the family from the swords in the plays of Tarot S.

See too

• Mass of air

• Psychrometry
• Humid air
• Air pollution and articles of the category: : Category: Air pollution

Nds-nl: Luch Simple: Air

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