Standard atmosphere

See also: Condition

This article treats standard atmosphere . The Temperature and the pressure of the Air vary from one point to another of the Earth, and according to time. However, these values have a great importance in many chemical processes and Physique S, in particular with regard to measurements. It is thus necessary to define “normal conditions of temperature and pressure” (CNTP), the “normal” term returning to “Norme” (arbitrary value of reference accepted by consensus), and not “usual”. One also speaks about “temperature and pressure normals” (TPN). Many values are given for these conditions.

One also speaks about “environmental conditions”. The “ambient” term is ambiguous, since the “usual” temperature depends on the climate and the season. It is thus necessary also to define the concept of “ambient condition of temperature and pressure”.

This brings to the more general definition of “standard atmosphere”. Indeed, the temperature and the air pressure vary according to the position on the sphere, of altitude and the moment (season, hour of the day, local conditions of Météorologie,…). It is thus useful to define “normal” values of pressure and temperature according to altitude.

Chemistry

In Chemistry, one meets two pressures of reference:

the pressure standard , noted p °: 1 bar = 10⁵ Pa ≈ 14,504 Psi, recommended by IUPAC;
the normal pressure : 1 atm = 760 mm Hg = 1,013 25·10⁵ Pa ≈ 14,696 psi, who is not used much any more .

There is no standard temperature. It is thus advisable always to specify the temperature considered.

The expression “Normal conditions of temperature and pressure” (CNTP) specifies a Température of 0 °C (273,15 K) and a Pression of 1 atm (definite as being 101,325 kPa, or 1,013 25 bar). One can then determine the volume which a mole of a perfect gas will occupy under conditions CNTP by using the law of perfect gases: 22,4 l/mol.

The expression “Conditions environmental of temperature and pressure” (CATP) specifies a temperature of 25°C (298,15 K) and a pressure of 100 kPa (1 bar). The “room temperature” is of 25 °C; if the pressure is not specified, a pressure of 100 kPa is implicitly supposed. Although there exist variations with this definition, most common is the temperature and the pressure to which the constant of balance of car-ionization of the Eau is 1,0·10^-14.

Standard atmospheres

When one is interested in notable variations of altitude, it is necessary to define values standardized according to altitude; they are the standard atmospheres.

The Terrestrial atmosphere is subjected to many variations of Température and Pression. Although the values vary according to the moment and from the position on the extremely complex sphere of manner (because of many parameters, like the relief, the Ensoleillement, the moisture, the Vent S, the temperature of the current sailors…), one defines however typical values of reference which depend only on altitude.

In a total way:

it is established a thermal Gradient between the ground, whose temperature is controlled by the Géothermie and the sunning (effect of black Corps, Greenhouse effect, effect Albédo), and the space;
it is established a gradient of pressure: the pressure is created by the Poids of the air located above the place considered; it is necessary to take into account the compressibility of the air as well as the variation of the Gravité with the Altitude.

At low altitude, the atmospheric pressure drops by 1 HPa each time one rises of 8 meters, and the temperature lowers approximately 1 °C each time one rises of 100 Mr.

Typical atmosphere ICAO

The International Civil Aviation Organization (ICAO) defines “typical atmosphere ICAO” (ISA for International Standard Atmosphere ) as being (with the sea level) 101,325 kPa, 15°C and 0% of moisture. These values are used to calculate various characteristics of aeronautical performance, such as the endurance, the Range operating, the air Speed and fuel consumption. To refer to a Baro-corrected altitude other than the sea level, the temperature is adjusted according to the adiabatic thermal Gradient prescribed (which is of -6,5°C/km for the first 11 km).

With regard to aeronautics:

With the sea level, the air is with 15 °C and 101,325 kPa;
the Troposphère extends from 0 to 11 km; the temperature decrease linearly of -6,5°C per km, it thus has a temperature of -56,5 °C to the Tropopause;
With the tropopause and low the Stratosphere, between 11 and 20 km of altitude, the temperature is constant and is worth -56,5 °C;
In the average stratosphere, between 20 and 32 km, the air is linearly heated +1°C by km, it thus reaches -44,5 °C to 32 km of altitude.

For the Meteorology, one extrapolates this model for higher altitudes:

In high stratosphere, between 32 and 47 km of altitude, the temperature grows linearly of +2,8°C by km, reaching -2,5°C to 47 km;
In the Stratopause, from 47 to 51 km, the temperature remains constant with -2,5°C.

For the Aéronomie, one extends this model until the Mésopause, to 85 km of altitude: the temperature decrease linearly and reaches -90°C at this altitude.

Table of values

The following values are obtained:

Atmosphere Standard Army Subway

The atmosphere Standard Army Subway , used in Ballistic, defines the conditions in the sea level as being 29,5275 mm Hg of pressure (99,9916 kPa), 59 °F (15°C), and 78% of moisture.

See too

Formula of barometric levelling
Barometer
standard State
Thermodynamic Balance
Téphigramme

Sources

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