Standard atmosphere
The standard atmosphere , normal atmosphere or standard atmosphere is a term from aviation and describes idealized properties of the earth's atmosphere .
The International Standard Atmosphere (Engl. International Standard Atmosphere , ISA) is from the International Civil Aviation Organization has been defined (ICAO). It represents an atmosphere in which the parameters air pressure, air temperature, air humidity and temperature decrease per 100 m altitude have values that are roughly equal to the mean values prevailing on earth. The international standard atmosphere thus roughly corresponds to the pressure and temperature conditions (15 ° C and 1013.25 hPa ) prevailing in mid-latitudes of 40 ° north .
The aim is to create an internationally uniform reference value and not to provide an exact description of the current, local atmosphere. The international standard atmosphere corresponds to the US standard atmosphere in 1976 up to an altitude of 32 km . Previously, a standard known as the International Standard Atmosphere was used. In Germany there was also the DIN 5450 standard atmosphere , in 1975 the standard atmosphere was specified in DIN ISO 2533.
application
aviation
The defined properties are z. B. needed to correct the performance values of an engine run in a test stand and to be able to make a statement about whether this engine produces enough thrust to be able to accelerate an aircraft sufficiently on the runway .
The use of a standard atmosphere is necessary to calculate the performance data of aircraft, engines and rockets. It is also used to calibrate pressure measuring devices such as B. Altimeter and Speedometer.
The following conditions apply to these at medium sea level:
parameter | metric system of measurement | imperial system of measurement |
---|---|---|
Humidity / H _{2} O | 0 rel. % | 0 |
Pressure p _{0} | 1013.25 hPa | 29.92 inHg |
Density ρ _{0} | 1.225 kg / m ^{3} | 0.002378 slug / ft ^{3} |
Temperature T _{0} | 15 ° C / 288.15 K | 59 ° F / 518.67 ° R |
Speed of sound a _{0} | 340 m / s | 1116.4 ft / s |
Gravity | 9.80665 m / s ^{2} | 32.174 ft / s ^{2} |
The classification of the atmosphere is also described. The following applies here:
- The temperature gradient from sea level to the tropopause is −6.5 K / 1,000 m (−3,564 ° F / 1,000 ft).
- The tropopause is at an altitude of 11,000 m (36,089 ft).
- In the stratosphere there is a constant temperature of −56.5 ° C (216.65 K; -69.7 ° F)
The temperature at an altitude below the tropopause can be calculated using the following formula:
Metric system:
Imperial system:
The unit of height (meter or foot) is shortened with the unit of length of the temperature gradient or results in an additional conversion factor.
Terrestrial refraction
Similar models
US standard atmosphere 1976
The ICAO (International Civil Aviation Organization) defined a generally applicable and binding standard atmosphere for aviation in 1976 with ICAO document 7488. This replaces a previous version from 1962. The following are defined as uniform conditions at sea level that correspond to an average level of moderate latitudes:
- Temperature T _{0} : 288.15 K (corresponds to 15 ° C)
- Air pressure p _{0} : 1013.25 hPa
The temperature profile with the height is defined according to the following table, with linear interpolation between the explicitly defined levels. The top level is also the upper limit of this model.
In addition, u. a. the following values are defined as constants:
geopot. Height h in m |
geometr. Height z in m |
Temperature T in ° C |
Air pressure p in Pa |
---|---|---|---|
0 | 0 | 15th | 101,325 |
11,000 | 11,019 | −56.5 | 22,632 |
20,000 | 20,063 | −56.5 | 5,474.9 |
32,000 | 32,162 | −44.5 | 868.02 |
47,000 | 47,350 | −2.5 | 110.91 |
51,000 | 51,413 | −2.5 | 66.939 |
71,000 | 71,802 | −58.5 | 3.9564 |
84,852 | 86,000 | −86.2 | 0.3734 |
- Gravitational acceleration g _{0} : 9.80665 m / s² (corresponds roughly to the real value at 45 ° latitude at sea level)
- Earth radius R _{earth} : 6 356 766 m (smaller than the true mean earth radius, in order to take into account the somewhat steeper decrease in gravitational acceleration due to the centrifugal force of the rotating earth when converting from geopotential to geometric heights )
- Molar mass M of the air: 28.9644 g / mol (the standard atmosphere contains no water vapor )
- Universal gas constant R * : 8.31432 J / (mol K)
The air density at sea level is calculated from this at 1.225 kg / m³.
It is assumed that the air fulfills certain principles in an ideal way, in particular the general gas equation . This allows the altitude-dependent air pressure to be calculated. This gives the international barometric altitude formula for the bottom layer . The barometric height formula with a linear temperature profile results for layers with a linear temperature profile .
The standard atmosphere works with standard pressure areas of the geopotential heights , since the acceleration due to gravity is assumed to be independent of altitude. At low altitudes these correspond pretty well with the usual geometric heights, but for greater accuracy at higher altitudes, instead of the geometric height z, an imaginary homogeneous gravitational field with the corresponding geopotential height h must be used. The relationship between the two is by the equation
given.
Jacchia reference atmosphere
The Jacchia reference atmosphere, which is particularly used in space travel , describes an atmosphere model that is defined for altitudes from 90 to 2,500 km for atmospheric values such as temperature, density, pressure and other values. In contrast to the international standard atmosphere, different values depending on latitude and season as well as geomagnetic and solar effects are also taken into account. In addition, another atmosphere model must be used for lower altitudes.
The Jacchia reference atmosphere model was first published in 1970 by Luigi Giuseppe Jacchia and updated in 1971 and 1977. It is based on air resistance measurement data from space flights and is mainly used for space travel.
NRLMSISE-00 model
The newer NRLMSISE-00 model from 2000 provides reference values for heights from the surface of the earth to space.
See also
Individual evidence
- ↑ ( Page no longer available , search in web archives: ICAO original document 7488, 3rd edition from 1993 (up to 80 km altitude) )
- ↑ ICAO standard atmosphere 1976 ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 89 kB) Archived copy ( memento of the original from September 16, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 480 kB)