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Concept
Lapse rate
Résumé
The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. Lapse rate arises from the word lapse, in the sense of a gradual fall. In dry air, the adiabatic lapse rate is 9.8 °C/km (5.4 °F per 1,000 ft). At Saturated Air Lapse Rate (SALR), where value is 1.1 °C/1000ft - 2.8 °C/1000ft as obtained from ICAO.
It corresponds to the vertical component of the spatial gradient of temperature.
Although this concept is most often applied to the Earth's troposphere, it can be extended to any gravitationally supported parcel of gas.
A formal definition from the Glossary of Meteorology is:
The decrease of an atmospheric variable with height, the variable being temperature unless otherwise specified.
Typically, the lapse rate is the negative of the rate of temperature change with altitude change:
where (sometimes ) is the lapse rate given in units of temperature divided by units of altitude, T is temperature, and z is altitude.
The temperature profile of the atmosphere is a result of an interaction between thermal conduction, thermal radiation, and natural convection. Sunlight hits the surface of the earth (land and sea) and heats them. They then heat the air above the surface. If radiation were the only way to transfer energy from the ground to space, the greenhouse effect of gases in the atmosphere would keep the ground at roughly .
However, when air is hot, it tends to expand, which lowers its density. Thus, hot air tends to rise and carry internal energy upward. This is the process of convection. Vertical convective motion stops when a parcel of air at a given altitude has the same density as the other air at the same elevation.
When a parcel of air expands, it pushes on the air around it, doing thermodynamic work. An expansion or contraction of an air parcel without inward or outward heat transfer is an adiabatic process. Air has low thermal conductivity, and the bodies of air involved are very large, so transfer of heat by conduction is negligibly small.
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