For every kilometer that a parcel of dry air is elevated adiabatically what approximate temperature decreases does it experience?
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When the parcel is raised adiabatically in dry air, the temperature decreases by 10°C/km.

Dry air also has less relative humidity compared to saturated air. Pressure and temperature decrease with increasing altitude. When the parcel of air rise high in the atmosphere through the dry air, its pressure will decrease.  

This means the parcel of the air's temperature at the surface of the earth will be higher than the parcel of air's temperature at a certain distance in the atmosphere. The rate of change of temperature in dry air is 10°C/km. When the parcel descents the temperature increases by the same amount. Thus, the above passage described the "adiabatic lapse rate".