In the lower troposphere of the outer planets, air is heated from the interior. If the infrared (IR) optical depth to space is high, then heat may not escape radiatively and instead the air rises convectively in order to transfer the heat upwards. This region of the atmosphere is called the troposphere (after the Greek word tropo for "turning"). As the air parcels rise they expand and, assuming negligible thermal contact with neighboring air masses, they cool adiabatically to give a temperature profile that may be calculated as follows. Consider a parcel of air moving vertically containing one mole of gas. If we assume that there is no net exchange of heat between the parcel and its surroundings, then the expansion may be considered to be adiabatic and reversible, and hence isentropic: that is,
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