In the troposphere, the temperature generally decreases with the altitude. The reason is that the troposphere’s gases absorb very little of the incoming solar radiation. Instead, the ground absorbs this radiation and then heats the tropospheric air by conduction and convection. Since this heating is most effective near the ground, the temperature in the troposphere gradually decreases with the increasing altitude, until the tropopause is reached.

The higher we go away from the surface, the thinner the atmosphere gets. The total heat content of a system is directly related to the amount of matter present, so it is cooler at the higher elevations.