Why is it colder at higher altitudes?

A view of clouds rolling over a mountain peak in Deomali, Odisha, Augsut 17, 2020.
| Photo Credit: Mohan Vamsi Somireddi

A: Measurements reveal that, as we go up into the atmosphere, the air temperature decreases with height from the ground up to about 15-20 km in the troposphere. Beyond this, it increases in the stratosphere up to a height of 50 km. Again, it decreases in the mesosphere up to 80 km and finally increases in the ionosphere.

Solar radiation heats the earth’s surface far more than the air through which it travels. The heat at the surface is transported by convective expansion into the atmosphere. An atmosphere conditioned by convective expansion will have relatively cooler temperatures at higher levels. This holds good in a hydrostatically stable atmosphere, under adiabatic conditions (i.e. when entropy is constant), which is true of planetary or stellar atmospheres.

But the real atmosphere is not static. It is dynamic with constant motion. The thermodynamic processes are not adiabatic as radiation from outside the atmosphere enters, and the atmosphere also radiates away the heat into outer space. Still the departures from hydrostatic condition and adiabatic condition are small and the temperature decreases with height.