Temperature and Altitude
Adapted from H & W Figure
Little Mass to Absorb Radiation
This diagram shows a typical profile of atmospheric temperature as a function of height in mid latitudes. Most striking is the decrease with height throughout the region known as the troposphere, which extends from the surface up to the tropopause, which in this diagram is at 10 km or 33,000’, around cruising altitude for modern jet aircraft.
Temperature is here measured in degrees Kelvin (K), which are degrees C + 273. Scientists use this transformation to the so-called absolute scale of temperature because 0 degrees is then the temperature at which the random motion of the molecules that we know as heat stops completely. The surface temperature is then 288 K or 15 oC, whereas at 10 km it is 223 K or -50 oC, a gradient of -6.5 oC/km (degrees per kilometer)..
This gradient arises because the air in the troposphere is frequently overturning, to allow the transfer by convection of the heat from absorbed solar radiation upwards from the surface to the ERL in the upper troposphere. Neither conduction nor infra-red radiation are efficient at transmitting heat through the lower troposphere, so the surface and the air in touch with it get steadily warmer until such air is less dense than the air above and overturns. Whereas in water this would lead to a uniform temperature, in the atmosphere it leads to a constant temperature gradient. As air rises there is less weight of air above it, and hence a lower pressure. In response, air expands, doing work against the remaining pressure of its surroundings and hence cooling to conserve its energy.