Heat transfer between ground and air

Heat is transferred through solid objects by conduction. For instance, if you heat one end of a solid rod in a fire, the other end becomes warm as heat is transferred by vibration of molecules in the rod.

Although molecules in a solid have limited movement, molecules in a liquid or gas can move over much greater distances in the process called convection. While purely random movement of individual molecules would take a long time to transfer heat, what happens in a gas like the atmosphere is that extremely large numbers of molecules have a somewhat co-ordinated movement superimposed on their random individual movement. This is a little like a flock of birds, where the flock moves in a coordinated manner, while within it individuals have their own independent movement.

Such 'parcels' of air containing an almost infinitely great number of molecules are called eddies. Close to the ground they are small, with movement restricted by the underlying surface. But higher up, where winds are stronger, they are larger and move more freely. The resulting increase in turbulence is very effective at transferring heat. Even in calm conditions, parcels of air rise to form the plumes called thermals. Soaring birds and glider pilots exploit these effectively to gain height.

During the day, when the ground is heated by the sun, small scale turbulence transfers heat from the ground to the air that is in contact with it. This warmed air then rises because its density is less, and is replaced by cooler air from above it. Wind turbulence enhances this effect so that surface heat is readily transferred through the air mass. At night a different situation prevails. Ground no longer heated by the sun cools by losing radiant heat to the upper atmosphere and to space, especially if the sky is cloudless. The air, particularly layers near the ground, becomes cold, even though turbulence is now transferring heat from above down to the cooling surface.

Cold air is denser than warm air, so the cold air near the ground sinks to the lowest level it can find, while the warm air remains above it. The wind turbulence now has great difficulty in stirring up the heavier cold air layers near the ground and replacing them with eddies of warm air from higher up. So the cold air becomes even colder, and the wind dies away. This reversal of the normal daytime decrease of temperature with height is called a temperature inversion.