Refer to figure.
During a calm and clear winter night, the Earth’s surface loses heat to space through long-wave radiation, a process known as radiative cooling. Because the night is clear, there are no clouds to trap outgoing infrared radiation, and because it is calm, there is little wind to mix the air layers. As a result, the ground cools rapidly, and the air directly in contact with it also cools. The air above, however, does not cool as quickly and remains relatively warmer.

This creates a situation where the temperature near the surface is lower than the temperature at some height above it. Normally, temperature decreases with height in the troposphere, but here it increases with height, which is the hallmark of a temperature inversion. Since this inversion is produced directly by the cooling of the ground, it is specifically called a ground inversion or sometimes a radiation inversion.

Frontal inversion occurs when warm air overrides cold air along a weather front. A turbulence inversion forms from mechanical mixing and turbulence rather than radiative cooling. A subsidence inversion occurs when large-scale sinking air compresses and warms the air aloft, creating a stable layer above cooler air. None of these mechanisms are relevant to a calm, clear night where the surface cools rapidly.