In most cases, fog formation occurs because the air temperature reaches the dew point as a result of a negative radiation balance. This leads to condensation, which manifests itself as fog. The negative radiation balance of the ground surface causes it to cool. In turn, sensible heat flow means that the air layers closest to the ground cool down the most, and fog formation therefore always begins close to the ground in such cases. Accordingly, “ground fog” is formed first (see image below).
This then expands into fog banks, which increasingly merge into more extensive areas of fog and ultimately lead to widespread fog. At the same time, the thickness of the fog increases. This is because the fog layer is practically opaque to long-wave radiation. Accordingly, in clear weather conditions, the greatest cooling occurs at the surface of the ground. In the case of fog, however, the greatest cooling occurs at the fog surface. In turn, this means that the temperature is lowest at the upper surface of the fog due to its strongly negative radiation balance, and turbulence begins to occur within the fog layer. The fog layer therefore undergoes moist adiabatic mixing.
Fog formation during the night due to negative radiation balance
Formation of ground fog in the evening
In the evening, a strong ground inversion develops due to the negative radiation balance (blue arrow). The red line gives a qualitative indication of how the temperature increases with altitude. At some point, the dew point is reached, leading to condensation and therefore the formation of “ground fog”.
Growth of fog layer at night
The ground fog becomes denser as a result. Accordingly, the level of the negative radiation balance moves to the upper surface of the fog, and the fog layer begins to undergo moist adiabatic mixing. The temperature therefore rises close to the ground and falls at the upper surface of the fog (red line).
Thick fog layer in the morning
As the night progresses, the fog layer continues to grow due to the ongoing negative radiation balance at its upper surface. As the volume of fog continues to grow, however, the growth slows down. The temperature is lowest at the fog top and increases (moist adiabatically) towards the ground surface at a rate of around 0.6 degrees/100 m.