Abstract
Radiative flux divergence (div R) recordings at heights between 1 and 11 m have been carried out before and in the initial stages of radiation fog. Temperatures, temperature gradients, radiation ground temperature, net radiation, relative humidity and wind speed were also measured.
The general pattern showed essentially either a high div R preceding fog formation (once shortβlived radiative cooling of over 30Β°C hr^β1^ was observed); or a high div R some time before, interrupted by a period of very reduced, or even negative div R (radiative warming), with resumed radiative cooling only immediately before fog formation.
It is reasoned that radiation and, to some degree, temperature phenomena during fog formation can be patchy in character and that some of the results observed on different occasions may actually happen on one and the same occasion. These and other indications suggest smallβscale advection to be important.
The results also demonstrate the important role of haze and suggest its vertical distribution rather than quantity to be the important factor.
The div R observations show the driving character of radiation and can readily explain the dissipation of latent heat of condensation and many observational facts which were difficult to interpret by conventional reasoning, such as the absence of fog under seemingly ideal conditions or the suddenness of its onset. A composite picture of radiation fog formation is given.