Microstructure of a Radiation Fog

Abstract

Abstract The relative humidity, temperature, droplet size distribution and transmittance of light at 632.8 nm were measured in a radiation fog. A new saturation hygrometer capable of measuring relative humidity between ∼95 and 105%, was used for the first time. Excursions of relative humidity into the supersaturation regime were brief and rapid. Estimated peak values of supersaturation were several tenths of 1%. The mean relative humidity in the fog was <100%. The importance of turbulence in controlling the fog was evident in the rapid fluctuations in all the measured variables, and in the fog's quasi-periodic oscillations which had a mean period of 18 min. Evidence suggested that turbulent mixing of nearly saturated eddies was the cause of fog formation and the broadening of the droplet size distribution.