Abstract
Some results on the drop size distribution of radiation fog near the ground at Asahikawa measured by gravity settling method are presented in Part 1. The concentration of small droplets with radii below 20 microns gradually increases until 10 to 50 minutes after the formation of the fog, then it has nearly constant value. At the base of the thick fog the large droplets with radii larger than 50 microns appear at the later stage of the development of the fog. In the shallow fog, on the other hand, the increase or decrease of the concentration of large and small droplets occurs concurrently.
The influence of air temperature on the structure of fog is also studied. It is found that when the air temperature is above freezing the mean volume radius is 12.5 to 15 microns In the majority of the cases when the air temperature is below -5°C the mean volume radius is about 7.5 microns. In the winter fog the low liquid water contents between 10-4 and 10-2g/m3 are frequently found. It is suggested that in winter fog a large number of very fine particles are present and obscure the visibility.
The measurements by means of a hand operated impactor and water-blue dye coated film reveal that many small droplets of radii less than 2 microns are present frequently in winter fog especially in smog and steam fog. Their results are shown in Part 2. It is further found that the local difference of the structure of fog and the effect of river steam are small in well developed radiation fog.
