The Productiveness of Fog Precipitation in Relation 
to the Cloud Droplet Spectrum 
JOHANNES GRUNOW 
Deutscher Wetterdienst, Meteorological Observatory Hohenpeissenberg, Germany 
Abstract—On Mount Hohenpeissenberg in Upper Bavaria (975 m NN) the atmos- 
pheric offer of fog precipitation is measured by a cylindric net of wires of 0.1 mm diam- 
eter, for which a nearly constant relation of deposit amount to wind velocity is found. 
The amount of deposited fog precipitation depends on (1) locality and exposition of 
the gage, and (2) weather situation. The efficiency of polar cold air, characterized by 
dominating small diameters of cloud droplets from 2 to 15 u, is seanty. Increasing pro- 
ductiveness results, when maritime warm air masses from temperate or subtropical 
zones pass. The cloud droplet spectrum is then characterized by a broader range of 4 to 
25 w diameter, with a maximum frequency from 8 to 14 uw. The deposits are heaviest 
with amounts of 2 to 3 mm/hr when persistent cloud decks form on the windward side 
of the Alps. The air masses have then often degenerated by continental influence. The 
droplet spectrum indicates a wide range from 5 to 60 » with a maximal frequent diame- 
ter of 12 to 18 uw. 
Introduction—Cloud air streaming against an 
obstacle precipitates a part of cloud droplets. 
The deposit on trees is known as fog drip or in 
freezing weather is visible as rime. In flat land 
the fog deposits are just sufficient for wetting 
needles and leafage. But in mountain regions, 
which rise at times into the cloud space, and 
in coastal mist belts, where moist-warm air 
passes from sea to land, considerable amounts of 
additional water by fog drip can be expected. 
Measuring method—The atmospheric poten- 
tial of fog precipitation can be comparably meas- 
ured with a specific fog gage as suggested by 
Tabata and coworkers [1953] for the research of 
sea fog on the coast of Hokkaido, Japan, and by 
the author [Grunow, 1952] for studying the pro- 
ductiveness of fog precipitation in mountain 
forests. Both types of fog collectors use the same 
principle, a system of wires. The Japanese pat- 
tern is that of a cylindrical wire screen; the Ger- 
man pattern that of a cylindrical wire net (Fig. 
1). The effect of these gages is found in the theory 
of the dust filter derived by Albrecht. The amount 
M of deposit on cylinder, in case of the fog gages 
on each single wire, is given by 
M = FvWtC 
It depends on the diameter and length of the 
wires through the factor F’, velocity of wind v, 
water content of fog W, time of deposit t, and an 
efficiency factor C which is influenced by the 
diameter of the wire, the diameter of droplets, 
and the wind velocity. The most favorable re- 
Fic. 1—View of the Hohenpeissenberg fog- 
collector; the cylindrical wire-net is mounted on 
a normal rain gage 10 cm in diameter 
110 
