1180 
dry, and consist of an unusually large number of very 
fine particles. These fogs are slow to be dissipated and 
very disagreeable to people living in regions where 
they occur. Of course a substantial part of such a fog 
must be solid particles of combustion. 
() 
a oa 
VISIBILITY (MILES) 
ow 
40 50 60 70 80 90 
RELATIVE HUMIDITY (%) 
Fic. 1.—The relation between visibility and relative hu- 
midity at Los Angeles Airport. (Adapted from Neiburger and 
Wuriele [16].) 
100 
Drop-Size Distribution and Liquid-Water Content. 
Among the most important physical characteristics of 
fog are the size distribution and water content of the 
particles comprising it. Although various indirect meth- 
ods which gave some idea of the drop-size distribution 
had been known for a considerable time, it remained 
for Houghton and Radford [13] to perfect a direct 
method for examining these droplets microscopically. 
They found that the maxima of the volume distribution 
curves obtained from forty sets of data taken in sixteen 
fogs (all of an advection nature, since the location was 
near the Atlantic Ocean) ranged from 12 to 90 u. 
An average of about 30 per cent of the liquid water 
contained in the fog fell within a 10-u band centered 
about the peak of the curve. Individual drop diameters 
were measured between the extremes of 2 and 130 up. 
It should be particularly noted that these figures refer 
to volume of water and not to frequency of drop 
diameter. For example, in a given fog the arithmetical 
mean of drop diameters observed was about 12 u while 
the volume mean diameter amounted to 40 yu. 
More recently, Heverly [10] has found droplet diam- 
eters (in fogs probably purely radiational in character) 
markedly less than the figures given by Houghton and 
Radford. In one particular case he cites a range of only 
1.5 to 15 » for drop diameters while in practically every 
case they examined Houghton and Radford found drops 
ranging as high as:60 or 80 yw. Heverly [10] observes, 
Tt appears that coalescence in drop measurements has been 
overlooked or, at least, underestimated by a number of ex- 
perimenters. In one instance, when an especially high den- 
sity of the droplet field was obtained, a great deal of coales- 
cence was microscopically observed. Coalescence took place 
instantly whenever droplets of water in the vaseline came 
into contact with one another. Unless the droplets are sepa- 
CLOUDS, FOG, AND AIRCRAFT ICING 
rated by a distance equivalent to several drop diameters, 
coalescence appears to be a serious factor and is, probably, 
the reason for some of the high medians of fog-droplet sizes 
reported in the literature. 
Neiburger and Wurtele found that the most frequent 
drop diameter for Los Angeles stratus was 14 yu, a value 
which seems to be in agreement with Houghton’s re- 
sults. Furthermore, these investigators found drops up 
to 75 or 80 yu, particularly near the base of the stratus, 
which would presumably correspond with the surface 
measurements in a fog at ground level. This small but 
quite possibly important difference in drop-size meas- 
urements could of course be due to a difference in 
techniques as was suggested by Heverly, but it seems 
more likely that there is a fundamental difference in the 
characteristics of the fogs examined. Houghton’s meas- 
urements were made almost entirely in pure advection 
fog uncontaminated by industrial pollution, while 
Heverly’s apparently were made in the Pennsylvania 
mountains where industrial pollution is strong and 
where fogs are almost entirely radiational in character. 
The liquid-water content of fogs is one phase of the 
physical aspects of the matter which appears to be open 
to little question. Radford’s diagram [13, p. 29], re- 
produced here as Fig. 2, relates the measurements of 
several independent investigators at various locations 
to the horizontal visibility, both on logarithmic scales. 
10.0 
= | : 
7 
Z = rina! 
= ° 
= 
é HERS 
= 10 + {Ht 
wt 
2 
iS aoe! 
oO H+ —— 
[o} 
N 
= — > 
2 
>) 
5° ; = = 
.0| PT 
10 - 100 1000 10000 
HORIZONTAL VISIBILITY (FEET) 
Fic. 2.—Relation between liquid-water content of fogs and 
visibility. (After Radford [13].) 
The fit of the curve is surprisingly good for this type 
of data. It is of interest to note from this curve that 
visibility will be reduced to less than 1000 m, thus 
creating fog, by as small an amount of liquid water as 
0.02 g m=. 
It seems likely that visibility in imdustrial areas 
would be considerably less for corresponding liquid- 
water contents than is shown by this graph. Willett 
[24] remarks, 
