178 
increased with rainfall intensity. For a rainfall rate of 
0.05 in. hr they give a volume-median diameter of 1.1 
mm with a maximum diameter of 4 mm; for 0.5 m. hr+ 
the volume-median diameter was 1.9 mm and the maxi- 
mum diameter was 5.5 mm; for 4.0 in. hr the volume- 
median diameter was 2.8 mm and the maximum di- 
ameter was 6.7 mm. The size and size distribution of 
raindrops can be expected to change from the cloud 
base to the ground because of evaporation, coalescence of 
drops, the variation of time of fall with drop size, and 
the fracture of large drops. For studies of the precipita- 
tion process it would be necessary to measure the rain- 
drop size in and immediately below the cloud. 
Hail. Hail is a special type of frozen precipitation 
associated with thunderstorms and characterized by 
extreme sizes much in excess of those of any other 
precipitation elements. A typical hailstone exhibits an 
onionlike structure when dissected, which has given 
rise to the belief that hailstones are formed by repeated 
excursions above and below the freezing level whereby 
successive layers of water are frozen onto it. A natural 
consequence of this theory was the inference that verti- 
cal velocities equalling the free-fall velocities of the 
hailstones exist in the atmosphere (over 100 mph in 
some cases). 
It is now generally accepted that the major growth 
of the hailstone is by the collection of supercooled water 
as the stone falls relative to the cloud. In the earlier 
stages the stone may well travel in a very irregular 
fashion, up as well as down, but the largest hailstones 
can hardly be supported by updrafts. The growth of 
the hailstone is essentially the same as the accretion of 
ice on aircraft. The layer structure is due to inhomo- 
geneities in the turbulent cloud. A similar layer struc- 
ture is commonly observed in ice deposits on aircraft. 
Large hailstones are favored by high vertical velocities, 
a large vertical extent of the supercooled portion of 
the cloud, and high liquid-water content. Schumann 
[47] has shown that the extreme values of these param- 
eters associated with cumulonimbus can lead to hail- 
stones of the observed sizes. Hailstones large enough 
.to damage aircraft have been reported in the clear air 
surrounding a thunderstorm, suggesting that stones for 
example, of one or two centimeters diameter may be 
discharged from the tops of thunderclouds. The termi- 
nal velocities of hailstones of 1 and 2 em diameter are 
about 12 and 16 m sec respectively. Extreme thunder- 
storm updrafts may readily exceed these velocities. 
Regardless of their direction of travel with respect to 
the earth, the hailstones are moving downward through 
the supercooled water drops at their terminal velocities. 
The thunderstorm updraft serves primarily to increase 
‘the length of the hailstone’s path through the cloud 
and thereby to increase its collection of ice. 
ARTIFICAL DISSIPATION OF FOG 
Although fog hampers all types of transportation, its 
effects on air transportation are most serious. Much of 
the impetus for the development of methods for arti- 
ficial dissipation of fog has come from aviation interests. 
Many unsuccessful attempts have been made to dis- 
CLOUD PHYSICS 
sipate fog, failure being due in most cases to a lack of 
understanding of the problem. Some successful experi- 
ments were not followed up because it was believed 
that instrument-landing systems would make fog dissi- 
pation unnecessary. With the advent of World War II 
the problem became acute, and the British developed 
a thermal method now called ‘‘Fido.” This system has 
been developed further, since the war, both in England 
and in the United States where an operational installa- 
tion has recently been made at a commercial air field. 
In spite of contmued advances in instrument-landing 
systems, most operators still feel that a cleared region 
for the final touchdown would greatly increase the 
safety of the landing. It seems evident that instrument- 
landing techniques will eventually be developed to the 
point where fog dissipation is completely unnecessary, 
but the writer would not care to speculate as to when 
this will come to pass. 
In general, fog can be dispelled by the evaporation of 
the drops or by the physical removal of the drops from 
the air. Most of the methods for accomplishing this 
have been discussed critically by Houghton and Rad- 
ford [23]. Those methods which were considered reason- 
ably feasible were (1) the direct application of heat, (2) 
the use of hygroscopic materials to ‘‘dry” the air, and 
(3) the dropping of electrically-charged particles 
through the fog. The first method is exemplified by - 
“Fido,” in which heat is released by the burning of oil 
in long lines of burners on either side of the runway. 
The second method was used successfully by Houghton 
and Radford. The third method was experimented with 
by Warren, who dropped charged sand on clouds with 
occasional success. 
There is now no doubt that fog can be dispelled by 
artificial means. Further experimentation with methods 
already proved practicable is desirable and there is still 
room for new ideas in both methods and equipment. 
The basic problem lies in the economies of fog dispersal. 
The mass of suspended water to be dealt with in even a 
relatively small volume is large, and it is mevitable that 
relatively large expenditures of energy will be required 
to remove it. As an example, the mass of water over an 
airport runway 6000 ft long and 300 ft wide to a height 
of 200 ft is about one to two tons, depending on the 
liquid-water content of the fog. In order to dissipate the 
fog by evaporation it is necessary both to supply the 
latent heat of vaporization and to lower the relative 
humidity of the air to cause the drops to evaporate 
rapidly. It is generally necessary to reduce the relative 
humidity to 90-95 per cent to meet the latter require- 
ment. The heat energy required to evaporate the water- 
drops and to reduce the relative humidity to 90 per 
cent in a fog at 10C, containing 0.1 g m~ of liquid 
water, is about 559 cal m7’. Of this amount, nearly 500 
cal m~? are required to reduce the relative humidity of 
the air. 
If we use the figures above, a total of some 5.7 x 10° 
cal would be required to clear the fog in the volume 
assumed above. This rather impressive number of calo- 
ries can be supplied by burning the modest amount of 
250 gal of oil at a combustion efficiency of about 70 
