ON THE PHYSICS OF CLOUDS AND PRECIPITATION 
per cent. The computed energy requirement is for 
optimum conditions which cannot be realized in prac- 
tice. Hven a light wind steadily brings in more fog, 
so that the heating must be continuous. It is not 
practicable to apply the heat uniformly, and in practice 
much of the heat is wasted in raising the temperature 
of some of the air much higher than is necessary. The 
concentrated heat sources produce convection currents 
which may carry the heat to unwanted heights and 
suck in additional fog from the sides. For these reasons 
the practical energy requirements are many times the 
computed minimum. Working installations are designed 
to burn on the order of 100,000 gal of oil per hour. 
Methods utilizing hygroscopic materials to dissipate 
fog are also evaporative processes and the basic energy 
requirements are the same as for the heating methods. 
Experiments reported by Houghton and Radford [23] 
indicate that operating systems require from five to ten 
times the theoretical minimum quantities of hygro- 
scopic material. 
Methods of physical removal of the fog drops, such 
as the charged-sand process, have smaller theoretical 
energy requirements than the evaporation methods. 
No basic energy requirement comparable to that given 
for the evaporative methods can be set and no experi- 
mental values are available. 
The two evaporative methods which have been sub- 
jected to full-scale tests, namely the burning oil and 
hygroscopic material methods, are demonstrably ca- 
pable of producing clearings of useful size. In both 
cases the costs of operation are relatively high and 
extensive installations are required. The use of hygro- 
scopic materials involves the hazards of corrosion and 
damage to electrical equipment, although these may 
be nearly eliminated by proper design. The existence 
of large oil burners along the sides of the runway is a 
potential hazard which can also be minimized by proper 
design. Because of cost and other practical considera- 
tions only limited clearings are feasible, so that auxil- 
iary instrumental methods must be available to guide 
the aircraft into the clearing. Neither method can deal 
with other conditions of poor visibility, such as dense 
snow, smoke, and dust. It must be concluded that fog 
dissipation by these methods is economically marginal 
and that installations are justifiable only in locations of 
extreme fog frequency or for urgent military purposes. 
The fact that all proved methods of fog dissipation 
require much more energy than the theoretical mini- 
mum offers some hope that more efficient methods can 
be found. There is certainly room for further work on 
methods such as the electrified-sand technique where 
there is reason to believe that the energy requirements 
are more modest. The ever-recurrent hope that a 
method will be found which will clear large areas of 
fog with the expenditure of small amounts of energy is 
incompatible with physical reality. 
CONCLUSIONS AND RECOMMENDATIONS 
Our present understanding of the physics of con- 
densation and precipitation is incomplete in many im- 
portant areas. The writer has attempted to point out 
179 
deficiencies in each phase of the subject during the 
detailed discussion. It is hoped that this will be of value 
to the reader, but it is felt that a more general ap- 
praisal of the situation is in order. In particular, an 
attempt will be made to present the writer’s views as to 
the relative importance of those phases of the subject 
which need further study. This requires a decision as to 
the most important contribution to meteorology as a 
whole which can be expected from the study of cloud 
physics. The author feels that the complete explanation 
of precipitation should be the dominant aim of the 
cloud physicist. Of the elements forecast, precipitation 
is probably the most important to the majority of 
people. Further, the complete explanation of the pre- 
cipitation process involves a knowledge of most of the 
topics in cloud physics. In making this decision the 
writer is acutely aware of the possibility that some new 
discovery will necessitate a refocusing of the entire 
cloud physics program. 
Knowledge of condensation nuclei and of condensa- 
tion in the liquid phase is incomplete in detail but is 
relatively satisfactory as compared to other parts of 
the field. The most important problem in this area is 
the investigation of the factors determining the breadth 
of the drop-size distribution. Knowledge of the ice 
phase in the atmosphere is inadequate. It is imperative 
that the nature and mode of action of freezing nuclei 
and sublimation nuclei be determined. This information 
is essential to an evaluation of and practical utilization 
of the ice-crystal theory of precipitation. It is equally 
imperative that a complete study be made of the 
growth of drops by collision in the gravitational field. 
It is the writer’s opinion that these problems should 
be attacked experimentally both in the laboratory and 
in the free atmosphere. The mechanisms of phase 
changes can be studied only in the laboratory, and the 
collision process is also a proper subject for laboratory 
investigation. However, the most complete laboratory 
study will not tell us what is happening in the atmos- 
phere. It is therefore essential that fight measurements 
be made to determine which precipitation process oper- 
ates under various conditions and to obtain a quantita- 
tive verification of the operation of the assumed proc- 
esses. No adequate instrumentation is available for 
flight observations, and instrument development is 
therefore an essential part of the program. Flight meas- 
urements are extremely costly and time-consuming and 
should not be embarked on without careful planning 
and adequate instrumentation. 
REFERENCES 
1. ApEn, A., ‘““Note on the Atmospheric Oxides of Nitrogen.” 
Astrophys. J., 90: 627 (1939). 
2. ArrKEN, J., Collected Scientific Papers. Cambridge, Uni- 
versity Press, 1923. 
3. ARENBERG, D. L., ‘‘Turbulence as the Major Factor in the 
Growth of Cloud Drops.’’ Bull. Amer. meteor. Soc., 20: 
444-448 (1939). 
4, Brrcrron, T., ‘On the Physics of Cloud and Precipita- 
tion.” P. V. Météor. Un. géod. géophys. int., Pt. 2, pp. 
156-178 (1935). 
