200 
dry-adiabatically, or the parcel may even be cooled by 
radiational heat loss (see below, also p. 203). 
Heat Conduction in the Interior of the Cloud. Cloud 
elements can be heated either by conduction or by 
radiation. Conduction inside a cloud can be neglected 
entirely if the cloud has a homogeneous horizontal 
temperature distribution and a moist-adiabatic or at 
least a uniform lapse rate of temperature. It is known 
through the investigations of Peppler [27] that nimbo- 
stratus clouds frequently have a “laminated” structure, 
in other words, they are characterized by numerous 
small superimposed inversions and isothermal layers. 
If this observation holds true for a closed nimbostratus 
mass and not for the dissolving region to the rear of it 
(as seen from the ground these two conditions cannot be 
distinguished), then the turbulent heat conduction in 
the vertical can lead to deviations from the moist-adi- 
abatic process in the interior of the cloud. However, 
no deviation from a smooth temperature distribution is 
to be expected in the interior of a cumulus cloud. 
Evaporation and Heat Conduction in the Environ- 
ment. The edge of the cloud is a mixing zone between 
the cloud itself and the environment, where, no doubt, 
there are considerable deviations from the ideal moist- 
adiabatic process as a result of the turbulent heat con- 
duction across the horizontal and vertical cloud bound- 
aries. Hvaporation of the droplets into the neighboring 
dry air and the increased water-vapor content of the air 
lead to a loss of heat from the border zones of the cloud, 
which then become colder than the environment. This 
process can be of importance on the edges of cumulus 
clouds and on stratus cloud surfaces which lie under 
inversions. This fact was pointed out by von Bezold 
[4] and later by Robitzsch. 
Radiation Processes. The heat transfer by radiation 
from and to the cloud is just as important as is heat 
conduction. In general, the clouds absorb only a small 
part of the sun’s radiation. About 50 to 70 per cent of 
the incoming radiation is reflected from the upper cloud 
surfaces [14]: most of the remaining 30 to 50 per cent is 
transmitted and only a very small portion is absorbed. 
This absorption is unimportant in comparison with the 
other thermal processes. However, it is the only process 
which shows only a minor decrease from the edge to the 
interior of the cloud; although insignificant, the ab- 
sorption is rather uniformly distributed throughout the 
entire cloud mass [1]. 
The long-wave radiation, however, acts in an en- 
tirely different way. The surface of a cloud can be con- 
sidered as a black body for the wave-length region from 
4 to 100 uw. Even very thin layers of water vapor and 
carbon dioxide in the surrounding air absorb almost 
totally at most of these wave lengths; however, between 
8 and 18 yn, even very thick air layers with an abundance 
of water vapor and carbon dioxide have almost no ab- 
sorption. Therefore, the clouds can produce strong 
surface emissions in this spectral range and lose much 
heat. On the undersurface of the cloud layer where the 
radiation of droplets is opposed by that of the usually 
warmer earth’s surface acting as a black body, the 
clouds receive more heat than they radiate and there- 
CLOUD PHYSICS 
fore become heated. Numerical values for this process 
can be found in the article on long-wave radiation in 
this Compendium.! In any case, these processes do not 
penetrate very deeply into the cloud mass; they influ- 
ence only a very thin surface layer, whose thickness can 
be estimated as about 10 m in the case of a cloud with 
high water content. Although these processes are char- 
acteristic of the surfaces of stratocumulus or cumulus 
clouds, the processes in thin altostratus or cirrus are 
entirely different. Here, the individual heat-radiating 
particles, the ice crystals or water droplets, are so far 
apart from each other that radiation exchange with the 
environment can take place even through thick cloud 
layers. The heat loss upward and the heat gain from 
below therefore take place rather uniformly in the en- 
tire mass of thin, veil-like clouds. Here, a clear-cut, 
moist-adiabatic process that can be defined thermo- 
dynamically is no longer possible, because the heat 
balance of the cloud is considerably influenced by 
radiation. In the lower layers of the troposphere, where 
the clouds have a relatively high water content, the 
rule holds true that the moist-adiabatic process is un- 
disturbed by radiation into some distance from the 
cloud’s edge. In the same way, other surface processes 
(such as evaporation and “outer” heat conduction on 
clouds) cannot have any effect within extensive cloud 
masses or in smaller clouds characterized by large verti- 
cal velocities. Thin and scattered clouds depart even 
further from the moist-adiabatic process. The effect of © 
radiation upon various cloud types will be treated 
below. 
Mixing Processes. It seems to be obvious that an air 
parcel which is lifted moist-adiabatically does not un- 
dergo any mixing with the surroundings, and one is 
easily inclined to take this for granted. However, there 
is no proof of this. In cumulus, the entire cloud mass is 
not uniformly lifted; the parcels in the upper portion of 
the cloud are retarded, whereas in the lower portion new 
masses move up and penetrate the higher layer. It is not 
at all certain in this process that the subsequent parcels 
from below move along the same moist-adiabatics. Prob- 
ably, their equivalent potential temperature is higher, 
and the mixing of the masses disturbs the thermody- 
namics. 
Furthermore, it must be considered that the require- 
ment of the absence of mixing also concerns the liquid 
and the solid constituents of a cloud. In the normal 
pseudo-adiabatic process we assume that all liquid 
water drops out as soon as it is produced. This, however, 
would mean a loss of mass; the ejected mass of liquid 
water takes its part of the entropy with it, that is, the 
process is no longer isentropic. Similarly, one must also 
consider that in a precipitating cloud the crystals and 
the growing water droplets are falling relative to the 
air in the cloud. Relatively speaking, they thus enter 
the individual ascending parcels of moist air from 
above, collect some cloud droplets, and then leave the 
air parcel. All in all, the process is by no means isen- 
tropic. 
1. Consult ‘“Long-Wave Radiation”’ by F. Moller, pp. 34— 
44, 
