4A RADIATION 
part in the use of the improved radiation diagram, but 
principally in the realization that the water-vapor con- 
tent of the stratosphere is much lower than was formerly 
assumed. For this reason—as mentioned above—the 
main emission level is shifted down to the altitude of the 
tropopause. The importance of reliable measurements 
‘of the water-vapor content of the stratosphere for these 
investigations cannot be over-emphasized, for a higher 
water-vapor content sharply reduces the emission at the 
level of the tropopause. So far only two or three meas- 
urements of the frost pomt have been published. They 
show an unchanged decrease above the tropopause 
[13] sometimes interrupted by thin saturated layers 
[5]; however, we do not know whether, for example, the 
water-vapor content over low-pressure areas is higher 
than shown by these measurements. There is consider- 
able evidence for this supposition, for otherwise how 
should the mother-of-pearl clouds observed in Norway 
develop in the rear portion of cyclones at an altitude of 
28 km, if the relative humidity remains at 1 per cent 
and less from an altitude of 14 km upward? If the 
humidity is greater, the radiation processes of the 
tropopause are reduced considerably. 
It is also very difficult to make any reasonable state- 
ments concerning the distribution with altitude of the 
upper cloud boundaries. It is here assumed that this 
boundary lies between 0 and 2 km in 15 per cent of all 
cases, between 2 and 5 km in 45 per cent, between 5 and 
8 km in 30 per cent, and between 8 and 10 km in 10 per 
cent of all cases. For the lower cloud boundaries, 80 per 
cent are assumed to lie between 0 and 3 km, and 20 
per cent between 3 and 10 km. Consideration of these 
values gives a cooling of the free atmosphere on com- 
pletely overcast days as shown in Fig. 5, curve B, that 
is, there is a “radiation screen” in the lowest layers, 
but above 2 km there is an increase of heat loss of about 
0.8 to 1 degree per day as compared to a cloudless at- 
mosphere. On the whole, however, the difference be- 
tween the cloudy and the clear atmosphere is small. 
The calculation made in 1935 (under the assumption 
of a somewhat different cloud distribution) gave the 
greatest cooling at an altitude of 4 km. This might well 
be taken as an indication of the importance for these 
investigations of more accurate data respecting the 
distribution of the clouds in the atmosphere not only 
on the average, but also for specific weather situations. 
An example from an altogether different climatic 
regime also shows this very clearly. An entirely differ- 
ent temperature and cloud distribution must be as- 
sumed for a winter month at the earth’s cold pole 
situated approximately in northeastern Siberia. On 
cloudless days (50 per cent of all days) the temperature 
at ground level is assumed to be —50C, rising to —25C 
at 1.5 km, and decreasing to —60C at 8-km altitude. 
In the presence of a cloud cover, the temperature be- 
tween 0 and 2 km is taken to be constant at —30C. 
The clouds, which are seldom very massive (there are 
only two days of precipitation per month), are assumed 
to be restricted to the layer between 0.5 and 2 km. 
On clear, as well as on cloudy days, an extremely strong 
cooling layer results between 1 and 2 km, with an 
average of 5C per day (Fig. 5, curve D). Aloft the 
cooling is comparatively slight. Thus the vertical ar- 
rangement of the heat balance in this continental winter 
climate deviates considerably from that of our middle 
latitudes under maritime influence. 
As a third example, a tropical atmosphere is repre- 
sented (Fig. 5, curve C). There is little difference up to 
7 km as compared to the temperate latitudes. The 
maximum of the outgoing radiation lies at an altitude 
of 13 km and is not affected by processes resembling 
heat conduction as in the temperate tropopause at 
10 km. This appears to be an approach to a solution of 
15 
HEIGHT (KM) 
TEMPERATURE CHANGE (°C PER DAY) 
Fie. 5—Temperature change of the atmosphere by water- 
vapor radiation in degrees per day. 
(A) Normal atmosphere in middle latitude, cloudless. 
(B) The same with an average distribution of clouds. 
(C) Tropical atmosphere, cloudless. 
(D) Temperature and cloud distribution at the cold pole. 
the old problem concerning the origin of the tropo- 
pause. In middle latitudes we find the maximum of 
heat loss in the region of the tropopause. There, it may 
be a fair assumption to consider the water-vapor radia- 
tion as the cause of the tropopause. In the tropics, 
the layer of strongest cooling lies about 4-5 km below 
the tropopause. Thus, it is most probable that the 
tropopause is produced in a different manner in the 
tropics than in temperate latitudes. Whether steady- 
state dynamic phenomena play a role here, or whether 
the CO>,-radiation becomes important, remains as yet 
unexplained. Explanations which distinguish between 
the tropopause in temperate and in tropical regions have 
also been developed by Goody [20]. Finally, further 
indication is given by the abrupt discontinuity in the 
altitude of the tropopause at lat 30°, which was sus- 
