316 
Except for the shallow layer at 345 mb all the air 
above 600 mb has a lower humidity mixing ratio than 
would correspond to saturation at the tropopause, while 
the observations at 345, 700, and 800 mb show that at 
these levels the air is only slightly more humid. 
If we assume that all the air was saturated and has 
moved continuously downward during any recent dy- 
namic movements, during which entropy and humidity 
mixing ratio have been conserved, we may compute, 
for each observation, the level from which the air has 
subsided. This is shown in Fig. 5a in which the 
temperature-pressure levels from which the air has sub- 
sided are shown joined to the corresponding observed 
temperatures by isentropic lines. It will be seen that 
there is a sharp discontinuity in the nature of the source 
air mass between 345 and 400 mb, the air above 345 
mb presumably being warm air and the air below 345 
mb polar air, most of which has subsided strongly. It 
should be noted that the demonstration of this feature 
depends entirely upon the water-vapour measurements. 
The temperature-height curve does not indicate any 
significant change in the layer between 345 and 400 mb 
or in any other layer. 
The thermal stability of the polar air mass from 
which the present air mass is derived is to be noted. 
This stability may be due to the cooling by radiation 
of the lower parts of the subsided polar air, and radia- 
tive adjustments of temperature may be the cause of 
the smoothness of the temperature-height curve. Al- 
ternatively, the air now found at the level of 600 to 
800 mb may have originated from relatively farther 
north, so that the coldest air has been selected for the 
greatest subsidence. On thermodynamic grounds it is 
to be expected that the coldest air would subside most 
strongly. There is little doubt that all the processes of 
the atmosphere are not adiabatic, as may be seen from 
the observations in the stratosphere. The apparent 
source from which the stratospheric air could have been 
obtained (by adiabatic compression from saturated air) 
is Shown in Fig. 5a, but this is a temperature-pressure 
relation which is not known to occur in the atmosphere. 
The Water-Vapour Structure of an Anticyclone. Dur- 
ing March 1945 an anticyclone persisted over Europe, 
its central position varying between the limits of south- 
ern England and southern France. Several ascents were 
made in the northern sector of this anticyclone. All the 
ascents showed a very sharp temperature inversion, the 
level of which varied from 900 to 800 mb, though it was 
most frequently found near 800 mb, higher than in most 
anticyclones. In the region of the inversion, and imme- 
diately above it, there was always a relatively dry 
layer, but this was usually only about 500 to 1000 m 
deep, with moister air above. The depth and dryness of 
this shallow layer varied considerably from day to day. 
One ascent (March 21, 1945) which showed this dry 
layer very strongly has already been published [6]. As 
further examples, two additional ascents (March 18 
and March 20, 1945) are shown in Figs. 6 and 7 to- 
gether with the corresponding sea-level and 500-mb 
charts. On March 18th the dry layer at the inversion 
THE UPPER ATMOSPHERE 
o——o TEMPERATURE 
400 ie o-—=-0 FROST-POINT 
MB TEPHIGRAM 18-MARCH-45 1600 GMT 
Fie. 6a.—Ascent in an anticyclone, March 18, 1945. Ascent 
made over southeastern England at approximately 51°N 
2°W. 
WEST 30° 20° toe o° 10° EAST 
20° EAST 
| 
Fig. 6¢—Contour chart from 500 mb at 0600 GMT, March 
18, 1945. Contour heights are given in feet and temperatures 
in degrees Fahrenheit. Figures in circles are wind speed in 
knots. Broken curves show isopleths of thickness of the layer 
750-500 mb. 
