WATER VAPOUR IN THE UPPER AIR 
Results of a balloon ascent to 30 km, which included 
dew- or frost-point measurements, have been given by 
Barrett and others [1] and other successful ascents have 
since been made. They have observed an increase of 
humidity mixing ratio with height in the stratosphere 
in the summer and a decrease in the winter. A moist 
layer has also been observed, but only a single layer 
and not a double layer as reported in their note. This 
work will be watched with great interest. 
On thermodynamic grounds, kinetic energy cannot 
be released in the atmosphere, except where low tem- 
perature is associated with low pressure. In the strato- 
sphere, therefore, the complex dynamic movements 
which occur in the troposphere would not be expected 
as the energy for them is not available. The explana- 
tion of the shallow moist layer which Barrett found at 
16 km will therefore be of special interest. The differ- 
ence between summer and winter rather suggests that 
the low temperatures now known to occur in the strato- 
sphere over the poles in winter may be a factor in main- 
taining the dryness of the stratosphere. 
The Tropopause. It was first suggested by Gold that 
the air at and immediately below the tropopause would 
be substantially saturated. Observations now show that 
the actual humidity is highly variable. Its average value 
is about 50 per cent. The lowest relative humidity ob- 
served at the tropopause is 3 per cent. When high hu- 
midities, including supersaturation with respect to ice, 
occur, the temperature is usually low enough for dens- 
or persistent condensation trails to be formed by aire 
craft. A simple account of the relation between aircraft 
condensation trails and the humidity and temperature 
of the air has been given by Brewer [3]. When humid- 
ity observations are being made, the condensation trails 
formed by the aircraft should always be noted, as they 
are a most useful check on the observations. 
Low relative humidities at or just below the tropo- 
pause could be due to a recent sinking movement of 
the tropopause, in which the air at the tropopause, and 
possibly in the stratosphere as well, moved downward. 
This process has often been suggested to explain the 
low tropopause heights which are found to the rear of 
depressions. The low humidities could also be caused 
by the incorporation of a substantial volume of strato- 
spheric air into the upper troposphere. 
It may be mentioned that the reverse process, the 
incorporation of moist tropospheric air into the strato- 
sphere, must be quite rare over southern England. The 
stratosphere air is so very dry that a small admixture 
of air from the troposphere would raise its frost point 
markedly. 
The Troposphere. In the troposphere the water- 
vapour distribution is extraordinarily complex. The 
atmosphere can be very moist, very dry, or very vari- 
able with intense stratification. The structure of the 
water-vapour distribution presumably arises from the 
complex dynamic movements which occur in the atmos- 
phere, and these are too poorly understood at present 
to permit a clear explanation of any particular observed 
distribution. The purpose of this section will be to pre- 
sent a small selection of the ascents which have so far 
315 
been made, to indicate the usefulness of dew-point ob- 
servations as a technique for dynamic investigation, 
and to show how the dew-point curve shows features 
which are otherwise difficult to identify. At present 
only isolated ascents have been made, and no analysis 
on a synoptic basis is possible. 
Subsiding Polar Air, May 3, 1944 (Figs. 5a and 5b). 
This ascent shows strongly subsiding polar air behind 
a vigorous depression. The surface synoptic situation 
is shown in Fig. 5b. The convective friction layer of 
moist air extends to about 820 mb, but at 800 mb the 
relative humidity is 5 per cent and cloud development 
is suppressed by the dryness of the air. The upper air 
is so dry that there is only about 0.2 mm of precipita- 
ble water in the whole atmosphere above 800 mb. 
(ep u 
6. ee. 
TaN oN 
STRATOSPHERE AIR OBTAINED 
BY ADIABATIG DESCENT 
SA 
Fic. 5a.—Typical frost-point hygrometer ascent in strongly 
subsiding polar air (1600 GMT, May 3, 1944). Ascent made over 
northeast England, at approximately 54°N 0°W. 
SYMBOLS USED IN SYNOPTIC CHARTS 
SURFACE CHARTS 
Oo-2/10 cLoud ©3-5/10 cLoup()6-7/10 cloud @)s-9710 cLoup @) 10/10 cLoup 
0° EAST 
WEST 30° 20° 10° o° 
Fria. 5b.—Sea-level synoptic chart at about 0600 GMT, May 
3, 1944. 
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