252 
an atmosphere consisting of N» and O2 below, to one of 
N»2 and O above [25, 57, 88, 90, 108]. Since low pressure 
is conducive to diffusive separation, it might be ex- 
pected that atomic oxygen would predominate in the 
highest regions of the atmosphere [75]. There is, how- 
ever, no evidence of it. Spectra of auroral streamers 
extending up to 1000 km and beyond show lines due to 
atomic oxygen and bands due to N2 with almost equal 
intensity. Presence of atomic nitrogen has also been 
reported in low-latitude aurorae [32, 41], but whether 
atomic nitrogen is as generally distributed as atomic 
oxygen is not yet known (see section on aurorae below). 
Temperature Distribution. The temperature distri- 
bution in the troposphere and the lower stratospheric 
regions is now well known from direct observations 
with the help of sounding balloons and smoke shells 
[50]. There is a falling temperature (lapse rate about 
5C km=) in the troposphere, the depth of which varies 
between 8 and 18 km. Above the troposphere the 
temperature is nearly constant. For the higher regions 
up to 120 km, we now have results of direct observation 
thanks to successful V-2 rocket flights. These obser- 
vations are now confined only to isolated places (in the 
United States) but it is hoped that in the near future 
reliable data based on rocket observations will also be 
available for the different latitudes at different hours 
of the day and night and in different seasons. A com- 
plete picture of the world distribution of temperature, 
at least up to Region E of the ionosphere, will thus be 
available. In Fig. 6 the temperature distribution ob- 
120 
100 
80 
{= 
2 
fb L 
= 60 
2 
rr 
ae 
40 
© DERIVED FROM PRESSURE- 
HEIGHT CURVE 
20 e FROM RAM PRESSURE 
A FROM SOUNDING BALLOON 
Oj = ee | ] 
150 200 250 300 350 400 
TEMPERATURE (°K) 
Fie. 6.—Temperature distribution with height. The rocket 
data are from the flight on March 7, 1947 at White Sands, New 
Mexico. The sounding balloon data were obtained within an 
hour of the rocket flight. For comparison, temperature dis- 
tributions as deduced from abnormal sound propagation ex- 
periments and from meteoric data are also given after Shep- 
pard [96]. 
tained from the rocket flight at White Sands, New 
Mexico, on March 7, 1947 is depicted [15]. For com- 
parison, the distributions obtained from results of in- 
direct observations (abnormal sound propagation, 
meteoric flashes, and noctilucent clouds), are also given. 
THE UPPER ATMOSPHERE 
It will be seen that the trend of the rocket-observation 
curve is the same as the general trend of the results 
from indirect observations. 
Some remarks about the origin of this peculiar tem- 
perature distribution in the middle and in the upper 
atmosphere and the indirect methods of investigation 
by which it had been surmised long before may be made 
here. The rise of temperature in the middle atmospheric 
region (30-50 km) is due to absorption by ozone [43, 
87]. The existence of any marked temperature vari- 
ation in the stratosphere was, however, not suspected 
until the famous work of Lindemann and Dobson on 
meteors [54]. These authors developed a theory of the 
appearance and disappearance of meteors and applied 
it to determine the density distribution in the middle 
atmosphere. Their findings could be reconciled with 
observed meteoric data only if it was assumed that an 
isothermal region of high temperature (about 100C) 
existed above 55 km. 
A later imterpretation of meteoric data by F.. L. 
Whipple [106] based on a different theory of meteors 
(first suggested by Sparrow [97] and subsequently de- 
veloped by Opik [83]) also confirms these results. The 
meteoric data, according to Whipple, can best be recon- 
ciled if it is assumed that there is a flat temperature 
maximum of about 375K near the 60-km level (as in ~ 
the case of the Lindemann-Dobson theory), a rapid 
drop to 250K near 80 km (as in the Taylor-Pekeris 
theory, see below), and a constant or slowly rising 
temperature up to about 110 km. A seasonal variation 
is also indicated. The upper atmosphere, under average 
midsummer temperatures, is raised 5.3 + 1 km above 
its height under average midwinter temperatures. 
Existence of high temperature in the middle atmos- 
phere, as indicated by the study of meteors, is also con- 
firmed by the study of abnormal propagation of sound 
waves. The sound of an explosion or the firing of a 
cannon can be heard not only in the neighbourhood of 
the source of sound but also at a greater distance, in 
an area separated from the audible zone by a silent 
zone. The phenomenon is caused by refraction of the 
sound waves by the region of high temperature in the 
middle atmosphere. Since the trajectory followed by the 
sound ray in the region of high temperature depends 
upon the temperature gradient of the region, it is pos- 
sible, from a systematic study of the abnormally prop- 
agated waves (the source being an artificial explosion), 
to deduce the gradient as well as the temperature. Such 
studies have been made in Germany, in England, in the 
United States, and recently in India [62]. 
In the region from 60 to 80 km there is no strong 
absorption of solar radiation. (Below 60 km there is 
absorption by ozone; above 80 km there are absorptions 
leading to dissociation of molecular oxygen and ioniza- 
tion of the atmospheric gases.) Hence, a falling tem- 
perature is to be expected in this region. The rocket 
curve shows this clearly, the temperature dropping to 
about 180K near the 80-km level. A drop was also 
inferred from indirect evidence long before rocket ex- 
periments. For example, the so-called noctilucent clouds 
which are observed in the region around 80 km are 
