GENERAL ASPECTS OF UPPER ATMOSPHERIC PHYSICS 
The hour angle h of the sun at its rising at any height 
H may be obtained as follows. We have 
cos Z = sin @sind + cos ¢ Cos 6 cosh, 
where Z = zenith distance of the sun, 6 = declination 
of the sun, and ¢ = latitude of the place of observation. 
From Fig. 2, Z = 90° + 6. 
No Ye 
Sa J 
A 
TG A 
La 
Fie. 2.—The height H at which the cylindrical shadow cast 
by the earth cuts the zenith is given by H = (a = 1). 
The effect of refraction—which accelerates the time 
of rising—has not been taken into account in the 
equation above. The true zenith distance of the sun’s 
centre when it rises at height H above the ground is 
given by 
Z = 90° +49 + 50’. 
Here 34’ has been allowed for horizontal refraction 
and 16’ for the semidiameter of the sun. 
Now, at actual noon the apparent time is 125 and 
the mean time is 124 + e, where « is the equation of 
time. Hence the mean time ¢ of sunrise at height 
above the ground is given by 
{$= 12>+e—A. 
Similarly, the hour of sunset at H is given by 
t= 12>+e+h. 
The values of e and 6 may be obtained from the Nautical 
Almanac. Hence the hours of sunrise and sunset at 
different atmospheric levels may be calculated with 
the help of these equations. 
Curves in Figs. 3a and 36 delineate (after Ghosh 
[40]) the variation of the hour of sunrise with height 
for the whole year at intervals of about a fortnight for 
the latitude of Calcutta (22° 32’ 48” N). Figure 4 
depicts (after Bartels [10]) the height above which the 
atmosphere is illuminated by solar rays at midnight 
during the course of the year in the latitudes 40° to 90°. 
249 
Useful curves depicting the hours of sunrise at dif- 
ferent latitudes on the surface of the earth and also at 
a given set of heights (¢.g., 50 km, 180 km, 250 km, and 
500 km) are also given by Bartels [10]. 
(a) 
400 
300 
= 
E 
ae 
© 200 
W 
x= 
100 
) ~ 
0300 0400 0500 0600 0700 
LOGAL MEAN TIME 
(b) 
400 DEG 29 
DEG 15 
vi yy owes 
NOV 
£ 300 Vi OCT 29 
:s A OcTI5 
5 VW ses 
im Bole WS avg 50 
= WSN _AUG IS 
Wenn JULY 30 
AA suLy4 
ae SQL DUNE 29 
As JUNE 15 
0 SSIws&k) 
0300 0400 0500 0600 0700 
LOCAL MEAN TIME 
Fig. 3.—Illustrating how the hour of sunrise at different 
heights in the upper atmosphere changes in the course of the 
year at the latitude of Calcutta. (After Ghosh.) 
1 
of WAN th dd TA Ss oO WN  D 
Fic. 4.—Tllustrating the heights above which the atmos- 
phere is illuminated at midnight in the course of the year at 
different latitudes. (After Bartels.) 
A PICTORIAL REPRESENTATION OF THE 
PHYSICAL FEATURES OF THE UPPER 
ATMOSPHERE 
In Fig. 5 an attempt has been made to depict the 
physical features of the upper atmosphere and some of 
the phenomena occurring there. 
Starting from the tropopause we note that the region 
