102 The Sky 
that in the stratosphere because with higher altitudes the 
air thins out rapidly until at the upper limit of the strato- 
sphere there is almost none at all—just the emptinéss, the 
vacuum of outer space. 
We will reserve the discussion of the lower regions of the 
atmosphere for those sections in which we discuss the winds, 
clouds, storms, etc., and will confine ourselves to a discussion 
here of those remote high regions of the upper air far beyond 
the reaches of weather changes. From ascents in airplanes 
and balloons, it is known that for the first 6 miles the tem- 
perature of the air falls off rapidly until it reaches a tem- 
perature of —67° (Fahrenheit). At this point we enter the 
stratosphere and discover with further ascent that the tem- 
perature remains constant many more miles at approximately 
this same low temperature of 67° below zero. As far as men 
and their instruments have been able to penetrate, the tem- 
perature remains constant. At very great altitudes it is sup- 
posed that the temperature of the air increases until at 50 to 
100 miles it reaches about 85° above zero (Fahrenheit). 
HOW HIGH IS UP? 
There are no clouds in the upper air, and so the only way 
that we know about its real extent is by observing auroral 
displays, duration of twilight, and meteors. One other evi- 
dence of the height of the atmosphere is the Kennelly-Heavi- — 
side layer, the effects of which have long been understood 
and carefully examined by radio engineers. The Kennelly- 
Heaviside layer is about 35 miles up in the daytime, about 
55 miles at night. It is completely invisible to your eye or to 
a telescope; the only way that we know that it is there at all 
is that it reflects radio waves, This property of reflecting 
— 
