The Upper Air 109 
colored; others are swift in their flight across the sky and of 
a soft bluish green; and most of them will appear to come | 
from a particular preferred point of the sky, different for 
each group, known as the radiant point. 
This, then, is the realm of the upper air, a frigid region far 
above our heads, to which, though remote, we owe much. It 
shades us from the harmful short-wave radiation of the sun; 
and it shields us from the direct impact of meteorites. But 
besides acting as a protector, it also provides awesome art 
exhibitions: it paints the northern lights in all their gaudy 
hues and the restful shadows of twilight. 
