The Upper Air 105 
clouds are still illuminated by the sun; finally even the 
highest clouds are shut off from sunlight as the sun continues 
to sink lower and lower—but twilight continues for some 
time even after that. The duration of twilight is explained by 
the fact that the sunlight is still striking air particles high 
above the highest clouds and that some of that light is being 
reflected back to earth. Eventually even this residual light 
disappears as the sun proceeds further on its way; but the 
duration of twilight shows that the atmosphere extends for 
a considerable distance above the highest clouds. 
NORTHERN LIGHTS 
Probably the most striking of all phenomena that take 
place in the upper atmosphere is a display of “aurora bore- 
alis’ called “Northern Lights,” when observed near the North 
Pole and “aurora: australis” when observed near the South 
Pole. Auroral displays occur as great glowing shifting pul- 
sating streaks of light in the night sky, in a wide variety of 
shapes—arcs, rays, curtains, draperies, bands, coronas and 
diffuse general glow. When faint, they may be of the order 
of the brightness of the Milky Way and are usually a whitish 
color. But when they are intense, they may be as bright as 
the full moon and exhibit many colors—yellow, red, green— 
colors which change rapidly from one to another. Usually the 
lower edges of the displays are well defined and bright, 
whereas the upper edges gradually fade away into invisi- 
bility. The rapid movements and changes and endless com- 
binations defy description. In the regions near to the poles 
a corona is sometimes observed as a ring or crown of light 
directly overhead in the sky with rays emanating from it on 
all sides like spokes of a wheel, growing and fading in 
