Bluer sky on top
 Q:
Why is the sky bluer on top than on the horizon?
[LPI/NASA] Sunrise over the South China Sea, seen from
space
A: Consider paths for
two sunbeams: one along the horizon and the other from directly overhead. The
overhead path encounters an air mass 1/38th of the mass along the horizon path.
So, fewer particles clutter the path from on top. Sunlight beaming in gets
scattered and therefore appears blue but it's less likely to get scattered again
and again. That's why the sky looks bluer on top than at the horizon.
You might think that if a little bit of scattering makes the
sky blue, then a whole lot of scattering makes the sky bluer. This, however, is
untrue, says physicist and meteorologist
Craig
Bohren, distinguished professor emeritus at the Pennsylvania State
University. If our atmosphere were ten times deeper, the sky would be white
everywhere — even straight up. On the other hand, if the atmosphere were a tenth
as deep, the sky would be black overhead and bluer toward the horizon.
Further Reading:
Craig Bohren,
Clouds in a glass of beer
D. Lynch and W. Livingston,
Color and Light in Nature
(Answered April 11, 2001; updated Oct. 25, 2007)
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