MadSci Network: Earth Sciences

Re: In miles, when does the earth's atmosphere change from blue to black?

Date: Wed Jun 16 13:03:34 2004
Posted By: In Koo Kim, Physical Atmospheric Chemistry
Area of science: Earth Sciences
ID: 1083955120.Es

I assume you know why the sky is blue. The answer to your question depends on
many things - the effects of solar zenith angle; the variance of the "blueness"
as you look directly above and at the horizons; varibility in different people's
vision (eyes are a biased spectrometer).  The point where you decide that the
scky is no longer blue will probably be different from another person's estimate.
The amount of scattered blue light you see decreases with altitude because of
the decrease in optical length and atmospheric pressure, both of which affect
the amount of scattered light.  Atmospheric pressure decreases exponentially
with altitude and optical length decreases linearly.  I have read that climbers
of Mt. Everest (about 5 miles) say that the sky directly above in the
afternoon appears a deep blackish blue. Also, I have flown in airplanes at
above 35000 feet (6.6 mi) and noticed that the sky appears nearly black.   From
there on up, I'm sure the sky gets blacker and blacker, faster and faster.
If you define "space," where one might say it certainly appears black, to
be the altitude where the pressure is less than 0.001 atmospheres, then
we're talking about 20 miles up. So depending on what you really consider
black to be, I would say the mid-day sky begins to appear black anywhere
between 7 to 20 miles up, depending on who you are.  This is only a best

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