MadSci Network: Earth Sciences |
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 guess...
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