Some years ago I had the chance to climb the Himalayas. Not the Everest of course, but the Annapurna Base Camp which is about 4km above sea level.
During the night of the stay I went out to do some stargazing and I found that the night was very bright, so bright that I was able to make out letters on a book. (Of course I didn't read.) I figured that since there was practically no light coming from the ground within several kilometers to illuminate the air, the light from the moon and the stars were comparatively bright (just the opposite of why we can't see stars clearly from a city).
But recently it hit me that, well, night is supposed to be dark. Anyone who lived outside cities will tell me that. Lack of light source from the ground will make the moon appear brighter because there will be greater contrast but it won't make moonlight reaching the ground any brighter, will it? So this confused me. How was that night in the Himalayas so bright? Is it an expected thing if I'm so high up, or are there some factors that might have made that particular night so bright? Or are nights outside cities normally bright regardless of the altitude (assuming clear sky and near-full moon)?
One thing I suppose could be the reason is that the thinner air due to the altitude meant there was less scattering of the light. I'm not sure if the air was thin enough to make such a difference though.
