<p>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.</p>

<p>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).</p>

<p>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 <em>appear</em> 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)?</p>

<p>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.</p>

CC BY-SA 4.0
2018-06-11T16:58:37.087Z
I'm glad you want to participate, but I don't think that this is about Earth Science at all. Rather, it's about the way human eyes adjust to light levels.Someone at [Psychology and Neuroscience SE](https://psychology.stackexchange.com) might be able to give you an expert answer.
Spencer
CC BY-SA 4.0
2018-06-11T17:26:20.207Z
This I very much doubt. Of course I have no physical measurements to show, but I believe the brightness was way beyond what eyes adjusting could account for. If that was the case then mankind wouldn't have needed torches to light the way when outdoors under clear skys.
Chaffee
CC BY-SA 4.0
2018-06-12T06:10:09.073Z
If there's a moon, especially a full or near full one, it will actually be fairly bright.  Then if the ground is mostly fairly light-colored rock, or snow, the light will reflect off that.  Even a starlit night can be fairly bright, once your eyes adapt.  For a really dark night, you need clouds, and to be a long way (100 miles isn't too much) from city lights that can reflect off the cloud base.
jamesqf
CC BY-SA 4.0
2018-06-13T14:30:10.513Z
Re *Anyone who lived outside cities will tell me that.* I won't tell you that. I grew up on a farm and went cross country skiing by moonlight many times when I was young. I've backpacked at night, far from any city lights, with the only source of light being the Moon. Your eyes need about 20 to 30 minutes to fully adapt to the nighttime lighting conditions, and after that, moonlight is more than enough to see by.
David Hammen
CC BY-SA 4.0
2018-06-13T18:32:50.370Z
it is not more light but in the thin air the contrast is better so it trick your brain to think it must be brighter.
trond hansen
CC BY-SA 4.0
2018-06-15T09:46:22.940Z
I don't know the precise numbers, but I doubt we'd build telescopes at high altitude if there wasn't some difference.   I've noticed the stars appear brighter when I've gone hiking and sleep at reasonably high altitude.  Part of that is probably reduced light pollution.   It's not just brighter stars, even though it looks that way.
userLTK
<atmosphere><mountains><moon>
Why is the night very bright in the Himalayas?
Chaffee
# Why is the night very bright in the Himalayas?

(Asked by: username_0 on 2018-06-11)

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.

**Tags:** atmosphere, mountains, moon

> **Comments:**
> **username_1**: I'm glad you want to participate, but I don't think that this is about Earth Science at all. Rather, it's about the way human eyes adjust to light levels.Someone at [Psychology and Neuroscience SE](https://psychology.stackexchange.com) might be able to give you an expert answer.  
> **username_0**: This I very much doubt. Of course I have no physical measurements to show, but I believe the brightness was way beyond what eyes adjusting could account for. If that was the case then mankind wouldn't have needed torches to light the way when outdoors under clear skys.  
> **username_2**: If there's a moon, especially a full or near full one, it will actually be fairly bright.  Then if the ground is mostly fairly light-colored rock, or snow, the light will reflect off that.  Even a starlit night can be fairly bright, once your eyes adapt.  For a really dark night, you need clouds, and to be a long way (100 miles isn't too much) from city lights that can reflect off the cloud base.  
> **username_3**: Re *Anyone who lived outside cities will tell me that.* I won't tell you that. I grew up on a farm and went cross country skiing by moonlight many times when I was young. I've backpacked at night, far from any city lights, with the only source of light being the Moon. Your eyes need about 20 to 30 minutes to fully adapt to the nighttime lighting conditions, and after that, moonlight is more than enough to see by.  
> **username_4**: it is not more light but in the thin air the contrast is better so it trick your brain to think it must be brighter.  
> **username_5**: I don't know the precise numbers, but I doubt we'd build telescopes at high altitude if there wasn't some difference.   I've noticed the stars appear brighter when I've gone hiking and sleep at reasonably high altitude.  Part of that is probably reduced light pollution.   It's not just brighter stars, even though it looks that way.  

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## 0 answers

