Theres a lot more going on over your head than you know.

For example, the atmosphere of the Earth thins out gradually the higher you go, and when you get to about 100 kilometers (60 miles) up, different physical processes become important. One of them is called chemiluminescence light produced by chemical processes. This can make the upper atmosphere glow in different colors. Its faint, and best seen from space where we conveniently keep several astronauts. Neuroscientist and amateur video maker Alex Rivest has collected pictures of this airglow taken by astronauts and made this eerie and beautiful time lapse video:

Alex took the original astronaut pictures and enhanced them somewhat to bring out the faint airglow. You can see it in lots of pictures taken from the space station, and Ive commented on it many times. One thing Ive been meaning to do, though, is find out what the physical process is thats causing the air to glow, and why it creates different colors you can clearly see green, yellow, and red glow in many of the pictures!

Alex comes to the rescue on that as well. On his blog, he discusses how he made the video and why the air glows (based on a somewhat terser explanation at the Atmospheric Optics website).

The way this works is simple in general, though complicated in detail much like everything else in the Universe! Basically, during the day, in the upper atmosphere ultraviolet light from the Sun pumps energy into oxygen molecules (called O2; two oxygen atoms bound together this is the stuff we breathe). This energy splits the molecules apart into individual atoms, and these atoms have a little bit of extra energy we say these atoms are in an excited state. Like a jittery person whos had too much coffee, they want to give off this energy. They can do this in a couple of ways: they can emit light, or they can bump into other atoms and molecules and react chemically with them.

If you have an excited oxygen atom sitting in space all by its lonesome, it can either dump that energy by emitting green light or red light. Usually, itll emit green light in less than a second after becoming excited, and itll emit red light on much longer timescales, like minutes. This is important, so bear with me.

At a height below about 95 km, the atmosphere is thick enough that collisions between atoms happens all the time. In fact, an excited oxygen atom doesnt have to wait very long (usually microseconds) before another atom or molecule bumps it. If collisions happen faster, on average, than about once every 0.1 seconds, then an oxygen atom doesnt have enough time to emit green light before getting smacked by another atom or molecule. When that happens, the other atom can steal its energy, and no green light is emitted. So below that height we dont see any green emission.

At heights of 95 100 km or so, collisions happen less frequently, giving the oxygen atom time to blow out a green photon (a particle of light). So at that height we do see the green glow. This layer is thin, like the shell of a bubble, and we see it as an arc due to limb brightening (which you can read about here if you want details). In the picture above, you can see it as a very thin green arc above the diffuse yellow glow (which Ill get to; hang tight). Normally it wouldnt be very bright, but looking along the edge of the shell is like looking through a very long slab that stretches for hundreds of kilometers. The light builds up, making it bright enough to see.

Higher up, above 100 km, the oxygen atoms are much farther apart because the density is lower. The odds of two of them colliding are a lot lower, so the time between collisions can be pretty long, long enough to give the oxygen atoms time to emit red photons. Thats why we see that red glow higher up, where the air is ethereally thin.

The rest is here:

The softly glowing night sky | Bad Astronomy