In this image, we are way east in California, a long way from the mountains. The sun has set in the valley where we are shooting from. The dark line, towards the horizon, is the place where the air below is in shadow, and the air above is still in sunlight. If you look at the larger image, you can see streaks of the mountains shadows in the air.
Definitely a nice place to watch the sun go down. … And looook at all the cotton.