A Fire in the Sky
One cloudy summer evening looking east I noticed an brilliant fire in the sky. This clearly wasn’t the sun hiding behind a cloud because I was looking east while the sun was setting in the west. Checking later, I discovered the sun had actually set five minutes before this photo was taken.
This is caused by the curvature of the earth. While the sun may have already set below the horizon for someone standing on the ground, clouds high in the sky may still catch rays of the sun since the horizon is further away the higher up you go.
You can observe the same effect for a fleeting minute in skyscrapers. The sun sets earlier for lower floors than upper floors. If skies are clear and you’re looking at just the right moment, you may be able to see a wave of darkness racing up the skyscraper. You can see the effect in this photo of a Toronto sunset.