under what circumstances can a red shadow be formed?
Weather systems typically move from west to east, and red clouds result when the sun shines on their undersides at either sunrise or sunset. At these two times of day, the sun's light is passing at a very low angle through a great thickness of atmosphere, the result of which is the scattering out of most of the shorter wavelengths — the greens, blues, and violets — of the visible spectrum, and so sunlight is heavy at the red end of the spectrum. If the morning skies are red, it is because clear skies to the east permit the sun to light the undersides of moisture-bearing clouds coming in from the west. Conversely, in order to see red clouds in the evening, sunlight must have a clear path from the west in order to illuminate moisture-bearing clouds moving off to the east. There are many variations on this piece of lore, but they all carry the same message.