The Green Flash

Clear, cold winter days in Alaska are conducive to seeing the green flash. Though a bit rare, the green flash happens often enough to pay to look for it just as the sun rises or sets. An observer in Fairbanks recently reported seeing the green flash at sunset when there was a strong temperature inversion (and much ice fog) in the Tanana Valley.

The green flash may last only a second or so near the time when the first edge of the sun comes up in the morning, or the last edge goes down in the evening. The green flash is the result of the atmosphere acting like a weak prism to bend each color of the sun's light by different amounts. The separation of color is small, but sometimes enough to remove from view the red and yellow light coming from the edge of the sun above the horizon. This leaves only blue and green light, but the blue tends to be scattered most by the air. So, if conditions are just right, a portion of pure green sun, the green flash, will be seen briefly. It might flash several times because of air motion in the light path. Sometimes, for the same reason, the green edge of the sun seems to detach itself and then is suddenly gone.

The green flash was familiar to the ancient Egyptians who apparently saw it often in the clear desert air. Observation of the green color at sunrise and sunset evidently led to the idea that the sun was green at night. Night was the Egyptian emblem of death. Hence, Osiris, the god of the dead and who was identified with the nighttime sun, was always painted green.