I saw a flash of green above the horizon at sunset. Was it real? What caused it? Will I be able to see it again?
Congratulations! The green flash is real but it is rarely seen, since it requires special conditions to be observed. The green flash is usually a band or vertical ray of green light just above the setting or rising sun, and can be green, violet, or blue. To see it, you need a clear, flat horizon and a haze free sky. An ocean works well - so do deserts.
The green flash is caused by rays of sunlight refracting (bending) in the atmosphere. Because refraction depends on the wavelength (color) of the light, blue, violet and green light are refracted more than yellow, orange, and red light. So at sunset, when the light has the most atmosphere to be bent by, the sun is surrounded by "shadows" of different colors, with the blue/violet/green shadows farther out. The red, orange, and yellow shadows are absorbed by the atmosphere, and the blue and violet shadows are scattered by the atmosphere, so the strongest shadow left is usually the green one. This effect is only strong enough to see for a few seconds during sunrise and sunset, hence the "green flash."
To see the green flash again, you'll probably need to watch a lot of sunsets on clear days over the ocean. It really is mostly just a matter of being in the right place at the right time.
This page was last updated June 28, 2015.