Explainer: green flashes over the Pacific at sunset

When Judy Flagg of Irvine looked at her photos of a sunset from Laguna Beach, she saw a pair of small green flashes just above the sun on the horizon. Mystery solved, via the Orange County Register's science blog. Astronomer Andrew T. Young at San Diego State University, who has a website on green flashes, says they happen when warm air sits on top of cold air, and we view the sunset while standing in the warm air and looking down into the cold air. “The sharp contrast in density where the warm air overlies the cold air acts like a lens, forming multiple and inverted images,” Young says. Refraction in the atmosphere at the horizon, meanwhile, "magnifies enough to make the green upper rim of the sun visible to the naked eye.”