I Didn’t Know That

Q: What is the green flash that is sometimes seen on the horizon at sunset?

A: It is actually the tail end of a rainbow, according to Sallie Baliunas, deputy director of the Mt. Wilson Observatory. When sunlight passes through a dense, thick layer of atmosphere, such as at sundown, it is frequently split into the colors of the spectrum, forming a rainbow. Red sets first, followed by orange and yellow, which leaves behind green and blue, she said. “On very clear days, I have occasionally seen a blue flash as well, but that is very rare,” she said. The flash can be seen from anywhere, such as the beach, where the horizon is visible, but is more visible from a mountaintop like Mt. Wilson because the sunlight passes through more air before reaching the viewer’s eyes.

Blocks of air over the Los Angeles Basin can have other lensing effects as well, so that it looks like little pieces of the sun are bitten off and floating. These sometimes make it appear that the sun has horns, for example.