The green flash, a momentary streak of light at sunset or before sunrise, has the sound of an urban legend. But green flashes are real – although rare.

MILA ZINKOVA The green flash, which occurs sometimes as the sun sets, is difficult to see and even more difficult to photograph.

Charles David Keeling, the Scripps researcher who helped launch the study of global warming with his landmark measurements of rising CO₂ levels, was a connoisseur of green flashes, according to his son Ralph.

“I saw dozens of green flashes with my dad,” said Ralph Keeling, who is a Scripps climate researcher himself. “He worked out exactly what days they were likely to happen.”

Green flashes occur when a green spot is visible for a few seconds above the sun, or a green ray shoots up from the sunset point. This occurs when the shorter wavelength colors of sunlight (blue and green) are refracted, as in a prism, in the lower, denser air of the atmosphere as the sun sinks.

“It's rare that you see a really good one,” Keeling said. “You need a super-clear horizon with no clouds and no haze. The sun needs to stay yellow and keep its shape all the way to the setting point. And even so, you don't always get the green flash. But if you don't have that, you won't get it.”

Much of the time in San Diego County, there are either low clouds or a smog layer or something over the water, Keeling said. Particles in the air scatter the refracted sunlight, which makes the light less likely to reach the viewer.

Green flashes are commonly seen at the coast, but they can also be seen when the viewer is above the horizon line, such as on a mountaintop or on an upper floor of a tall building surrounded by flat land.

Darin Toohey, a professor of atmospheric and oceanic sciences at the University of Colorado, Boulder, has photographed several green flashes. He said the flash looks better in person.

“People want it to look super green,” he said. “But the camera never really does justice to it.”

– ROBERT KRIER