This fact comes to you just in time for St. Patrick’s Day— the phenomenon of the green flash. Not to be confused with the fictional superhero character, a green flash is a wonder in which part of the sun appears to suddenly change color for about one or two seconds. The brief flash of green light is seen more often at sunset than sunrise.
This fleeting spectacle is caused by the refraction of sunlight, which is particularly significant at sunset and sunrise when the light travels through more of the Earth’s atmosphere. According to Cornell University, the atmosphere bends the sunlight passing through it, separating the light into its different colors, similar to how a prism bends and splits sunlight into rainbows.
The various colors of light bend in different amounts based on their wavelengths; shorter wavelengths, such as blue, violet, and green, refract more strongly than longer wavelengths, like yellow, orange, and red. As such, blue and violet light is scattered by the atmosphere while red, orange, and yellow are absorbed, leaving the green light the most visible during the few seconds when the sun sets below or rises above the horizon.
However, according to Andrew T. Young at San Diego State University, green flashes are not always green.
Sometimes, when the air is extremely clear, enough blue or violet light rays will make it through the atmosphere and create a blue flash instead of green. Nonetheless, green is the most common hue reported and caught in images.
There are four categories of green flashes: inferior mirage, mock mirage, subduct flash, and green ray, according to an article published by the American Association for the Advancement of Science (AAAS). However, nearly all green-flash sightings fall into the first two categories.
Inferior mirage flashes are oval and flat and occur close to sea level when the water’s surface is warmer than the air above it. In contrast, mock mirage flashes, on the other hand, occur higher up in the sky and when conditions on the surface are colder than the air above. The flashes appear to be thin, pointed strips being sliced from the sun, and they last about one to two seconds.
Subduct flashes are created when the sun appears to form an hourglass shape due to a phenomenon called atmospheric inversion, which occurs when a layer of warm air traps cool air and moisture close to the ground. In this rare circumstance, the upper section of the sun may appear green for up to 15 seconds.
The rarest type of green flash is known as a green ray. In this instance, a beam of green light shoots straight up a few degrees from the green flash immediately after the sun sets for about a second. It’s caused by the combination of hazy air and an unusually bright inferior, mock, or subduct green flash.
While there isn’t a perfect recipe for a condition that will guarantee a green flash sighting, the best way to potentially observe one is to go somewhere that provides a clear view of the horizon and is free of pollution, such as over the ocean, according to Young.
Green flash sightings frequently occur at the ocean, where more of the atmosphere is visible, and the line of sight is virtually parallel to the horizon. Flat prairies and deserts also tend to have the appropriate conditions for a green flash sighting. Sometimes, conditions are so perfect that a rare double-green flash can be seen.