If you’re in the right place at the right time, you might just get to witness the elusive green flash as the sun passes the horizon. A relatively rare optical phenomenon, green flashes can only be observed under the correct amalgamation of atmospheric conditions, surrounding landscape structures, and sheer luck.
Green flashes are only observable during sunrise and sunset, as they appear when the last sliver of sun seems to be peeling away from the horizon line. They are usually seen over large bodies of water or desert, as they require clear air and an unobstructed view of the horizon. But, they can also be seen when the horizon is viewed from an elevated position like an airplane, mountain, or tall building.
As a result of experiencing consistent ideal green flash conditions, they are most regularly viewed in Cerro Paranal, Chile. This 2,635-meter (8,645-foot)-high mountain in the Chilean Atacama Desert is also home to the European Southern Observatory's Very Large Telescope. Conditions on the mountain are so green-flash-friendly, that they observatory was even able to capture an image of a green flash on the moon.
While the length of a green flash is dependent on the speed at which the sun is setting or rising, usually between one and three seconds, the longest recorded observation lasted for a whopping 35 minutes. Witnessed by Admiral Byard’s 1929 expedition at the Little American base in Antarctica, the massively elongated sighting was a result of the sun in the polar regions setting incredibly slowly.
Despite being referred to definitively as “the green flash”, there are actually four unique phenomena that give off a similar green flash effect.
I-Mir, or inferior mirage, is the most common, and it can be seen just as the sun dips below the horizon. Looking like a flattened oval, it occurs when there is warm air over the ocean with a strong temperature gradient near the water surface.
M-mir, or mock mirage, is similar to the inferior mirage, but is produced by an atmospheric temperature inversion with a kink of a few degrees in the temperature gradient above the surface. It produces a pinched effect, with a thin point to the area of green. This is best seen when viewed from an elevated level.
Arguably the most spectacular and the longest lasting green flash, the sub-duct flash can be visible for up to 15 seconds. It causes a green hourglass effect that’s caused by a much narrower height gradient, with the observer being below the atmospheric inversion occurring near the surface.
Rarest of all is the green ray, which produces a beam of green light shooting up from the horizon that lasts several seconds. This phenomenon is not well documented, and there are no known pictures of it occurring.
While different types of mirages are caused by slight atmospheric variations, the reason green flashes can be observed relates to the way light refracts through the Earth’s atmosphere. Depending on the wavelength of light, blue, violet, and green colors are refracted more than yellow, orange, and red. When the sun is at the horizon it’s shining through a much denser atmospheric layer; yellow, orange, and red wavelengths passing through here are most likely to be absorbed by the atmosphere. The remaining blue and violet waves are scattered, leaving the strongest green wavelength to create the flash – although, blue flashes can sometimes occur too.