The Green Flash Phenomenon and How to See It

The Elusive Green Flash of the Sun

The green flash most often appears as a thin band of green at the top of a sunrise or sunset.
The green flash most often appears as a thin band of green at the top of a sunrise or sunset. © Roger Ressmeyer/Corbis/VCG / Getty Images

The green flash is the name of a rare and interesting optical phenomenon where a green spot or flash is visible at the top edge of the sun at sunrise or sunset. Although less common, the green flash may also be seen with other bright bodies, such as the Moon, Venus, and Jupiter.

The flash is visible to the naked eye or photographic equipment. The first color photograph of the green flash was taken at sunset by D.K.J.

O'Connell in 1960 from the Vatican Observatory.

How the Green Flash Works

At sunrise or sunset, light from the sun travels through a thicker column of air before reaching the viewer than when the star is higher in the sky. The green flash is a type of mirage in which the atmosphere refracts sunlight, breaking it into different colors. The air acts as a prism, but not all colors of light are visible because some wavelengths are absorbed by the molecules before the light reaches the viewer.

Green Flash Versus Green Ray

There is more than one optical phenomenon that can make the Sun appear green. The green ray is a very rare type of green flash that shoots up a beam of green light. The effect is seen at sunset or just afterward when the green flash occurs in a hazy sky. The ray of green light is typically a few degrees of arc high in the sky and can last for several seconds.

How to See the Green Flash

The key to seeing the green flash is to view sunrise or sunset at a distant, unobstructed horizon.

The most common flashes are reported over the ocean, but the green flash can be viewed from any altitude and over land as well as sea. It's regularly seen from the air, particularly in an aircraft traveling west, which delays sunset. It helps if the air is clear and stable, although the green flash has been observed as the sun rises or sets behind mountains or even clouds or a fog layer.

Slight magnification, as through a cell phone or camera, generally makes the green rim or flash visible on top of the sun at sunrise and sunset. It's important to never view the unfiltered sun under magnification, as permanent eye damage may result. Digital devices are a safer way to view the sun.

If you're viewing the green flash with your eyes rather than a lens, wait until the sun is just rising or has partially set. If the light is too bright, you won't see the colors.

The green flash is generally progressive with respect to color/wavelength. In other words, the top of the solar disc appears yellow, then yellow-green, then green, and possibly blue-green.

Atmospheric conditions can produce different types of green flashes:

Type of FlashUsually Viewed FromAppearanceConditions
Inferior-mirage Flashsea level or low elevationsOval, flattened disc, Joule's "last glimpse", usually 1-2 seconds durationOccurs when the surface is warmer than the air above it.
Mock-mirage Flashmore likely viewed the higher it's seen above the inversion, but brightest just above the inversionThe upper rim of the sun appears as thin strips. Green strips last 1-2 seconds.Occurs when the surface is cooler than the air above it and the inversion is below the viewer.
Sub-duct Flashat any height, but only within a narrow range below the inversionThe top part of an hourglass-shaped sun appears green for as long as 15 seconds.Seen when the observer is below an atmospheric inversion layer.
Green Raysea levelA green beam of light appears to shoot up from the top center of the sun as it sets or just after it sinks below the horizon.Seen when a bright green flash is present and there is hazy air to produce the column of light.

Blue Flash

Very rarely, refraction of sunlight through the atmosphere may be sufficient to produce a blue flash. Sometimes the blue flash stacks on top of the green flash. The effect is best seen in photographs rather than with the eye, which is not very sensitive to blue light. The blue flash is so rare because blue light is generally scattered by the atmosphere before it reaches the viewer.

The Green Rim

When an astronomical object (i.e., the Sun or Moon) sets on the horizon, the atmosphere acts as a prism, separating the light into its component wavelengths or colors. The upper rim of the object may be green, or even blue or violet, while the lower rim is always red. This effect is most often seen when the atmosphere contains a lot of dust, smog, or other particles. However, the particles that make the effect possible also dim and redden the light, making it tricky to see.

The colored rim is very thin, so it's difficult to discern with the naked eye. It can be seen better in photographs and videos. The Richard Evelyn Byrd Antarctic expedition reported seeing the green rim and possibly the green flash, lasting for about 35 minutes in 1934.