If you see a green flash, there is no need to duck for cover; Lord Voldemort isn’t trying to kill you. A green flash is a very rare sliver of green light seen immediately after sunset or immediately before sunrise, usually over the ocean.
Green flashes are a consequence of light from the sun being refracted as it enters the atmosphere. Shorter wavelengths of light (like violet) follow closer to the curvature of the Earth than longer wavelengths (like red) when the sun is right at the horizon. Therefore, as the sun sets, the longer wavelengths (first red, then orange, then yellow) are the first disappear from view for the observer because they don’t follow the curvature of the Earth as closely as the shorter wavelengths (green, blue, violet) and therefore have a lower incident angle to the observer. However, because this process occurs extremely quickly and the sun is just a sliver at the horizon, we need to have a mirage in the lower atmosphere to distort the sun and magnify these green wavelengths.
You may be wondering why we don’t get blue or violet flashes. Blue and violet are scattered throughout the atmosphere (that’s why the sky is types of light are generally scattered throughout the atmosphere as the sun sets. Otherwise, we’d see violet flashes. On very rare occasions with an extremely strong mirage, you may be able to catch a blue flash with a camera.
There are four general types of green flashes: the inferior-mirage, the mock-mirage, the sub-duct, and the green ray. The inferior and mock-mirage flashes are by far the most common, making up 99% of all green flashes, with the inferior making up approximately 2/3rds of all green flashes. (citation)
Inferior-mirage green flash:
As the name suggests, inferior-mirage green flashes occur in the presence of an inferior mirage. Inferior mirages occur when the surface is significantly warmer than the surrounding air. Inferior mirages are commonly found over roads during hot summer days, when the air temperatures near the asphalt are significantly warmer than the surrounding atmosphere. They are found over oceans when the surrounding atmosphere is significantly colder than the ocean and the temperature at the ocean’s surface.
When there is a strong inferior mirage, we see an “omega sunset,” so-called due to the apparent shape of the sun as it sets. The sequence below shows the progression of an omega sunset. As you can see in the top-right image, the base of the sun becomes vertically stretched and merges into the mirage as it approaches the horizon. This vertical stretching magnifies the apparent size of the sun, and when the sun is just about to set below the horizon, the apparent size is big enough to see the last sliver of green light reaching the observer.
The whole sequence looks something like this:
Mock-mirage green flash:
The mock-mirage green flash is a little more complicated than the inferior mirage green flash. It occurs when the viewer is above a superior mirage, which occurs when there is a strong temperature inversion and inverts/elevates the apparent object relative to the real object. This puts them in a so-called “mock-mirage,” which is similar to a superior mirage in that it elevates and flips the apparent object but differs because the mirage is not viewed through a duct (more on that later).
The image below shows how light reaches the observer when they are standing above a temperature inversion (grey line). Light that crosses the inversion is refracted more than light that does not, and the more time the light spends in the inversion, the more it is bent. In the below image, light that reaches the observer -4 minutes below their altitude crosses the -8, -6, and -10 minute marks, resulting in an inverted apparent object above the actual object for the observer. The refraction of light through the inversion amplifies and magnified image.As a result, light that would reach the observer from an angle higher above the horizon is refracted below other angles that spend less of their path traveling in the inversion. In the
Because light that spends a greater amount of time in the inversion is bent more than light that spends less time in it, vertical differences between the different light directions are magnified. The end result of this is that you get a vertically stretched, inverted mirage that can give you a green flash if the atmosphere is clear enough.
Here’s another diagram from Andrew T. Young showing the entire sequence of a mock-mirage green flash.
Sub-duct green flash:
Sub-duct green flashes are the most spectacular green flashes, but are extremely rare. While mock-mirage or inferior-mirage green flashes last for a few seconds, sub-duct green flashes can last for well over 15 seconds.
Sub-duct green flashes appear when the observer is under a very strong, abrupt inversion that creates an atmospheric duct. A duct is an area with an inversion so strong that light entering the inversion from certain angles is guided along a path parallel to the curvature of the Earth. The duct persists below the inversion, with the depth depending on the strength and thickness of the inversion.
Atmospheric ducts are associated with superior mirages, and the stronger/deeper the inversion, the stronger/deeper the duct. The image below shows a superior mirage and atmospheric duct looking north to Vancouver Island from the Strait of Juan de Fuca. This was taken during a mild summer day with temperatures near 70 in the Victoria area. With water temperatures in the low 50s, surface temperatures were much cooler than temperatures slightly above the surface, leading to a strong inversion and atmospheric ducting.
Because atmospheric ducting strongly bends wavelengths and shorter wavelengths (like violet) are bent more than longer wavelengths (like red), getting just below an atmospheric duct allows for these wavelengths to be separated to their maximum extent, resulting in a very pronounced green flash as the sun sets.
Here’s a modeled sequence of a sub-duct flash (once again, credit to Dr. Andrew Young) from 133 meters, just below the region where any ducting of the visible spectrum takes place.
Very little is known about the elusive “green ray.” As the name suggests, green rays are like green flashes, but with rays of light instead of a single point. There have been various accounts of them throughout history, with the first being from M.E. Mulder in July 1907 looking across the Bristol Channel. Green rays are hypothesized to be typical (i.e. – inferior mirage of mock-mirage) green flashes that have been scattered against the background sky by aerosols over the ocean. Aerosols would give the flash the appearance of a larger, green glow higher above the horizon. Additionally, green rays may form when a wave blocks an unusually bright green flash, allowing light emanating from that flash to be seeing well above the horizon.
Next time you are by the sea near sunset, keep an eye out for a green flash! They are rare, but that makes them all the more special.
Inferior-mirage green flash from Brittany, France
Mock-mirage green flash from San Diego, California
Sub-duct green flash setup, but too hazy to see a green flash
Written by Charlie Phillips – charlie.weathertogether.net. Last updated 12/1/2017
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