A green flash over the Pacific off San Francisco on December 29, 2006.
Retrieved from the NWS
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.
Longer wavelengths (such as green light) are refracted more than shorter wavelengths (such as red light) and reach the observer at a higher angle. As a result, red is the first color to disappear from view during a setting sun, while violet is the last. Blue and violet are scattered by the atmosphere, but green is not.
The difference in refraction between red and green light under normal conditions is too small to allow a green flash to form, so the atmosphere must contain an inversion to further refract the light.
Credit: HyperPhysics (Georgia State University)
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:
Inferior-mirage green flash looking west from Madagascar
Credit: Vic and Jen Winter, 2001
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.
In an inferior mirage, the mirage is below the the actual object and is inverted.
Credit: Keith C. Heidorn, PhD. aka “The Weather Doctor”
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.
Images showing the progression of an omega sunset. The vertical stretching that is first seen in the top-right image stretches the very top of the sun as it sets, magnifying the last sliver of green light from the sun at the horizon and making it visible to the observer. Even though this was a textbook omega sunset, it didn’t produce a green flash because the atmosphere was too hazy and scattered out any green light.
Credit: Dr. George Kaplan of the U.S. Naval Observatory. Retrieved from Andrew T. Young’s Green Flashes Website
The whole sequence looks something like this:
The stages of an inferior mirage green flash
Credit: Andrew T. Young
Mock-mirage green flash:
Development of a Mock-Mirage Green Flash at sunset in San Francisco.
Credit: Wikimedia user Brocken Inaglory
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).
Diagram of a superior mirage
Credit: Keith C. Heidorn, PhD. aka “The Weather Doctor”
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
Light reaching the observer during a mock-mirage green flash. Note how the light reaching the observer is bent as it passes through the inversion (grey line). 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.
Credit: Andrew T. Young
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.
Diagram showing the evolution of a mock-mirage green flash.
Credit: Andrew T. Young
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 Duct diagram
Credit: Andrew T. Young
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.
A superior mirage and atmospheric duct looking north to Vancouver Island/Victoria from the Strait of Juan de Fuca.
Picture taken August 3, 2011, by Charlie Phillips
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.
Hypothetical light dispersion at a viewpoint very close to the duct. The incident blue and violet light are in the duct and therefore do not reach the observer below it.
Credit: Andrew T. Young
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.
A simulation of a sub-duct green flash. For more detail, see Dr. Andrew T. Young’s article on sub-duct green flashes
Credit: Andrew T. Young
Green Ray
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.
Videos:
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
References:
Cowley, L. (n.d.). Mock Mirages and Green Flash Formation. Retrieved November 29, 2017, from https://www.atoptics.co.uk/atoptics/gfmmform.htm
Geerts, B. (2000, April). The Green Flash. Retrieved November 29, 2017, from http://www-das.uwyo.edu/~geerts/cwx/notes/chap02/greenflash.html
Heidorn, K. C. (n.d.). The Weather Doctor. Retrieved November 29, 2017, from http://www.islandnet.com/~see/weather/doctor.htm
Nave, R. (n.d.). Green Flash. Retrieved November 29, 2017, from http://hydrogen.physik.uni-wuppertal.de/hyperphysics/hyperphysics/hbase/atmos/redsun.html
Phillips, C. R. (2011, September 19). Green Flash. Retrieved November 29, 2017, from http://charlie.weathertogether.net/2011/09/20/green-flas/
Phillips, C. R. (2011, August 04). Mirages! Retrieved November 29, 2017, from http://charlie.weathertogether.net/2011/08/05/mirages/
Young, A. T. (n.d.). An Introduction to Mirages. Retrieved November 29, 2017, from http://aty.sdsu.edu/mirages/mirintro.html
Young, A. T. (n.d.). Ducts. Retrieved November 29, 2017, from http://aty.sdsu.edu/explain/atmos_refr/duct.html
Young, A. T. (n.d.). Green Rays. Retrieved November 29, 2017, from http://aty.sdsu.edu/observing/greenray.html
Young, A. T. (n.d.). The green flash, celestial phenomenon between ship and shore. Retrieved November 29, 2017, from http://aty.sdsu.edu/papers/Zenit/GF.html
Young, A. T. (n.d.). The Sub-Duct Green Flash. Retrieved November 29, 2017, from http://aty.sdsu.edu/explain/simulations/sub-duct/SDGF.html