“Did you know when the Sun sets just right over the ocean, on a clear night, you can see a green flash in the sky?” my great aunt asks as we sit on the balcony at her apartment in Clearwater, Florida. We are sipping wine as we enjoy the first post-vaccination family vacation, “it is an amazing sight, and everyone starts applauding to see it happen” she continues. As an atmospheric science researcher, my thoughts immediately erupted with questions. What causes this phenomenon? Does it occur in places other than Florida? Are there certain conditions that favor this green flash appearing over others? In this post, I will answer some of these questions on that idyllic Florida sunset.
History of green flash sightings
The “green flash” at sunrise and sunset has been widely observed and documented across the world, from the Mediterranean to Australia, and not just the United States. Archaeological evidence suggests that the green flash was observed in ancient Egypt, and Celtic folklore also references “soillse bheó” (living light) [1]. In 1882, Jules Verne wrote about the green flash in his novel “Le Rayon Vert” (The Green Ray) that, “a green which no artist could ever obtain on his palette, a green of which neither the varied tints of vegetation nor the shades of the most limpid sea could ever produce the like! If there is a green in Paradise, it cannot be but of this shade, which most surely is the true green of Hope” [2].
Figure 1. Photograph of the green flash off the coast in San Diego, California on Dec. 13, 2017 [3]
Scientific journals documenting this phenomena date back to the 1800s, and in an article in Nature by R.T. Omond in 1887, they report they saw the green flash while on board a ship on a perfectly clear night just after the Sun set over a distant sea horizon. They hypothesized that they thought the cause of the green light was the Sun shining through the ocean water [4]. A few years later, T. Archibald Dukes reported a flash that looked like a “green fire” in Warlingham, Surrey as the Sun set behind a hill and a cloud (besides this cloud, it was reported the sky was clear) [5], debunking Omond’s theory that the green flash was due to sunlight moving through the ocean. More scientists became interested in documenting the green flash in the early 1900s and predicting its cause. Was it an astronomical event—in other words, did it have something to do with how planets were lined up with the Sun? Was it a meteorological event having solely to do with weather conditions? Was it due to retinal fatigue from staring at the Sun? Or was it due to something else? [6].
Science finds the answer
Through a series of experiments in the first half of the 20th century, scientists were able to determine that the main factors contributing to the green flash were atmospheric refraction (or bending) and scattering of light. The green flash only occurs when the observer has a clear view for miles with the line of sight almost parallel to the horizon, with no clouds, haze or fog [7, 8]. Therefore, the conditions tend to be more favorable in flat areas, such as near the sea or in a prairie. The flash occurs when the Sun appears to be almost entirely below the horizon, with the edge of the Sun still visible. As the Sun sets lower in the sky, the Sun’s rays spread out near vertically as they pass through an increasingly thicker atmosphere. The light path through the atmosphere is the longest at sunrise and sunset, and the ozone layer helps to refract and separate the different colors of light [9]. To conceptualize this, think of the atmosphere as a prism that refracts and scatters light (Figure 2). As the Sun sets, the light coming through the atmosphere up towards our line of sight are spread out into its different colors. The yellow and orange colors are absorbed by water vapor, and molecules in the air scatter violet light [2]. The remaining light is red and blue-green. The red light tends to shoot overhead due to its long wavelength, but blue-green color has a steeper ripple, which is why, under the right conditions, we can see the green flash as the Sun sets over the horizon [7] (see Figure 3). Aerosols (particles suspended in air) can warp and deflect the blue-green color, which is why the green flash is so elusive—the green flash only occurs when aerosol content is very low. The green flash typically occurs for a short time, around one second. However, it was reported in 1929 by Richard E. Byrd that a green Sun persisted for 35 minutes during an expedition to Antarctica [9].
Figure 2. The top image demonstrates how a prism separates the different colors (or wavelengths) of light [10]. The middle image shows how the atmosphere acts as a prism, affecting the colors we perceive [10]. The bottom image likewise demonstrates how the atmosphere acts as a prism [11].
Figure 3. Different colors of light have different wavelengths and frequencies. Here, we can see that the red light as a longer wavelength compared to green light. The green light has a higher frequency (or more “ripples”) compared to red light [12].
The Sun is not the only celestial body that can cause the green flash—it can also be observed at the rising of Venus and of bright stars [1]. However, this may require a telescope to observe [13].
It should be noted, it is dangerous to look directly at the Sun, and therefore when looking to watch the “green flash” one should glance only for an instant when the tiny sliver of the top of the Sun is disappearing [8]. The danger is enhanced when using an optical device—so if trying to capture on camera, do not look through your lens—use a timer or a video to record the event.
Summary
In summary, the green flash can be observed anywhere on Earth at sunrise or sunset, given the right conditions. These conditions involve a view of the horizon for many miles (over sea, land or even low altitude clouds), clear weather conditions and low aerosol content. The green flash tends to occur most frequently over the ocean—which is why many Floridians try to look for it each night.
Figure 4. Green Flash by Explain xkcd [14]
O'Connell, D. J. K. (1958). The green flash and other low sun phenomena. Ricerche Astronomiche, 4. http://adsabs.harvard.edu/pdf/1958RA......4.....O
Nave, R. (n.d.). Red Sunset. HyperPhysics – Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html
McPhate, M. (2018). When the California sun turns green. The California Sun. https://medium.californiasun.co/green-flash-san-diego-947cc485f46b
Omond, R. A. (1887). Green Light at Sunset. Nature 35, 391. https://doi.org/10.1038/035391b0
Dukes, T. (1890). The Green Flash at Sunset. Nature 42, 127 https://doi.org/10.1038/042127a0
Duffield, W. G. (1914). The green flash at sunset and sunrise. The Observatory, 37, 452-453. http://adsabs.harvard.edu/pdf/1914Obs....37..452D
Powell J. (2018) Atmospheric Factors and Features. In: Rare Astronomical Sights and Sounds. The Patrick Moore Practical Astronomy Series. Springer, Cham. https://doi.org/10.1007/978-3-319-97701-0_8
Redfern G.I. (2018) The Sun, Sunsets, Sunrises and More. In: Cruise Ship Astronomy and Astrophotography. The Patrick Moore Practical Astronomy Series. Springer, Cham. https://doi.org/10.1007/978-3-030-00958-8_5
Christensen, L. L., & Horálek, P. (2019). Light Phenomena Over ESO’s Observatories IV: Dusk and Dawn. The Messenger, 178, 51-56.
King, B. (2018). Quest for the Green Flash. Sky & Telescope. https://skyandtelescope.org/observing/quest-for-the-green-flash/
LeVin, A., LeVin, D.J. (2016). The Green Flash. Visit Turks & Caicos Islands. https://www.visittci.com/weather-and-astronomy/sun/what-is-the-green-flash
Biology. Provided by: OpenStax CNX. Located at: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8. License: CC BY: Attribution. License Terms: Download for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8
Lawrence, P. (2021). The green flash of Venus: what it is and how to see it. BBC Sky at Night Magazine. https://www.skyatnightmagazine.com/advice/skills/green-flash-venus-what-is-how-see/
Explain xkcd. (2019). 766: Green Flash. https://www.explainxkcd.com/wiki/index.php/766:_Green_Flash