The green flash is an intriguing occurrence that involves the sudden appearance of a green spot or streak over the rising or setting Sun, Moon, or other celestial bodies. While not especially rare, many people have never seen it. Also, it’s not really a “flash”. Instead, the top of the Sun appears green just before it rises or sets.
What Is the Green Flash?
The green flash is an optical phenomenon in the atmosphere that briefly appears around the time of sunrise or sunset. Either there is a green spot or ray above the Sun or else the top of the solar disc is green. Typically, the green flash manifests in relation to the Sun, but it also occurs with the rising or setting Moon, Venus, or Jupiter.
How the Green Flash Works
The phenomenon is a type of mirage that occurs due to atmospheric refraction and dispersion. Light bends or refracts as it passes through Earth’s atmosphere, separating into its component colors much like in a prism. The degree of refraction depends on the light’s wavelength. Shorter wavelengths (blue and violet light) refract more than longer ones (red, orange, and yellow light). However, the blue and violet light usually scatter out of view, leaving the green light as the most dominant short-wavelength color.
Different Types of Green Flash
There are several types of green flash, each with its unique appearance:
- The Inferior-Mirage Flash: This is the most common type and appears as a brief green oval on the upper edge of the solar or lunar disc. It typically happens when the surface is warmer than the air just above it. This is the type of green flash usually visible over the ocean. It usually lasts one or two seconds.
- The Mock-Mirage Flash: This version occurs when an observer is situated above an inversion layer, such as on a mountain or tall building. The surface is colder than the air above it. The mock-mirage flash produces a segmented or “stepped” Sun, with a green flash visible on the uppermost step. The mock-mirage flash usually lasts a second or two.
- The Sub-duct Flash: This flash appears as a green top of an hourglass-shaped Sun. The Sun’s (or Moon’s) lower edge is obstructed, typically by the horizon, but the upper edge is still visible. While it is visible at any altitude, the observer is below an atmospheric inversion. This type of flash lasts up to 15 seconds.
- The Green Ray or Duct Flash: The rarest of the green flashes, it manifests as a green ray extending upward from the horizon. When it occurs, it often lasts longer than the other types. It happens under unusual atmospheric conditions where the light is refracted within a “duct” or hazy inversion layer. Sea level is the best place for spotting this event.
How to See the Green Flash
You can see the green flash anywhere, providing the conditions are right:
- Low elevation: You don’t have to be at a low elevation, but the Sun must be very close to the horizon, either during sunrise or sunset. The rising or setting of the Moon or a large planet works too.
- Clear, unobstructed view: The observer needs a clear, unobstructed view of the horizon. A few clouds are okay.
- Stable atmospheric conditions: Weather conditions play a crucial role. The air must be relatively clear of large particles that scatter light, such as dust or smog. A little haze actually favors the sub-duct flash and the green ray.
- Thermal stratification of the atmosphere: The air temperature must change markedly with altitude. This condition usually exists over oceans and large bodies of water.
The Green Ray and the Blue Ray
The terms “green flash” and “green ray” are often used interchangeably, but the two terms don’t mean quite the same thing. The green flash is a general term encompassing several types of optical events that appear green and occur briefly at the horizon around sunrise or sunset. In contrast, the green ray is a specific type of green flash that looks like a green beam shooting up from the setting or rising Sun.
The blue ray and blue flash are related phenomena that occasionally occur under similar conditions as the green flash, but they are rarer due to the increased atmospheric scattering of blue light. Sometimes the blue flash appears as a blue flash stacked above a green flash.
The Green Rim
The green rim is a thin edge of green light on the top of the solar or lunar disc just before it rises or sets. But, the upper rim can be blue or even violet instead of green. The lower rim is always red. The effect is common. Watching a sunrise or sunset through a cell phone usually reveals the phenomenon.
Interesting Green Flash Facts
- The longest observed green flash lasted 35 minutes, as seen by the Richard Evelyn Byrd Antarctic expedition in 1934. In general, the event lasts longer closer to the poles because sunrise and sunset are slower.
- Aircraft pilots often see the green flash, especially traveling west so that sunset is delayed.
- The flash is not a single shade of green. The color progresses in terms of wavelength, starting out yellow-green, then green, and sometimes blue-green and blue.
- Jules Verne popularized the phenomenon in his novel “Le Rayon Vert” (The Green Ray), in which characters believe that anyone who sees a green flash will never err in matters of the heart.
References
- Camuffo, D. (1990). “A meteorological anomaly in Palestine 33 centuries ago: How did the sun stop”. Theoretical and Applied Climatology. Springer Wien. 41 (1): 81–85. doi:10.1007/BF00866205
- Ozerov, Ruslan P.; Vorobyev, Anatoli A. (2007). “Wave Optics and Quantum–Optical Phenomena”. Physics for Chemists. ISBN 978-0-444-52830-8. doi:10.1016/B978-044452830-8/50008-8
- Verne, Jules (1882). The Green Ray. Translated by M. de Hauteville. S. Low, Marston, Searle, and Rivington.