Occurrence and Visibility
Green flashes are optical phenomena that occur shortly after sunset or before sunrise. Observers may witness a green (emerald) dot appearing for approximately 1-2 seconds above the Sun's upper rim or as a green ray shooting upward from the sunset point. Green flashes can also be observed when the Moon or bright planets such as Venus and Jupiter rise or set near the horizon.
Green flashes can be observed from any altitude, including from airplanes. They are more commonly seen from locations with unobstructed views towards the horizon, such as over bodies of water.
Optics and Atmospheric Effects
Green flashes are an optical phenomenon primarily caused by the refraction of white sunlight as it passes through Earth's atmosphere. The atmosphere preferentially bends different wavelengths of light, separating them into colors. Blue light is refracted the most, followed by green, yellow, and red.
As the Sun dips below the horizon until only about 1/60th of its diameter remains visible, a sequence of color bands can be seen near its rim, progressing from red to blue. As the red and then yellow bands disappear, the green and blue bands remain. However, the blue band scatters extensively in the atmosphere, making it harder to see. The green band (being between blue and yellow in the spectrum) is less prone to scattering and is therefore more likely to be noticeable for a few seconds.
Earth's atmosphere, with its varying air densities, also refracts light. This is why rainbows sometimes appear around the Sun or mirages are visible in the distance, according to Jan Null, a meteorologist in California. The refraction becomes most pronounced as the Sun nears the horizon because the sunlight is entering the thickest part of the atmosphere at a shallow angle. This is when green flashes can occur.
Types and Variations
Most green flashes fall into two types. The first type appears just before the Sun disappears. However, the type Null encounters more frequently occurs while the Sun is still visible above the horizon. "You see it from the top of the Sun's disk," he said.
On rare occasions, blue or even violet flashes are observed under exceptional atmospheric conditions.
Visibility Requirements
To witness a green flash, observers need favorable viewing conditions. Firstly, they must have a clear view of the Sun near the horizon, such as from a coastal or elevated vantage point, as mentioned by Jean-Luc Courtial, an astronomer at CNRS, an academic and research organization in France. Null notes that elevated coastal areas like San Francisco have higher chances of green flash sightings on temperate days because of warm air layers forming above the cooler waters. These air layers aid in the refraction of sunlight.
The composition of the atmosphere also plays a role in green flash visibility. Particles in the air can scatter blue and violet light, making green light more prominent. Courtial demonstrates this with a simple experiment: adding powdered milk to a water tank and shining a white bicycle light through it. "With the right particle concentration, you get an emerald green," he said.
Observers should also attempt to view green flashes on clear days with unobstructed views of the Sun. Note that it is crucial to avoid looking directly at the Sun without proper eye protection.
Duration and Persistence
Green flashes typically last less than a second. However, in rare instances, they can be visible for a minute or two. Null has rarely observed such long-lasting events despite studying green flashes for 45 years. He attributes these extended periods to stable atmospheric conditions.
Summary
Green flashes are fascinating atmospheric phenomena that occur due to the refraction of sunlight by Earth's atmosphere. They are typically green in color and appear shortly after sunset or before sunrise. While green flashes are not uncommon, they require specific viewing conditions, including unobstructed views of the horizon and clear atmospheric conditions. By understanding the science behind these optical events, observers can enhance their chances of witnessing the elusive green flash.