Green flash

Near the horizon, sunlight travels through a long path in Earth's atmosphere. Refraction bends the rays, and because the refractive index of air varies with wavelength, colours are displaced by slightly different amounts. The Sun's image can therefore develop narrow colour fringes at its upper and lower edges. As the disk disappears below the horizon, the green part of the upper edge may remain visible briefly after much of the rest of the disk has vanished. Atmospheric scattering and absorption affect the colours that survive; blue and violet are often strongly diminished, allowing green to stand out. The phenomenon is an atmospheric optical effect, not a burst of green light emitted by the Sun.

Green flashes are most often sought at sunset, though they can also occur at sunrise. A low, unobstructed horizon gives the observer a view of the Sun's final edge; clear air generally improves the chance of seeing the brief colour. The effect commonly lasts only a moment, so a distant sea horizon is a popular viewing location, but a horizon formed by cloud tops or mountains can also provide a sightline. Atmospheric temperature layers can create mirages and reshape the apparent solar image, sometimes making the flash more distinct or changing its form.

Observers describe several forms rather than one uniform flash. An inferior-mirage flash occurs close to the horizon; a mock-mirage flash can appear as a narrow strip at the Sun's edge; a subduct flash is associated with an hourglass-shaped solar image under an atmospheric inversion; and a green ray is a rarer beam-like appearance extending above the horizon. These descriptions refer to different atmospheric image distortions and viewing conditions. The green colour is typical but not inevitable: in very clear air, blue or violet may sometimes be visible instead.

A green flash can be difficult to predict because it depends on both the sightline and the changing atmosphere near the horizon. It is best observed when the Sun is naturally close to the horizon and its disk is about to disappear, without looking directly at the bright disk while it is still above the horizon. Binoculars, telescopes, cameras with optical viewfinders, and other magnifying optics must never be aimed at the Sun without appropriate certified solar filters, since concentrated sunlight can cause serious eye injury.