The green flash and green ray are meteorological optical phenomena that sometimes occur transiently around the moment of sunset or sunrise. When the conditions are right, a distinct green spot is briefly visible above the Sun's upper limb; the green appearance usually lasts for no more than two seconds. Rarely, the green flash can resemble a green ray shooting up from the sunset or sunrise point.
The development of a green flash at sunset in San Francisco
A green flash in Santa Cruz, California
The stages of a green flash
A mock-mirage green flash observed in San Francisco, California
Atmospheric optics is "the study of the optical characteristics of the atmosphere or products of atmospheric processes .... [including] temporal and spatial resolutions beyond those discernible with the naked eye". Meteorological optics is "that part of atmospheric optics concerned with the study of patterns observable with the naked eye". Nevertheless, the two terms are sometimes used interchangeably.
A colorful sky is often due to indirect sunlight being scattered off air molecules and particulates, like smog, soot, and cloud droplets, as shown in this photo of a sunset during the October 2007 California wildfires.
When seen from a high altitude, as here from an airplane, the sky's color varies from pale to dark at elevations toward the zenith.
Purple sky on the La Silla Observatory.
An occurrence of altocumulus and cirrocumulus cloud iridescence
