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Alpenglow (fromGerman:Alpenglühen,lit. 'Alps glow';Italian:enrosadira) is anoptical phenomenon that appears as a horizontal reddishglow near thehorizon opposite to theSun when the solar disk is just below the horizon.[1]
Strictly speaking, alpenglow refers to indirectsunlightreflected ordiffracted by the atmosphere aftersunset or beforesunrise. This diffuse illumination creates soft shadows in addition to the reddish color. The term is also used informally to include direct illumination by the reddish light of the rising or setting sun, with sharply defined shadows.
When the Sun is below the horizon,sunlight has no direct path to reach a mountain. Unlike the direct sunlight around sunrise or sunset, the light that causes alpenglow isreflected off airborneprecipitation,ice crystals, orparticulates in the lower atmosphere. These conditions differentiate between direct sunlight around sunrise or sunset and alpenglow.[2]
The term is generally confused to be any sunrise or sunset light reflected off the mountains or clouds, but alpenglow in the strict sense of the word is not direct sunlight and is only visible after sunset or before sunrise.[1]
After sunset, if mountains are absent,aerosols in the easternsky can be illuminated in a similar way by the remaining scattered reddish light above the fringe ofEarth's shadow. Thisbackscattered light produces a pinkish band opposite of the Sun's direction, called theBelt of Venus.[3]
Alpenglow in a looser sense may refer to any illumination by the rosy or reddish light of the setting or rising Sun.[4][5]
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