thumb|top: reflected alpenglow; bottom: direct sunrise alpenglow from Table Mountain to Mount Baker, WA, USA.
thumb|upright=1.2|Solar ray 1 is the lowest from the Sunthe Sun is set. Solar ray 2 is reflected in the (snow) clouds to the observer.
Alpenglow (from ; ) is an optical phenomenon that appears as a horizontal reddish glow near the horizon opposite to the Sun when the solar disk is just below the horizon.
Description
Strictly speaking, alpenglow refers to indirect sunlight reflected or diffracted by the atmosphere after sunset or before sunrise. 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.
Reflected sunlight
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 is reflected off airborne precipitation, ice crystals, or particulates in the lower atmosphere. These conditions differentiate between direct sunlight around sunrise or sunset and alpenglow.
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.
Direct sunlight
Alpenglow in a looser sense may refer to any illumination by the rosy or reddish light of the setting or rising Sun.
See also
- Golden hour (photography)