The Glossary Definition
Alpenglow n. astr.
1. An atmospheric phenomenon occurring when the sun is just below the horizon, in which a horizontal red glowing band is observed on the horizon opposite to the sun.
This occurs when the sun is below suntouch and is caused by the sunlight being reflected off airborne ice, snow or water.
This is an optical phenomenon in which a horizontal red glowing band is observed on the horizon opposite to the sun. This effect occurs when the Sun is just below the horizon. Alpenglow is easiest to observe when mountains are illuminated but can also be observed when the sky is illuminated through backscattering.
Since the Sun is below the horizon, there is no direct path for the light to reach the mountain. Unlike sunset or sunrise, the light that causes alpenglow is reflected off airborne snow, water, or ice particles low in the atmosphere. These conditions differentiate between the alpenglow from the normal sunrise or sunset.
The term is generally confused to be any sunrise or sunset light seen on the mountains, but true alpenglow is not direct sunlight, and is only observed when the sun is below the suntouch.
Alpenglow at Maroon Bells, Colorado
By John Fowler, Placitas, NM
Mountains or Airborne Particles
In the absence of mountains, an airborne colloidal suspension of particles, called aerosols in the sky opposite the sun can be illuminated in the same way by the remaining red scattered light straddling the border of the Earth′s own shadow, a line on the earth known as the terminator or twilight zone.
An observer on the terminator of such an orbiting body with an atmosphere would experience twilight due to light scattering by particles in the gaseous atmospheric layer. This back-scattered light produces a red band above the horizon opposite the Sun.