Alpenglow (from German: Alpenglühen) is an optical phenomenon. When the Sun is just below the horizon, a horizontal red glowing band can sometimes be observed on the opposite 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. Instead, light reflects off airborne snow, water, or ice particles low in the atmosphere. It is this circumstance that separates a normal sunrise or sunset from alpenglow.
Although the term may be loosely applied to any sunrise or sunset light seen on the mountains, true alpenglow is not direct sunlight and is only observed after sunset or before sunrise.
In the absence of mountains, the aerosols in the eastern portion of the sky can be illuminated in the same way by the remaining red scattered light straddling the border of the Earth's own shadow (the terminator). This back-scattered light produces a red band opposite the Sun.