Alpenglow Maroon Bells, Maroon Lake, Colorado

Alpenglow (from german: Alpenglühen, lit=Alps glow; it, enrosadira) is an

optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the sun or moon wit ...

that appears as a horizontal reddish glow near the

horizon The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether ...

opposite to the Sun when the solar disk is just below the horizon.

Description

Strictly speaking, Alpenglow refers to indirect sunlight

reflected Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in ...

or diffracted by the atmosphere after

sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon due to Earth's rotation. As viewed from everywhere on Earth (except the North and South poles), the equinox Sun sets due west at the moment of both the spr ...

or before

sunrise Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning. The term can also refer to the entire process of the solar disk crossing the horizon and its accompanying atmospheric effects. Terminology ...

. 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

off airborne

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hai ...

ice crystals Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth ...

, or

particulates Particulates – also known as atmospheric aerosol particles, atmospheric particulate matter, particulate matter (PM) or suspended particulate matter (SPM) – are microscopic particles of solid or liquid matter suspended in the air. The ...

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. After sunset, if mountains are absent,

aerosol An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of ant ...

s in the eastern sky can be illuminated in a similar way by the remaining scattered reddish light above the fringe of

Earth's shadow Earth's shadow (or Earth shadow) is the shadow that Earth itself casts through its atmosphere and into outer space, toward the antisolar point. During the twilight period (both early dusk and late dawn), the shadow's visible fringe – somet ...

. This

backscatter In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, ...

ed light produces a pinkish band opposite of the Sun's direction, called the Belt of Venus.

Direct sunlight

Alpenglow in a looser sense may refer to any illumination by the rosy or reddish light of the setting or rising Sun.

References

Atmospheric optical phenomena {{optics-stub

Description

Reflected sunlight

Direct sunlight

See also

References