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Alpenglow (from german: Alpenglühen, lit=Alps glow; it, enrosadira) is an
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 i ...
opposite to the Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
when the solar disk is just below the horizon.
Description
Strictly speaking, Alpenglow refers to indirectsunlight
Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
reflected or diffracted
Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
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 spring ...
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
Al ...
. 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
Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
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
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 hail. ...
, 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, n ...
, 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 ter ...
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 (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...
s in the eastern sky
The sky is an unobstructed view upward from the surface of the Earth. It includes the atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from outer space.
In the field of astronomy, ...
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 – sometimes ...
. 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, a ...
ed light produces a pinkish band opposite of the Sun's direction, called the Belt of Venus
The Belt of Venus (also called Venus's Girdle, the antitwilight arch, or antitwilight) is an atmospheric phenomenon visible shortly before sunrise or after sunset, during civil twilight. It is a pinkish glow that surrounds the observer, extend ...
.
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)
In photography, the golden hour is the period of daytime shortly after sunrise or before sunset, during which daylight is redder and softer than when the sun is higher in the sky.
The golden hour is also sometimes called the "magic hour," es ...
* Belt of Venus
The Belt of Venus (also called Venus's Girdle, the antitwilight arch, or antitwilight) is an atmospheric phenomenon visible shortly before sunrise or after sunset, during civil twilight. It is a pinkish glow that surrounds the observer, extend ...
References
Atmospheric optical phenomena {{optics-stub