Afterglow

An afterglow is a broad high arch of whitish or rosy light appearing in the sky due to very fine particles of dust suspended in the high regions of the atmosphere. An afterglow may appear above the highest clouds in the hour of deepening twilight, or reflected from the high snowfields in mountain regions long after sunset. The particles produce a scattering effect upon the component parts of white light. The true alpenglow, which occurs long after sunset or long before sunrise, is caused by the backscattering of red sunlight by aerosols and fine dust particles low in the atmosphere. It is an afterglow caused by direct illumination of atmospheric particles by sunlight as it refracts and gets scattered through the Earth's atmosphere. The high-energy and high-frequency light is scattered out the most and the remaining low-energy, low-frequency reaches the observer in the horizon at twilight. The backscattering of this light further turns it pinkish-red. This period of time is referred to as the blue hour and is widely treasured by photographers and painters as it offers breathtaking imagery. The afterglow persists till the Earth's shadow (terminator line) takes over the sky of the observer as nightfall and the stars appear, with planet Venus being the brightest star visible in the night sky just opposite to the Belt of Venus at the anti-solar point.

After the eruption of the volcano Krakatoa in 1883, a remarkable series of red sunsets appeared worldwide. These were due to an enormous amount of exceedingly fine dust blown to a great height by the volcano's explosion, and then globally diffused by the high atmospheric currents. Edvard Munch's painting The Scream possibly depicts an afterglow during this period.

See also

• Belt of Venus
• Earth's shadow
• Gegenschein
• Sunset
• Red sky at morning

External links

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Wikisource has the text of the 1911 Encyclopædia Britannica article Afterglow.

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