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Crepuscular rays of light shining through clouds in the sky



During the day

Dawn is the beginning of morning twilight.

Except for light that comes directly from the sun, most of the light in the day sky is caused by scattering, which is dominated by a small-particle limit called Rayleigh Scattering. The scattering due to molecule sized particles (as in air) is greater in the forward and backward directions than it is in the lateral direction.[6] Scattering is significant for light at all visible wavelengths, but it is stronger at the shorter (bluer) end of the visible spectrum; meaning that that the scattered light is more blue than its source, the sun. The remaining sunlight, having lost some of its short wavelength components, appears slightly less blue. Although violet, not blue, is the color with the shortest visible wavelength; the spectrum of light emission from the sun is not constant at all wavelengths, and additionally is partially absorbed by the high atmosphere, so there is less violet in the light. Human eyes are also less sensitive to violet than blue.[4]


The sky can turn a multitude of colors such as red, orange, purple and yellow (especially near sunset or sunrise) when the light must pass through a much longer path (or optical depth) through the atmosphere. Scattering effects also partially polarize light from the sky, most pronounced at an angle 90° from the sun. Scattered light from the horizon travels through as much as 38 times the atmosphere as light from the zenith, causing it to lose blue components, causing a blue gradient: vivid at the zenith, and pale near the horizon.[8] Because red light also scatters if there is enough air in between the source and the observer, these longer wavelengths of light will also scatter significantly, making parts of the sky change color during a sunset. As the amount of atmosphere nears infinity, the scattered light appears more and more white.[9]

The sun is not the only object that may appear less blue in atmosphere. Far away clouds or snowy mountaintops may appear yellowish; as the effect is not so obvious on clear days, but very pronounced when clouds are covering the line of sight reducing the blue hue from scattered sunlight.[9] At higher altitudes, the sky trends to darker colors, since scattering is reduced due to lower air density; an extreme example is the moon, where there is no atmosphere and no scattering, making the sky on the moon black even when the sun is visible.[10]


Dusk and dawn

The Crescent Moon remains visible just moments before Sunrise

The intensity of the sky varies greatly over the day and the primary cause differs as well. When the sun is well above the horizon, direct scattering of sunlight (Rayleigh scattering) is the overwhelmingly dominant source of light. However, in twilight, the period of time between sunset and sunrise, the situation is more complicated. Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena stemming from different causes,[12] most of which occur when there is a temperature inversion (when temperatures increase with altitude rather than the normal decrease in temperature with altitude). Green flashes may be observed from any altitude (even from an aircraft). They usually are seen at an unobstructed horizon, such as over the ocean, but are possible over cloud tops and mountain tops as well. A green flash also may be observed in association with the Moon and bright planets at the horizon, including Venus and Jupiter.[13][14]

The Earth's shadow is the shadow that the Earth itself casts on its atmosphere, which is often visible from the surface of the Earth, as a dark band in the sky near the horizon. This atmospheric phenomenon can sometimes be seen twice a day, around the times of sunset and sunrise. When the weather conditions and the observer's viewing point permit a clear sight of the horizon, the shadow can be seen as a dark blue or greyish-blue band. Assuming the sky is clear, the Earth's shadow is visible in the opposite half of the sky to the sunset or sunrise, and is seen right above the horizon as a dark blue band. A related phenomenon is the "Belt of Venus" or "anti-twilight arch" (sometimes written as "antitwilight arch"), a pink band that is visible above the dark blue of the Earth's shadow, in the same part of the sky. There is no clear dividing line between the Earth's shadow and the Belt of Venus; one colored band shades into the other in the sky.[15][16]

Twilight is divided into three segments according to how far the sun is below the horizon in segments of 6°. After sunset the civil twilight sets in, and ends when the sun drops more than 6° below the horizon. This is followed by the nautical twilight, when the sun reaches heights of −6° and −12°, after which comes the astronomical twilight defined as the period from −12° to −18°. When the sun drops more than 18° below the horizon the sky generally attains its minimum brightness.[17]


During the night

Main article: Night sky




Use in weather forecasting

Marestail white clouds appeared over Dhaka, Bangladesh when significant development of flooding was underway in many parts of the country.


Tropical cyclones

Picture of the sky in the eye of a tropical cyclone


Use in transportation

An F-117 Nighthawk stealth strike aircraft in flight
Main article: Flight


Significance in mythology

Jupiter, Ancient Roman sky deity
See also: Astrology and Sky father



Split Sky just after Sunset 
High Desert Summer Sunset 
Twilight Sky after Rain 

See also


  1. ^ John Tyndall (December 1868). "On the Blue Colour of the Sky, the Polarization of Skylight, and on the Polarization of Light by Cloudy Matter Generally". Proceedings of the Royal Society 17: pp. 223–233. doi:10.1098/rspl.1868.0033. JSTOR 112380. 
  2. ^ Lord Rayleigh (June 1871). "On the scattering of light by small particles". Philosophical Magazine. 41, 275: pp. 447–451. 
  3. ^ J.G. Watson (June 2002). "Visibility: Science and Regulation". J. Air & Waste Manage. Assoc 52: pp. 628–713. doi:10.1080/10473289.2002.10470813. Retrieved 2007-04-19. 
  10. ^ National Weather Service, Anchorage, Alaska (2005-07-15) Chapter 3: Radiation and Temperature National Oceanic and Atmospheric Administration. Retrieved on 2012-02-15
  11. ^ Darula, S., Kittler, R. (2002). "General Sky Standard Defining Luminance Distributions" (PDF). Proc. Conf. eSim 2002, September 11th–13th, 2002, Montreal. Retrieved August 26, 2013.
  12. ^ Young, A. (2006). "Green flashes at a glance". San Diego State University page. Retrieved 2009-03-05. 
  15. ^ Les Cowley (2009-08-02). "Earth's shadow". Les Cowley. Retrieved 2012-02-15. 
  16. ^ David K. Lynch, William Charles Livingston (July 2001). Color and light in nature. Cambridge University Press; 2 edition. p. 38,39. ISBN 978-0-521-77504-5. Retrieved 2012-02-15. 
  17. ^ Dr. Irv Bromberg (2011-04-04). "The Duration of Twilight". University of Toronto. Retrieved 2012-02-15. 
  22. ^ Central Pacific Hurricane Center (2006-07-23). Tropical Cyclone Observations. National Oceanic and Atmospheric Administration. Retrieved on 2008-05-05.
  24. ^ J. Gurevitch, S. M. Scheiner, and G. A. Fox (2006). Plant Ecology, 2nd ed. Sinauer Associates, Inc., Massachusetts. 

External links