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Sky luminance and colors
Clouds made orange by a sunset
When seen from altitude, as here from an airplane, the sky's color varies from pale to dark at elevations approaching the zenith
Light from the sky is a result of the scattering of sunlight, which results in a blue color perceived by the human eye. On a sunny day Rayleigh scattering gives the sky a blue gradient dark in the zenith, light near the horizon. Light that comes in from overhead encounters 1/38th of the air mass that light coming along a horizon path encounters. So, fewer particles scatter the zenith sunbeam, and therefore the light remains a darker blue. The blueness is at the horizon because the blue light coming from great distances is also preferentially scattered. This results in a red shift of the far lightsources that is compensated by the blue hue of the scattered light in the line of sight. In other words some of the red light scatters also and if it does at a point at a great distance from the observer it has a much higher chance of reaching the observer than blue light. At distances nearing infinity the scattered light is therefore white. Far away clouds or snowy mountaintops will seem yellow for that reason; that effect is not obvious on clear days, but very pronounced when clouds are covering the line of sight reducing the blue hue from scattered sunlight. This can be observed at the bottom part of the picture on top of the article.
Another thing worth mentioning is that the scattering due to very small particles (molecule sized) is almost random. The scattering in a 90 degree angle is still half of the scattering that reflects or goes forward. This causes the blue sky to be almost evenly colored and thin clouds to form a white area around the sun, because the big particles the clouds are made of are scattering preferentially only at low angles. The color of the clouds is also due to scattering and a cloud at a small distance has the white color because all the light from these clouds is scattered multiple times in the mass of particles and no wavelenght effects will be observed.
The sky can turn a multitude of colors such as red, orange, purple and yellow (especially near sunset or sunrise) and black at night. Scattering effects also partially polarize light from the sky, most pronounced at an angle 90 from the sun.
Sky luminance distribution models have been recommended by the International Commission on Illumination (CIE) for the design of daylighting schemes. Recent developments relate to ll sky models for modelling sky luminance under weather conditions ranging from clear sky to overcast.
Gallery
Skies o'er us
Turbulent skies  
The sky's zenith appears centered in this daytime photograph taken looking up though trees  
Sunset at Knysna (Western Cape, South Africa)  
Early sunset view from a plane  
See also
Air
Diffuse sky radiation
Extraterrestrial skies
Night sky
Rayleigh scattering
Sky brightness
Skygazing
Sunlight
References
^ Tyndall, John (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 of London 17: pp. 223233. doi:10.1098/rspl.1868.0033. http://links.jstor.org/sici?sici=0370-1662%281868%2F1869%2917%3C223%3AOTBCOT%3E2.0.CO%3B2-X&size=LARGE. 
^ Rayleigh, Lord (June 1871). "On the scattering of light by small particles". Philosophical Magazine 41, 275: pp. 447451. 
^ Watson, JG (June 2002). "Visibility: Science and Regulation". J. Air & Waste Manage. Assoc 52: pp. 628713. http://scholar.google.com/scholar?hl=en&lr=&q=cache:aulPiqN6uTUJ:www.awma.org/journal/pdfs/2002/6/Crit_Review.pdf+. Retrieved 2007-04-19. 
^ Why is the sky Blue?
^ Why is the sky bluer on top than at the horizon
^ eSim 2008 (May 20th - 22nd, 2008) General Sky Standard Defining Luminance Distributions
External links
Wikimedia Commons has media related to: Sky
Day sky images
Night Sky images
Sky photo gallery
Why is the sky blue?
Categories: Basic meteorological concepts and phenomena | Observational astronomyHidden categories: Articles to be expanded from September 2009 | All articles to be expanded
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This article was published on 2010/11/01