WW2010
University of Illinois

WW2010
 
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Online Guides
 
  introduction
 
> meteorology
 
  remote sensing
 
  reading maps
 
  projects, activities

Meteorology
 
  introduction
 
  air masses, fronts
 
  clouds, precipitation
 
  el nino
 
  forces, winds
 
  hurricanes
 
  hydrologic cycle
 
> light, optics
 
  midlatitude cyclones
 
  severe storms
 
  weather forecasting

Light, Optics
 
  introduction
 
  mechanisms
 
> air, dust, haze
 
  ice crystals
 
  water droplets

air, dust, haze
 
> crepuscular rays
 
  blue skies, blue haze
 
  sunsets

User Interface
 
  graphics
> text

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Crepuscular Rays
sun rays converging on the horizon

Crepuscular rays occur when objects such as mountain peaks or clouds partially shadow the sun's rays. The name crepuscular means "relating to twilight" and these rays are observed at sunrise and sunset. Crepuscular rays appear to diverge outward from the setting sun, and are visible only when the atmosphere contains enough haze or dust particles so that sunlight in unshadowed areas can be scattered toward the observer.

[Image: crepuscular rays (33K)]
Photograph by: Holle

The light rays are actually parallel, but appear to converge to the sun due to "perspective", the same visual effect that makes parallel railroad tracks appear to converge in the distance. Crepuscular rays are often red or yellow in appearance because blue light from the sun is selectively scattered out of the beam by air molecules.

Light rays scattered by dust and haze occasionally appear to converge toward the "antisolar" point, (the location on the horizon opposite the point where the sun is setting). These rays, called anti-crepuscular rays, originate at the sun, cross over the sky to the opposite horizon, and appear to converge toward the antisolar point because of perspective.

[Image: anticrepuscular rays (80K)]
Photograph by: Mckee

In the photo above, the sun is near the horizon behind the observer and sunlight is reflecting off the small cloud in the top right corner of the picture. Mountains and clouds behind the observer are responsible for the shadows in between.



mechanisms
Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.

blue skies, blue haze