Posts Tagged ‘atmospheric optics’

Good Morning Wisconsin!

May 7, 2013

Crepusculur rays originating from clouds below the horizon.

The above image is an unusual example  of crepuscular rays. The name comes from their typical appearance during crepuscular hours (those around dawn and dusk), when the contrasts between light and dark are the most obvious. Crepuscular comes from the Latin word crepusculum meaning twilight.

A key component for crepuscular rays is the casting of a shadow. The shadow can be due to most anything, typically clouds. The result is sunlight broken into darkened areas (in the shadow) and lighted areas. The above image was attained prior to sunrise with the crepuscular rays caused by clouds below the horizon.

The visible beams always appear to spread out from the light source. In the above image the rays appear to diverge  from the base of the silo.  When I noticed the crepuscular rays forming, I changed my position allowing the rays to appear to be related to the farm buildings.

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Bilateral Sun Dogs

March 28, 2013
Bilateral sun dogs in Plover, Wisconsin

Bilateral sundogs in Plover, Wisconsin

Sundogs can appear as a colored patch of light to the left or right of the sun, 22° distant and at the same distance above the horizon as the sun, typically in ice halos. The ‘dogs’ in this image have red colors towards the sun with greens and blues beyond.  This is the same sequence of colors seen in rainbows.

Like sun pillars, this atmospheric phenomenon is due to ice crystals. Where as sun pillars owe their existence to plate like crystals, sundogs are due to light reflecting through hexagonal crystals.  If the crystals are oriented with their flat faces horizontal, a sundog is observed. If the hexagonal crystals are randomly oriented, a halo  is formed.

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