Posts tagged ‘22 degree halo’

February 4, 2014

Halos and Sundogs

22° Halo phenomenon

Wispy cirrus and veil-like cirrostratus clouds lingered over Osage Hills State Park for most of the day yesterday.  The millions of tiny ice crystals that make up these high clouds can refract the sun’s rays into halos of light and rainbow-like designs as they pass through.

A little after noon, I noticed a halo around the sun called a “22̊  halo” (22 degrees is the visual distance of the halo from the sun).  The halo remained visible for several hours as a nearly complete circle.

Towards evening, another phenomenon called a “sundog” — A bright patch of colored light at the same level as the sun, but at a distance from it — came into view on both sides of the sun.  Sundogs usually occur at 22̊  like the 22̊  halo, but these were at the less common angle of 46 degrees.

Both of these phenomena occur when sunlight passes through optically pure and perfectly formed ice crystals shaped like six-sided plates or columns.  To further complicate matters, these ice crystals must be suspended in the cloud layer in a uniform orientation.  The angles at which the sunlight enters and exits each crystal dictates what kind of effect is created.

Very specific conditions are necessary to create these features, yet they are actually fairly common.  Unfortunately, we rarely see them because we don’t spend much time looking at the sky.

To learn more about halo phenomena, as well as fantastic information about all cloud types, check out my favorite book on the subject:  The Cloudspotter’s Guide – The Science, History, and Culture of Clouds by Gavin Pretor-Pinney.