Monday, May 23, 2011

A Good Day for Sundogs and Better for Halos

Where I live, May usually brings 90+ F degree days that makes seeing the rings and arcs from atmospheric ice crystals rare, but this May has been unusually cold , making for some great halos. I saw this one yesterday:
Colorful arc with ends that split on the left and right.
Lower center is my hat blocking the sun.

The lower section of the arc was not as bright nor as colorful.
My hat is barely shown at the top, where it is blocking the sun.

Most 22 degree halos I see have little color and are a single halo, rather than a split halo shown here next the sundog:
Sundog, outer ring, and inner 22-degree ring. The outer ring leads into the sundog,
suggesting some common optical property at work.

According to Atmospheric Optics, the sundog gets closer to the inner, 22-degree halo as the sun is lower to the horizon. These photos were taken around 3:30 PM, when the sun is still rather high in the sky, and accordingly, the sundog is noticably out from the 22-degree ring.

This ring was also observed by a friend and fellow sky-watching enthusiast who took this photo, probably a mile from me:

Photo by Shaun Brodie

Shaun's photo is a bit more useful in that he included the sun, from which we can trace the arcs of the two rings. Shaun duplicated and rotated his photo to create an extension of the ring. He manipulated the contrast and then drew the approximate tangents that conform to the outer ring.

According to Shaun, "The center of the secondary arc lies on the circumference of the arc/circle around the sun." I took a slightly different approach and agree with his conclusion:

An update(24 May): Per Atmospheric Optics, the outer ring is a circumscribed halo, which is also the high sun version of the upper and lower tangent arcs:
Circumscribed halo

Keep looking up.


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