A Marvelous Night for a Moondance

January 10, 2014

Hexagonal ice crystal in a 22˚ halo. CC BY-SA 2.5 from Wikimedia Commons.
When it is cold enough for hexagonal ice crystals to form in the sky, you can sometimes see the 22˚ halo. Its angular radius, as the name suggests, is about 22˚, which means that it is about half as wide as a rainbow (which has an angular radius of around 42˚).

To make the 22˚ halo, light only gets refracted, which just means that it gets bent when entering and leaving the ice crystal.

Water droplet in a rainbow. Public domain from Wikimedia Commons.

In contrast, to make a rainbow the light also has to reflect off the back surface of the water droplet.

This has a few effects:

On 2014-01-10 there was a magnificent lunar halo in Tahoe.

So that you can see the reversed colors and dramatically larger size of a rainbow, here is a photo I took in 2009 of a brilliant rainbow in DC. You can also tell from the shadows that the sun is behind me in this photo, in contrast to the halo.

January 10, 2014

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