What causes a halo around the moon?

I recently saw a ring or halo around the moon.  What caused that?

We recently received a good many phone calls to the KTRE Weather Center asking this very question.  You will occasionally see what appears to be a halo around the moon when we have a thin cloud cover high in the sky.  These high, thin clouds are made up of very tiny ice crystals.  The moon light shining through these tiny ice crystals creates a halo appearance around the moon.

Here are some websites where you can see pictures of a halo around the moon and read more about it: