What causes a lunar halo and how frequent are they?
Halos are optical atmospheric phenomena that happen with the moon and also with
the sun. They are mostly due to the refraction of moonlight (or sunlight) that
passes through ice crystals present in thin clouds at the atmosphere(colored
halos). Reflection of the light from the ice crystal faces (white halos) can
also contribute to that beautiful effect. They are pretty common and can be
seen whenever these clouds are present at the convenient position and the
skies are clear.