Moon Ring

On any full moon day one can see a bright ring around the moon if the sky is clear.  This is not an actual ring around the moon rather it is an illusion or a halo effect.  

In the upper atmosphere, in the absence of sunlight, the temperature would be much low to the extent water vapour in that layer could easily form tiny  ice crystals or a layer of thin cloud.  

 When the bright light from the moon pass through this thin cloud with an inclination of 22 degrees, the ice crystals in the thin filmy cloud diffract the light resulting in a halo like appearance.  As a result in addition to the direct moonlight, one could also see diffracted moonlight in a circle 22 degrees away from the moon. This is popularly known as ‘moon ring’.

It is a belief that bigger halo is an indication of an imminent storm or a rain.  Sometimes people used to count the number of stars inside the moon ring to predict the number of days of the approaching storm or rain.