Irregular Moon Coronae

Colourful coronae around the sun and the moon are caused by light diffraction at water droplets in medium level clouds. Normally these coronae are circular like this case, as they indicate that the droplets in a cloud are all of about the same size. If only a 10% of the droplets vary in size, the corona already appears blurred. (see left figure)

In this observation, made by Bertram Radelow at Davos, Switzerland, in the evening of October 10, 2011 at moonrise, the corona showed very brilliant colours but nevertheless was rather “egg-shaped”. Bertram Radelow said the corona appeared in very thin clouds which could not be seen with the naked eye. Probably there were thin orographic clouds (as can be seen here). In these clouds, droplets are often largest in the centre of the cloud and getting smaller gradually towards the rim, so that the colours are preserved because in a given plane the droplets are still of about the same size.

In this example it looks as if two layers of clouds with different sized droplets are positioned above each other, causing a kind of break in the gradient of the corona.

More pictures: 1 – 2 – 3