October 19, 2005
The above photo showing an oval shaped, cloud corona was taken in early August (around 6:15 p.m. local time), from Austin, Texas. Some scattered rain showers were in the area during the afternoon and early evening. The Sun remained just behind the cloud at the center of the picture, and this ring of light crowned the top of the cloud. While the effect was quite striking in person, the photo isn't as dramatic since it was taken with my cell phone.
A cloud corona is a diffraction phenomenon, which, like any coronae or iridescent cloud, forms as a result of interference of sunlight (or moonlight) about cloud water droplets. Cloud coronae can have almost any shape because the cloud can have almost any shape. Therefore, the distribution of water dropets can be spatially complicated and not very uniform. With the more prosaic circular corona, water droplets are nearly uniform in size.Related Links: