Coronas are formed by light being diffracted or bent. Coronas are also formed, as light passes through a thin layer of water droplets, which are usually produced by altostratus or altocumulus clouds. The only time there might be a deformation of a corona, is when the water droplets are not approximately the same size. Then the colors may appear smeared and lose their sharpness. Coronas are often in middle or high latitudes and can be seen all year long.
Iridescence is another optic which is very pleasing to the eye.
This occurs when patches of mid-level clouds such as altostratus are tinted with colors of the spectrum. Iridescence can be produced by either sun or moonlight. However, iridescence created during sunlight will produce stronger colors. This is also, often associated with a frontal system. It forms when cloud droplets are too small for a corona to form or if the sun or moon is not directly behind the cloud, which it is with the coronas.