Sometimes it occurs that small cloud cap forms above a cumulus or cumulonimbus cloud. These caps, wich are similate to a veil, are called pileus (cap) and indicate that the air above the cumulus cloud is very humid. The humidity is near the saturation point, so that a cloud can form. If this cloud cap is near the sun and the glare of the sun is in an ideal case reduced by the cumulus cloud covering the sun, iridescent colours appear in the cloud cap.
The intense colour of a pileus cloud indicates that the water droplets in the cloud are very small and of a uniform size.
Such an iridescent pileus cloud could be observed by Gabriele Schröder on June 6, 2015, at 6.50 P.m. in Schneeberg in the Erz Mountains. The phenomenon appeared in three different parts of the cloud within 10 minutes. Especially interesting is above all the shadow in this picture, which was cast by the lower cumulus cloud and projected upon the clouds. Faint rays can also be seen behind the cloud, indicating that also the surrounding air is very humid.