Ice rainbow cap over “Alberto”

May 30 tropical depression, “Alberto” was over Illinois – at this time Chris Moore took this picture from the city of Stritor. According to the author, the setting sun illuminated the cirrus clouds from behind, and colorful rainbow rings wound around the cloud ridge on the western edge of the storm.

This pileus, or “cap” on top of a cumulonimbus cloud. It occurs when strong vertical air streams break through the layer of isothermia. The appearance of a rounded icy “cap” is a sign that the atmosphere is dominated by strong ascending currents, and the humidity is quite high. Humid air above the cloud cools and condenses, which leads to the formation of pileus. Since the “caps” are formed very quickly, the droplets of water inside them are usually of the same size, and this is a suitable condition for iridescence. Homogeneous droplets scatter the passing sunlight, and we can see bright pastel tones.

Usually “caps” appear on warm summer days, when the sky is decorated with towering cumulonimbus clouds. However, in this case, “Alberto” created all the conditions for their appearance in late spring.