Iridescent Pileus & Shadows

Captured by Dávid Hérincs. "It was a warm, muggy day and the air was very unstable. A thunderstorm with iridescent pileus formed late afternoon near my village of Egyházasrádóc, Hungary. The iridescence was visible only 1-2 minutes, so I was very lucky to see it."

©D�vid�H�rincs, shown with permission

The story of thin and translucent pileus or cap cloud starts with the development of lower and much thicker cumulus on a hot and humid day.

Warm air rises from sun heated ground. It expands and cools as it climbs. If its humidity is sufficient then moisture condenses out to form cumulus cloud. As the day proceeds the cloud grows upwards with yet more condensation. The condensation releases heat which accelerates further the upward motions in the unstable mass. In extremis we get a thunderous cloud mass towering several kilometres.

All this pushes the air layers above the cloud tops upwards. If a layer is moist then the upward travel causes expansion, cooling and sudden condensation into a thin cloud of droplets - a pileus cloud.

The rapid condensation into pileus is important. The droplets are all of similar size and do not have time to evolve. Ideal material to diffract the high altitude sunlight into bright iridescence.

A ragged shadow extends upwards and to the right of the cloud tops.

All is not what it seems. The downward sun rays cast the shadow onto a lower layer of thin cloud. To the eye it looks as though the shadow is above the upper cloud.

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Air pressure decreases with height. An ascending isolated pocket of moist air expands and adiabatically cools because expansion requires work to be done against the surroundings. With sufficient cooling some of the pocket's water vapour condenses out into small droplets