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At the time of taking this photograph in Vic, in Osona, for La Vanguardia's Readers' Photos, at sunset, to the northeast, powerful cumulonimbus clouds can be observed. And on top you could see a hat made of several layers of pileus.
Pileus is a cap-shaped accessory cloud that sits above the top of cumulus and cumulonimbus, and sometimes several pileus are seen superimposed, as in this image.
Pileus are formed by strong updrafts with moist air from low altitudes, causing the air to cool below its dew point.
This type of clouds are also reminiscent, due to their shape, of the lenticular clouds that form over some mountains or mountain ranges.
Pileus tend to change shape quickly. It is a small, horizontal cloud that can appear above a cumulus or cumulonimbus cloud, thus giving the characteristic appearance of a cap or cup.
