April 2017

The hat of Freedom

A ‘pileus’ is a brimless felt hat that was worn by the Ancient Greeks and later the Romans. It is also the name of the smooth cap of cloud we’ve chosen as Cloud of the Month for April.

This fleeting pileus formation was spotted Frank Leferink (Member 41121) over Arapahoe County, Colorado, US. It appears above the top of a towering Cumulus cloud. The Cumulus is growing rapidly up through the unstable atmosphere. This will soon develop from the current congestus stage into a fully-fledged Cumulonimbus storm cloud. Violent updrafts of air within are fuelling its growth, and these cause the pileus to form. They are lifting, and so cooling, a moist airstream passing overhead. Where the airstream is lifted most, it cools enough for some of its moisture to condense into droplets that appear as the smooth, delicate cap of cloud. The unstable atmospheric conditions also ensure that the pileus is a very short-lived formation. As the Cumulus continues to grow with unstoppable force, its brilliant white mounds soon envelope the delicate cap as they reach up towards the blue.

For the Ancient Romans, the pileus hat was a symbol of liberty. It was given to slaves when they were released from servitude. To wear a pileus was to show everyone you were finally free.

Pileus above Cumulus congestus, spotted by Frank Leferink (Member 41121) over Arapahoe County, Colorado, US.

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