March 2024

Cap Cloud – High Mountain Fashion

Low clouds are often shaped by the terrain over which they form, especially when that terrain is raised into hills or mountains. Cloud formations that are influenced by raised terrain are known generally as orographic clouds. A classic, and particularly stylish, orographic formation is the cap cloud. 

It resembles a hat perched on a mountain summit, and it is a specific form of the lenticularis cloud species. Lenticularis clouds are disc- or lozenge-shaped formations that develop in the vicinity of hills and mountains. They’re mostly seen downwind of the raised terrain. Only when a lenticularis forms directly on, or just above, a peak is it called a cap cloud.

It is not easy to fashion a cloudy hat like the one spotted here by Edward Huijbens over Herðubreið volcano, Iceland. You need the steady hand of a moist airstream that is flowing in a stable atmosphere. That means the meteorological conditions in which the wind is blowing are settled, not stormy. This ensures the flow of moist air up the slopes doesn’t keep on rising and build into shower clouds. The sartorial splendour of the cap depends on this rising air, which forms cloud droplets as it cools over the peak, then sinks back down beyond, where the droplets evaporate away again.

For mountain headwear to be cut with a bold, oversized brim like this one, it needs to be shaped over the right summit of mountain – or, in this case, volcano. The terrain needs to rise on an isolated peak that is steep and pointed. The elevated style of the resulting cloud incorporates an element of illusion. It looks to be poised motionless on the mountain head, but the cloud is in fact in a state of constant change. Its droplets form at its upwind side as the air climbs the mountain flanks, and they zoom along in the wind, travelling the width of the cap’s broad brim before evaporating away at its downwind side.

The ultimate in mountain haute couture, a cap cloud is an atmospheric testament to the beauty of change.

Stratocumulus lenticularis cap cloud spotted by Edward Huijbens over Herðubreið volcano, Iceland. View this in the photo gallery.

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