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Cumulus congestus with pileus appear over Charleston, West Virginia on June 1, 2004. Pileus 'caps' form when a rapidly rising convective cloud encounters a thin layer of humid air at high altitudes. As the convective cloud top pushes toward this layer, the resulting increase in pressure above it causes the moisture in the layer to condense into a smooth cloud. The smooth cloud eventually balloons upward beneath the rising convection until the cloud below eventually bursts through. There will often be several of these moist layers that will each create pileus caps on a developing thunderstorm cloud as it rises upward through the atmosphere. Image from 4.7MP digital camera.

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