Cumulonimbus Calvus and Pileus

These isolated multi-cellular cumulonimbus calvus clouds appeared in skies at Noelridge Christian Church in Cedar Rapids, Iowa on the evening of Thursday, July 7, 2011. The left panel shows a large bank of the calvus clouds in the north sky around 6:05 PM. Its center was located approximately 50 miles away, near the town of Oelwein in southern Fayette County. Cloud tops at places reached 35,000 feet. The right panel shows a calvus with a pileus cap. This cloud, shot at 5:50 PM, was located about 30 miles to the northeast, in southern Delaware County. Pileus are made of ice crystals high in the troposphere directly above another cloud. The pileus layer forms when the cumulus cloud thrusts a moist layer of air above it upwards, causing condensation in the layer. The cumulus cloud may eventually grow to penetrate the pileus layer.