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Pileus cap on sunset storm
This storm was probably the most impressive one of the day visible from Charleston. I shot these images from my front porch of this cumulonimbus over Sutton, WV, about 50 miles to the northeast. I anticipated the formation of the pileus cap and was able to get a full sequence of its development and 'punch through'. Lightning was occasionally flickering out of the side of this storm, but it was too sporadic and too bright outside to try and capture any of it. As darkness fell, the storm weakened and disappeared behind low clouds that moved in from the west. In the last image, a jetliner passes in front of the storm (click each image for a larger version).
When a growing cumulonimbus cloud surges upward, it pushes through the upper layers of the atmosphere with great force. Occasionally there will be one or more thin layers of moisture in the upper atmosphere that, when forced upward by the rising updraft from a growing storm, will cool and condense into a 'cap' cloud directly above the top of the cumulonimbus. This formation is called a pileus cloud, and can often be seen in the early stages of a developing storm.