This active cumulus castellanus cloud is indicative of extremely unstable conditions in the lower levels of the atmosphere. It is growing vertically far more quickly than horizontally, and it is growing into a cold atmospheric level already containing moisture (the cirrostratus behind it.) This cloud went on to become part of a vigorous, hail-producing thunderstorm.

2004 D. Windrim

Source: english wikipedia, original upload 21 May 2004 by Dwindrim en:Image:Castellanus.jpg

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