A collection of so-called "mammatus" clouds made for a dramatic evening sky over the Fredericksburg Agricultural Fair in the eastern US state of Virginia on Thursday. The stunning formation, which features a series of pouches hanging underneath the base of a cloud, is a meteorological phenomenon that typically occurs after severe thunderstorms. It indicates high levels of atmospheric instability, which is why aviators are advised to steer clear -- no matter how beguiling the scene.
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