Cumulonimbus clouds, aerial photograph taken at 10, 000 metres. Cumulonimbus clouds (two seen at centre) are large and turbulent clouds that carry rain or hail. These are developing and growing in size and will form flattened anvil-shaped tops as they hit a layer in the atmosphere known as the tropopause. The resulting structures are known as hammerheads or thunderheads, and are a clear sign of an impending thunderstorm.

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