"Distant thunderheads reared quivering against the electric sky and were sucked away in the blackness again"
Thunderheads forming
GNU Free Documentation LicenseThunderheads forming - Credit: Fir0002/Flagstaffotos

Thunderheads are a common name given to cumulonimbus clouds, spectacular cloud formations which may occur before or during a thunderstorm.

They usually form at quite low heights, growing vertically instead of horizontally with the base of the cloud extending for several miles across.