"where thunderheads were standing sheared off from the thin dark horizon"
Thunderheads above the Grand Canyon
Creative Commons AttributionThunderheads above the Grand Canyon - Credit: Talbot Troy

Thunderheads are cumulonimbus clouds, spectacular cloud formations that can be seen just 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.