2 adjectives to describe thunderhead

Overhead, the Eternal Painter was throwing in the smoky purple of a false thunderhead, sweeping it away with the promise of a downpour, rolling in piles of silver clouds and drawing them out into filmy fingers melting into a luminous blue.

The clouds were rolling up in the north and west in lofty thunderheads, pearl-white in the hot sun, with great blue valleys and gorges below, filled with shadows.

2 adjectives to describe  thunderhead