Use 'thundercloud' to describe the sky's appearance when a storm with thunder is likely. We canceled the hike because of the thunderclouds gathering above.
Pro Tip 2/3
Dramatic Visual
Thunderclouds are often used in films and books to create a tense atmosphere anticipating a storm. The hero stood defiantly, silhouetted against the menacing thunderclouds.
Pro Tip 3/3
Not Just Rain
Thunderclouds imply more than rain; they suggest lightning and thunder are possible. When thunderclouds appear, it's best to avoid swimming in open water.