A big dark cloud that is carrying a storm inside it — the kind you see building up on a hot afternoon, tall and heavy and grey-black, before the rain and thunder arrive. The word is a warning word: when someone says 'look at those thunderclouds', they mean the weather is about to turn bad. Scientists have a technical name for these (cumulonimbus), but ordinary people say thundercloud. It is countable, and often plural, because a storm usually brings several. English also uses it for a feeling: 'a thundercloud passed over his face' means he suddenly looked very angry, and 'thunderclouds are gathering' means trouble is coming — a fight, a crisis, bad news. That second use is common in newspapers. You may also see it written as two words, 'thunder cloud', though one word is more usual.

雷雨雲、烏雲（快下雷雨前那種又厚又黑的雲）