Classifier for rain, storms, fevers, anger: cơn

Cơn is the classifier for things that come over you, or over the sky, for a while and then pass: cơn mưa (a shower), cơn bão (a storm), cơn sốt (a bout of fever), cơn đau (a spell of pain), cơn giận (a fit of anger).

Like any classifier, it goes between a number or này / đó and the noun: một cơn mưa lớn (a heavy shower; in Hà Nội mưa to), cơn bão này (this storm).

Often you don't need it at all. Trời mưa means it's raining, and Em bị sốt means I've got a fever. Use cơn when you talk about one particular shower or bout: when it came, how long it lasted, how bad it was. Cơn mưa hồi chiều lớn quá. That shower this afternoon was so heavy.

You'll hear it in the weather news, where storms are numbered: cơn bão số 3. Trận is close and also goes with rain and storms, trận mưa; it's the word for a match, too: trận bóng đá.