Vapor and vapour describe the same thing: fine droplets or gas suspended in the air, such as steam or mist. Vapor is the spelling used in American English, while vapour keeps the British u. Think of the kettle: American writers say the vapor rose from the kettle, British writers say the vapour rose from the kettle, and both mean the same cloud of steam.
The core difference
Vapor and vapour are the same word describing mist, steam, or a gaseous form of a substance; only the spelling changes between American and British English.
- vapor — American spelling for mist, steam or gaseous form: Vapor rose off the hot springs in the cold morning air.
- vapour — British spelling of the same word: Vapour hung over the fields at dawn.
How to tell them apart
Both spellings cover the same senses: tiny droplets suspended in the air, like steam from a kettle, and the gaseous form a liquid or solid takes on, as in water vapo(u)r condensing into clouds. The American spelling simply drops the u that British English keeps, the same pattern seen in color/colour and favor/favour.
The confusion tends to surface in scientific and weather writing, where a document might quote a US source using vapor and a UK source using vapour side by side. As with other -or/-our pairs, consistency within a single piece of writing matters more than which variant you pick.
