Il y a de l'eau dans le gaz

Literally: There is water in the gas

Example: “Les deux associés ne se saluent plus ; il y a de l'eau dans le gaz.” — The two partners don't even say hello anymore; storm clouds are gathering.

English equivalent: Storm clouds are gathering

Origin: A kitchen metaphor from the early 20th century, with two candidate origins. The first is the familiar disaster of a pan boiling over on a gas ring: first steam, then the flame drowns, then the unlit gas keeps coming and will go up if nobody reaches the tap — it starts by smoking and ends in an explosion, which is exactly the shape of a quarrel. Claude Duneton prefers the second. The coal gas once piped into houses carried a lot of water vapor, which in the right conditions condensed and pooled in the pipes until it blocked them. There was then, quite literally, water in the gas, announced by the flame turning orange before it died — and with dinner no longer cookable, the row that followed needed no metaphor at all.