Il fait très chaud.
Eel feh treh shoh
It is very hot.
Simple
Il fait une chaleur étouffante, hydrate-toi bien.
Eel feh ewn sha-LUR ay-too-FAHNT, ee-drat-twah BYAN
It's stifling heat, hydrate well.
étouffante = stifling; hydrate-toi = hydrate yourself
On cuit ! C'est la canicule, impossible de sortir entre midi et deux.
On kwee! Seh lah kah-nee-KEWL, am-po-SEE-bluh duh sor-TEER ahn-truh mee-dee ay duh
We're cooking! It's a heatwave, impossible to go out between noon and two.
On cuit = we're cooking; canicule = heatwave (official French term); midi et deux = noon and 2pm, the siesta rule
More French Weather Phrases