Il fait un froid de canard
If you want to say the weather is chilly, one common expression is il fait un froid de canard (it is duck cold).
This phrase refers to duck hunting, which takes place around late autumn or early winter, during the migration period when hunters have to stay still in the cold weather waiting for their prey to come close.
Some say the expression may be inspired by very cold days when lakes and ponds are frozen over, meaning ducks are more exposed and vulnerable, or by the chilly moments at dawn or dusk when the birds are landing on or taking off from a lake and an easy target.
Un temps de chien
Horrible weather (both wet and cold), is referred to as un temps de chien (dog weather) – not very polite about man’s best friend! Our ancestors’ poor opinion of dogs is also reflected by the expression il fait un temps à ne pas laisser un chien dehors (it’s weather you wouldn’t [even] leave a dog out).
Faire un froid de loup
A slightly less common expression for bitter cold is faire un froid de loup (to be wolf cold) which, in areas like Franche-Comté, referred to weather when there was a danger that wolves would come out of hiding in search of food – and farmers needed to be wary for their livestock.
On such a day you may remark ça caille! This comes from cailler, meaning to curdle (ie: it is so cold your blood is starting to thicken in lumps…), not la caille – the quail.
Trempé comme une soupe
If you go out in dog weather you will probably end up trempé comme une soupe – literally, soaked as a soup. It sounds rather obvious that soup is wet – but this is said to go back to the medieval meaning of soupe, which was originally a slice of bread soaked in broth, not the liquid itself.
Un temps de Toussaint
Weather that is not only cold, but gloomy and grey is un temps de Toussaint – referring to the festival on November 1 linked to remembrance of the dead, due to sombre associations and the typical weather at that time of year.
Frais, froid, or frisquet
And finally, remember if you want to say the weather is cool, the word is frais (fresh) – which is that bit less bitter than froid. A relaxed alternative, meaning ‘a little bit chilly’, is frisquet.