There's an R in the month
The weather is cold.
The winter months between September to April have Rs; May to August don't. The phrase is often used in the advice 'Don't eat oysters unless there is an R in the month'. Henry Buttes made a reference to this as early as 1599, in Dyets drie dinner:
"The oyster is unseasonable and unwholesome in all months that have not the letter R in their name".