Definition of "mistral"

I saw him, the sea gray under the mistral and foam-flecked, watching the vanishing coast of France, which he was destined never to see again; and I thought there was something gallant in his bearing and dauntless in his soul.

1919, W[illiam] Somerset Maugham, “chapter 48”, in The Moon and Sixpence, [New York, N.Y.]: Grosset & Dunlap Publishers […]

The mistral had been blowing for three days now and the sea showed more white than blue

1973, Patrick O'Brian, HMS Surprise