Hen scarts and filly tails, Make lofty ships wear low sails.
Robert Chambers explains in Popular Rhymes (1847) that Certain light kinds of clouds are thus denominated, from their supposed resemblances to the scratches of hens on the ground and the tails of young mares. They are held as prognosticative of stormy weather.
Another traditional version is: Mares’ tails and mackerel scales mak tall ships tak in their sails.