A mackerel sky or buttermilk sky is an indicator of moisture (the cloud) and instability (the cirrus-cumulus form) at intermediate levels. If the lower atmosphere is stable and no moist air moves in, the weather will most likely remain dry. However, moisture at lower levels combined with surface temperature instability can lead to rainshowers or thunderstorms should the rising moist air reach this layer. In the winter it is often said to precede snowstorms and flurries. Mackerel skies are spoken of in the popular bywords, "Mackerel in the sky, three days dry," "Mackerel sky, mackerel sky. Never long wet and never long dry," and the nautical weather rhyme, "Mare's tails and mackerel scales / Make tall ships carry low sails." The phrase 'mackerel sky' came from the fact that it looks similar to the markings of an adult king mackerel. Reference: Wikipedia