MACKEREL SKY

sky dappled with small cirrocumulus or altocumulus clouds that look like the markings of mackerel

A mackerel sky is an indicator of moisture (the cloud) and instability (the cumulus form) at intermediate levels (2400-6100 m, 8000-20,000 ft). 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 fluries. There is an old saying, "Mackerel sky, mackerel sky. Never long wet and never long dry." The phrase 'mackerel sky' came from the fact that it looks similar to the markings of an adult king mackerel.
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Noun
1. a sky filled with rows of cirrocumulus or small altocumulus clouds
(hypernym) sky

mackerel sky /'mækrəl'skai/
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