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A mackerel sky or buttermilk sky describes a sky mostly covered by altocumulus clouds. It is rare with altocumulus and extremely rare in its cirrocumulus form. The occurrence of these clouds is an indicator of moisture and instability at intermediate levels (). Rainshowers or thunderstorms may occur should any lower cumulus clouds reach the layer of the altocumulus form. However, the most common reason for the occurrence of a mackerel sky is an old, disintegrating frontal system. The cloud was probably originally altostratus and has been broken up into altocumulus as the weather front disintegrates (usually as a result of encountering an area of high atmospheric pressure). Little, if any rain most often follows a mackerel sky. Another common place that it is found is in the warm sector of a depression preceding the cold front and associated showery weather, however usually here it is obscured by lower stratus clouds. 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'' is an old English folkname, coming from the species of mackerel familiar in British waters, as the clouds look similar to the markings of an adult Atlantic mackerel. ==In popular culture== *Hoagy Carmichael wrote the song "Ole Buttermilk Sky." *In the film "Plenty", mackerel sky is mentioned and shown. *In the book "Under a Graveyard Sky", the "graveyard sky" referred to is a dark Mackerel sky. The author, John Ringo, confirmed this on his Facebook page. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Altocumulus mackerel sky」の詳細全文を読む スポンサード リンク
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