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Altocumulus mackerel sky
|Appearance||Clumps and rolls of clouds that resemble mackerel scales|
|Precipitation cloud?||No, but may signify approaching precipitation.|
A cumulus is a common term for a sky with rows of cirrocumulus or altocumulus clouds displaying an undulating, rippling pattern similar in appearance to fish scales; this is caused by high altitude atmospheric waves.
Cirrocumulus appears almost exclusively with cirrus some way ahead of a warm front and is a reliable forecaster that the weather is about to change. When these high clouds progressively invade the sky and the barometric pressure begins to fall, precipitation associated with the disturbance is likely about 6 to 12 hours away. A thickening and lowering of cirrocumulus into middle-étage altostratus or altocumulus is a good sign the warm front or low has moved closer and it may start raining within less than six hours. The old rhymes "Mackerel sky, not twenty-four hours dry" and "Mares' tails and mackerel scales make lofty ships to carry low sails" both refer to this long-recognized phenomenon.
Other phrases in weather lore take mackerel skies as a sign of changeable weather. Examples include "Mackerel sky, mackerel sky. Never long wet and never long dry", and "A dappled sky, like a painted woman, soon changes its face".
It is sometimes known as a buttermilk sky, particularly when in the early cirrocumulus stage, in reference to the clouds' "curdled" appearance. In France it is sometimes called a ciel moutonné (fleecy sky); and in Spain a cielo empedrado (cobbled sky); in Germany it is known as Schäfchenwolken (sheep clouds), and in Italy the clouds are described as a pecorelle (like little sheep).
Mackerel sky over Erlangen, Germany
Mackerel sky over Heidelberg, Germany
Altocumulus mackerel sky clouds over Burlington, Canada.
- Downing, L. L. (2013). Metereology of Clouds. p. 154. ISBN 9781491804339.
- Ahrens, C. Donald; Henson, Robert (2015). Metereology Today. Cengage Learning. p. 153. ISBN 9781305480629.
- Wong, Chi-wai. "Mackerel sky, not twenty-four hours dry". Hong Kong Observatory.
- "Ontario Regional Marine Guide". Environment Canada. Archived from the original on 2015-12-03.
- "Mackerel sky". Weather Online. Retrieved 21 November 2013.
- "Making heads of mares' tails". NASA Earth Data.
- Klocek, Dennis (2010). Climate: Soul of the Earth. SteinerBooks. p. 32. ISBN 9781584204589.
- Hamblyn, Richard (2011). The Invention of Clouds: How an Amateur Meteorologist Forged the Language of the Skies. Pan Macmillan. p. 240. ISBN 9780330537308.
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