Altocumulus mackerel sky

Mackerel sky
Altocumulus mackerel sky
Abbreviation	Ac
Symbol
Genus	Alto- (mediumhigh) -cumulus (heaped)
Altitude	2400-6100 m (8000-20,000 ft)
Classification	Family B (Medium-level)
Appearance	Clumps and rolls of clouds that resemble mackerel scales altogether
Precipitation cloud?	No, but may signify approaching precipitation.

A mackerel sky or buttermilk sky is an indicator of moisture and instability at intermediate (altocumulus) or high (cirrocumulus) levels. The phrase 'mackerel sky' came from the fact that it looks similar to the markings of an adult king mackerel and this phrase is generally only used if a significant proportion of the sky is covered in altocumulus or cirrocumulus. This usually produces perspective effects as the clouds become smaller towards the horizon. 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." In the winter they are often said to precede snowstorms and flurries. Mackerel skies have therefore become synonymous with bad weather approaching. However while altocumulus does rarely form instead of the more usual altostratus ahead of a frontal system, it is more likely that a so-called mackerel sky signifies the break up of an altostratus layer and leads to brighter conditions, and so the forecasting value of mackerel skies often depend on whether it was cloudy or clear prior to their appearance. However even if it was clear prior to their appearance, it does not always mean rain is approaching as altocumulus is often associated with a weak frontal system where the usual altostratus has become broken, and in this case there is a chance the weather may remain dry, although it is more likely that there will be light drizzle from stratus clouds.

Cirrocumulus mackerel skies are much rarer than altocumulus mackerel skies because cirrocumulus generally only forms in patches and does not cover the entire sky. Very fine stratocumulus undulatus can also produce a mackerel sky at low levels.