The Mackerel sky is a variation of Altocumulus and is named for its resemblance to fish scales. Early Mariners first named the altocumulus mackerel clouds and is spoken of in the phrase “Mackerel in the sky, three days dry" Whether or not it is an indicator of three days of dry weather is certainly up for debate. As with altocumulus formation in general a mackerel sky is produced by the lifting of a large mass of moist air, usually by an approaching cold front. This would indicate disturbed weather approaching! (However if the low level air remains stable then the weather should remain dry) Therefore, it is more likely that the other frequently quoted phrase, “mackerel sky, mackerel sky. Never long wet and never long dry." is more likely to be accurate as a cold front would indicate a short-lived but possibly intense spell of wet weather, followed by frequent showers. These clouds are generally to be found at intermediate levels between 2400 - 6100 m (8000-20,000 ft). The exact cause and mechanism for the formation of altocumulus is not yet firmly established, but it is likely that wind shear, akin to that which produces undulatus cloud formations, is the cause. With a mackerel sky the wind shear produces a more intricate pattern of small waves, which produces the much finer texture of the clouds. The pattern resembles the scales on a mackerel, thus, the name. In Germany and France however they are more frequently known as 'sheep cloud', as their pattern can resemble a flock of sheep, also they are occasionally known as 'buttermilk sky.' And while we are talking about fish...The weather does have a big impact on fishing, so a successful fisherman will need to know how to take advantage of the elements. Although it might be thought that a dry and warm summer's day is the best to go fishing on, and as a family pursuit this is probably the case, but for the hardy sporting fisherman the best weather is actually warm and overcast with a slight wind. Hot weather can make fish sluggish and they tend to steer clear of bait, or just lose their appetite. Therefore it is common to see fishermen huddled under their umbrellas early in the morning, or late at night, when the fish are more active. A gentle wind also breaks the surface and can conceal the fisherman and his line, and often people will fish into the wind and some breeds of fish actually like the wind and will feed more when the surface is broken. There is even a wind based fishing lore - and several derivatives of - “Wind in the East - the Fish bite the least. Wind in the West - the Fish bite the best.” However on the open seas fishing can be dramatically curtailed by a large swell and dangerous seas. Cold weather also brings about problems of its own. Fish tend to go deeper and remain more dormant to prevent heat loss and adjust to the scarcity of food. Sea anglers may find that the fish can be more active in the winter and this is because the changes in the seasons tend to affect the oceans over a longer time period. But most anglers, I am sure, will understand the vagaries of the climate and will know in their own particular favourite fishing spot exactly when is the best time to fish, beneath a mackerel sky or not!
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