Associated Weather: May indicate an approaching frontal system.
The mackerel sky variation of altocumulus is named for its resemblance to the scales of a fish. It is almost certain that the name originated among early mariners, and it may have considerable antiquity.
As with altocumulus formations in general, mackerel sky is
produced by the lifting of a large, moist air mass, usually by an approaching cold front, combined with instability at cloud level. The exact causes of this pattern have not been firmly
established, but it is likely that a form of wind shear, similar to that which produces undulatus formations, is the cause. In this variation, the wind shear gives rise to a more intricate pattern of small waves, which produces the much finer texture of mackerel sky.
Because it is often created by an approaching frontal system,
mackerel sky has long been associated in folklore with
deteriorating weather conditions. More often than not, this
formation, like other middle-level clouds, is a good indicator
of changing weather, although, as in all such cases, the front
may pass some distance away from the observer, resulting in
little change in local conditions.