9th September 2005

Altocumulus Mackerel Sky

Source: Excerpt from The Book " Weather "

Altocumulus Mackerel Sky

  • Distribution: Worldwide.

  • Height: 6500 to 16,500 feet.

  • Cause: Lifting of a large air mass, followed by condensation combined with instability and wind shear.

  • 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.


  • Acknowledgement due: John W. Zillman, William J. Burroughs,
    Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.


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