An old English saying, “Mare’s tails and mackerel scales make lofty ships to carry low sails,” highlights a long-held understanding of summer sky patterns as indicators of impending weather shifts. This traditional wisdom about distinctive cloud formations, now supported by meteorological science, continues to offer valuable clues about the UK's often unpredictable summer weather.
Mackerel skies typically refer to either cirrocumulus or altocumulus clouds. Cirrocumulus clouds are high, white, and wispy, appearing in small, regular, patchy rows, resembling the scaled pattern of a mackerel. Altocumulus clouds are similar but thicker and grey, forming at lower altitudes. A simple guide to differentiate them is that cirrocumulus patches appear narrower than a finger held at arm's length, while altocumulus patches look wider, roughly three fingers.
Mare’s tails, scientifically known as Cirrus uncinus, are high-altitude clouds composed of ice crystals. They are characterised by a dense, comma-shaped head followed by fainter, swept-back plumes, genuinely resembling a horse's tail. These elegant formations are a result of ice crystals being stretched into long, delicate streamers by varying wind speeds at different altitudes.
Both mackerel skies and mare’s tails are significant indicators of an approaching warm front or a low-pressure cyclonic-storm system. As a warm front advances, it pushes moisture high into the atmosphere, creating the specific conditions for these distinctive cloud patterns to form. Mackerel skies are a product of atmospheric turbulence, while mare’s tails are formed by the dispersion of ice crystals in varying wind conditions.
Historically, these cloud signals were crucial for mariners, who would heed the warning by lowering their sails to prepare for the strong winds that often accompanied these weather systems, preventing their tall ships from being overbalanced. This age-old observation remains accurate today, providing an early alert for potential changes in wind and precipitation across the UK.