Weatherwatch: How English summer clouds can warn of trouble ahead | Meteorology
“Mare’s tails and mackerel scales make lofty ships to carry low sails,” runs an old English saying about summer skies.
Mackerel skies are cirrocumulus or altocumulus clouds in regular but patchy rows, resembling the light and dark-scale pattern on a mackerel. The cirrocumulus version is white and wispy, altocumulus is grey and thicker. One easy rule is that cirrocumulus is narrower than a finger at arm’s length, altocumulus more like three fingers.
Mare’s tails are correctly known as Cirrus uncinus or “curly hook”. These are high-altitude clouds formed of ice crystals, with a dense, comma-shaped head trailing a series of fainter, swept-back plumes. They really do resemble a horse’s tail.
Both cloud types signal the imminent arrival of a warm front or a low-pressure cyclonic-storm system. As a warm front advances, it pushes moisture to high altitudes and creates these distinctive patterns. Mackerel skies are the result of turbulence, mare’s tails result from ice crystals being swept out into long faint plumes by variation in wind speed at different altitudes.
The old saying turns out to be accurate. Sailors of tall ships were right to lower the sails, reducing the sail area to avoid being overbalanced by the strong winds when they saw these warning clouds in the summer sky.
