Weatherwatch: How English summer clouds can warn of trouble ahead

Cirrocumulus clouds above woodlands in Dunsden, Oxfordshire. Photograph: Geoffrey Swaine/Rex/Shutterstock View image in fullscreen Cirrocumulus clouds above woodlands in Dunsden, Oxfordshire. Photograph: Geoffrey Swaine/Rex/Shutterstock Weatherwatch: How English summer clouds can warn of trouble ahead Mackerel skies and mare’s tails signal arrival of warm fronts that push moisture to high altitudes and creates distinctive clouds “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. View image in fullscreen Altocumulus clouds. Photograph: Dave Porter/AlamyMare’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 . View image in fullscreen Cirrus uncinus, or mare’s tails, in Wokingham, England. Photograph: George Anderson/WMOThe 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 . Explore more on these topicsMeteorology Weatherwatch UK weather England features Share Reuse this content Most viewed Most viewed