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.