An old English saying warns: “Mare’s tails and mackerel scales make lofty ships to carry low sails.” In other words, if the sky looks like a fish and a horse had a baby, you might want to batten down the hatches.

Mackerel skies - cirrocumulus or altocumulus clouds arranged in patchy rows resembling the scales of, you guessed it, a mackerel - come in two flavors: white and wispy (cirrocumulus, narrower than a finger at arm’s length) or grey and thicker (altocumulus, more like three fingers). Mare’s tails, technically Cirrus uncinus or “curly hook,” are high-altitude ice crystals with a comma-shaped head and swept-back plumes. They look exactly like a horse’s tail, because nature has a sense of humor.

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, creating these distinctive patterns. Mackerel skies result from turbulence; mare’s tails form when ice crystals are swept into long faint plumes by varying wind speeds at different altitudes.

Turns out, the old saying is accurate. Sailors of tall ships were right to lower sails and reduce sail area to avoid being overbalanced by strong winds. So next time you see a mackerel sky or a mare’s tail, you can impress your friends with your nautical wisdom - and maybe take in the laundry.