‘MARE’S TAILS and MACKEREL SCALES make tall ships carry low sails’

Ancient travelers new this phrase well, it was a warning heeded by sailors throughout the ages. But the mackerel scales referred to are not fishy ones, and nor do the mare’s tails anything to do with horses. Both, in fact, are nicknames for clouds.

Mare’s tails are cirrus clouds found high in the atmosphere, which are pulled into long streamers resembling the tail of a horse. Mackerel scales are altocumulus clouds which look like fish scales. Both cloud types are present just before bad weather, so if a sailor noticed these formations he would know that within 12 to 36 hours the weather would be too rough to be out on the open water – so he would lower his sails and head for dry land.

What is a Cirrus Cloud?

The name Cirrus (Ci) derives from the Latin cirrus meaning curl of hair, tuft or wisp. Cirrus clouds are high-altitude cloud formations occurring between about 5 to 13km (16,600 to 40,000ft). They are thin featherlike white, silky patches or fine, narrow bands. Shaped by strong winds in the upper atmosphere they may be curved, hooked, fairly straight or randomly entangled. They may be appearing grey when dense and seen against the light, and yellow, orange, pink, purple and reddish when illuminated by the lowering sun.

Cirrus are composed of minute ice crystals, in regions where air temperature is lower than -20°C or -30°C. They may be caused by turbulence and wind shear, or by upper-tropospheric convection. Sometimes they are just blown out ice-crystals spreading from the top of a waning storm cloud. Nowadays another method of cirrus formation is from the condensation trails of aircraft, often persisting for hours and spreading to cover large portions of the sky.

The Sailors Had It Half Right

Cirrus often indicate the leading edge of a warm front and degrading weather conditions, especially if they are spreading out from the west or south-west. However, cirrus can also be fair weather clouds. If they appear irregular and patchy, slowly shifting from the east, dissolving, they are indicators of increasing high pressure and dry, sunny and weather.

Relying on cloud-type lore, ancient sailor’s weather predictions were only right about half the time. With modern technology it seems ironic that the accuracy of forecasts hasn’t improved much since antiquity. For those #WARHorse trail riders who head out on a long ride only after consulting a weather app you might confirm that forecast with a glance upward. And if you spot any mare’s tails overhead, be certain to pack rain gear in your saddle bag – just in case.