photo #5132908

Kitty McCoy (Cloud Appreciation Society Member 52,344) spotted this mackerel sky over Fairfax, Virginia, US. The richly textured Cirrocumulus cloud layer is a high cloud that forms at altitudes of 6-12 thousand metres (20-40 thousand feet). Its grainy texture results from this cloud’s thousands of tiny cloudlets, cloud clumps composed, typically, of a mixture of supercooled water droplets and ice crystals. They look small because they are a long way up. Some are arranged into stripes, known as the cloud variety undulatus, which is where the mackerel name comes from due to the similarity to the fish’s stripes. In maritime folklore, mackerel skies have long been associated with the arrival of unsettled weather. We don’t know if a change in conditions followed for Kitty, but this formation’s forecasting credentials are generally quite good. Its undulatus ridges suggest shearing winds up at its high altitudes. These, along with the arrival of moister air up there, demonstrated by the cloud spreading across the sky, do tend to happen ahead of the arrival of a weather front that brings wind and precipitation.