This form of the high ice-crystal cloud Cirrus is named from the Latin for ‘having vertebrae’ because its icy streaks fan out like ribs from a central spine to resemble the skeleton of a fish.

The cloud forms with the winds up at cloud level flowing down the length of its spine. The cloud’s ice crystals likely fall through ‘shearing’ winds – which is when wind velocity varies markedly with altitude – and they typically fall from faster into slower wind speeds. The air that the ice crystals fall into below likely contains more moisture. This would account for why the cloud streaks spread outwards to form the branching ribs of the cloudy skeleton. As they fall and trail behind in the slower winds, the ice crystals act as catalysts for the formation of more ice, and so they become more plentiful within the lower, moisture airflows. In other words, the falling ice crystals encourage more crystals to form as they fall. This combined effect of the falling crystals trailing behind and spreading out likely gives the formation of Cirrus its distinctive skeleton appearance. The atmospheric conditions needed for vertebratus to develop mean that it is fairly rare.

Similar formations can develop a little more frequently from aircraft condensation trails. Here, the spine of the cloud begins as a contrail that happens to be aligned to the flow of the high winds, and whose water droplets freeze into ice crystals that fall below into slower, moister airflows. When a vertebratus formation develops from a manmade cloud like this, it is classed as Cirrus vertebratus homogenitus.

Image: Spotted over over the Catalina mountains, Tucson, Arizona, US by Richard Friedman.

Uncinus is a species of Cirrus where the streaks of the high cloud have a hooked appearance at one end. To be a true uncinus, these hooks should look like the handles of walking sticks, and should not take the form of rounded tufts or clumps. Those would be more likely to be the floccus form of Cirrus and trails of virga falling from Altocumulus.

Described in maritime folklore as ‘mare’s tails’, uncinus have traditionally been associated with the arrival of unsettled weather. There is some truth to the phrase ‘mare’s tails and mackerel scales make tall ships carry low sails.’ The hooked ends of these clouds indicate that strong winds up at the high-cloud level vary greatly with altitude – being much faster above the cloud than they are below. Such ‘shearing’ wind conditions do indeed herald the arrival of a weather front.

Before the start of the First World War and the advent of high-altitude flight, our skies appeared very different from the way they do today – there were no condensation trails, or contrails, which form in the exhaust of aircraft.

There’s no confusing these man-made clouds with the natural ones. Following the aircraft’s path, contrails tend to appear as long, straight slashes of white across the blue. In the vicinity of airports, however, they can sometimes form large loops, due to the stacking formation of aircraft waiting to land.

The length of time contrails remain in the sky – or indeed whether they form at all – varies greatly depending on the air conditions up at cruising altitude. When it’s cold enough and moist enough, the water vapour contained in the plane’s hot exhaust gases mixes with the very cold air to condense and form ice crystals. In some conditions, these soon evaporate. In others, they can persist for hours, the ice crystals absorbing water vapour from the surrounding air to grow in size and spread out in the high winds. In this way, contrails often encourage the formation of Cirrus, Cirrocumulus and Cirrostratus ice-crystal clouds.