Cirrus uncinus – The Punctuation Marks of the Sky

The highest of the ten main cloud types, Cirrus clouds are translucent streaks of falling ice crystals with an often flowing, brush-stroked appearance. Their ice crystals form by a process known as deposition, where water changes directly from gas (water vapour) to solid (ice) without ever forming liquid water droplets like those found in lower clouds. This cloud’s delicate and graceful forms appear when moist air high in the troposphere gently ascends. And they’re not just pretty to look at. There is a distinctive hooked form of Cirrus that indicates a change in weather is on its way.

Cirrus uncinus, spotted here over South Portland, Maine, US by Philip Carlsen (Member 50,125), is the species of Cirrus whose Latin name means ‘hooked’. It’s easy to see why. The overlapping streaks of cloud have curls at one end. They resemble elongated commas – quite different from the solid-looking dots of cloud in Philip’s image, known as Cirrocumulus floccus, that are more like full stops, or periods. Cloudy punctuation marks like Cirrus uncinus reveal the movements of the high winds. Their comma forms show that distinctly faster winds are blowing up where the ice crystals start falling compared to those further below. The abrupt difference in airflows is known as wind shear, and it can be an indicator of weather in store. Cirrus-cloud-forming moisture high in the troposphere and a wind pattern that hooks the clouds like this are signs that a weather front’s coming that could bring rain later in the day.

You’ll want to watch the clouds over time, however, to say whether the cloud commas are significant. They’re more likely forecasting a change in weather if, over time, the Cirrus uncinus are thickening and joining into a continuous diaphanous layer of ice crystals known as Cirrostratus. Your confidence can grow as this cloud in turn thickens into the mid-level layer Altostratus and eventually Nimbostratus, which is the cloud responsible for light-to-moderate precipitation that often lasts for hours.

Next time your sky fills with thickening Cirrus uncinus, take a breath to enjoy the delicate cloud commas. Then watch over time to see if yours do indeed presage a transition from one type of weather to another.

Cirrus uncinus spotted over South Portland, Maine, US by Philip Carlsen (Member 50,125). View this image in the photo gallery.