Supercilium – Skybrows Aiming to Dazzle

Moist airstreams flowing over hills or mountains can lead to a range of different cloud formations – many of them dramatic-looking. Clouds like these form where the airflow cools as it rises in a peak over the raised terrain or downwind from it. When wind is flowing in stable atmospheric conditions, a disc-shaped formation, known as a lenticularis cloud, typically appears at the peak of the rising and dipping flow. This is when the airflow rises and dips like a smooth wave. But on rarer occasions, this wave of flowing air tumbles over itself and becomes more turbulent. That is when dynamic and fleeting eyebrow-shaped cloud features can appear – we like to think of them as skybrows – like these spotted by Modestino Carbone (Member 7,416) over Campomaggiore di Montevergine, Mercogliano, Italy.

Skybrows don’t have an official Latin name – at least, not yet. That’s because they’ve yet to be recognised as a cloud type by the people who matter, the World Meteorological Organization. This hasn’t stopped us from proposing a classification name, for we think these cloud features warrant official recognition. We’ve started calling them supercilium, which is Latin for – you guessed it – ‘eyebrow’. Clouds reveal the hidden movements of the air, and none more so than these. They show the chaotic, tumbling, breaking airflows that can develop in the lee of steep mountain peaks.

Modestino’s skybrows are also showing multi-coloured highlights. These pastel hues are the optical effect known as cloud iridescence. They tell us that the cloud droplets are forming only briefly in the turbulent air before evaporating away again, which ensures that they remain tiny and uniform in size. These are the conditions needed for a cloud to form iridescent colours. The sunlight shining through the cloud is separated into soft bands of colour as it is diffracted, or scattered, by the tiny, regularly sized droplets.

Hopefully, clouds like Modestino’s iridescent formations will one day be officially recognised with a Latin classification like our proposed ‘supercilium’. In the meantime, we’ll have to refer to them as party-loving skybrows.

Altocumulus ‘supercilium’ showing cloud iridescence spotted over Campomaggiore di Montevergine, Mercogliano, Italy by Modestino Carbone (Member 7,416). View it in the photo gallery.