The Eyebrow Cloud That’s Waiting to be Made Official

This cloud formation should have a Latin name, but it is yet to be classed as an official cloud type. It can appear in the turbulent airflows downwind of mountain peaks, and we think it looks like eyebrows in the sky.

This would be a good cloud to be added to the list of types because it has a distinctive appearance that’s easily differentiated from other formations. Also, pilots would likely want it to have a name for practical reasons: so that they can learn to stay well away from it. This cloud reveals where turbulence in the mountain airflow is particularly chaotic and violent, which is where no glider pilot wants to fly.

The turbulence develops as part of the rising and dipping flow of air as winds pass over mountains. Much of the airflow is smooth, rising to pass over the peak and dipping back down again beyond, like water flowing over a rock in a stream. But just as the water flowing in a stream can break and foam at a particular point beyond the obstacle, so can the wave of the airflow break at a particular point downwind of the mountain peak. Where this happens, if it does, depends on the shape of the terrain and the speed of the wind. Often the breaking wave of air is invisible. Sometimes, it produces a churning, roll-like cloud described by pilots as a rotor cloud. Sometimes, when the air tumbles over itself, it makes the distinctive shape of eyebrow clouds.

We’ve had examples of this unnamed formation sent in by members from around the world, including examples over the Sierra Nevada of California, US, the mighty Himalayas of Nepal, and the alpine peaks of Switzerland like The Eiger mountain and The Jungfrau. We even have a Latin name in mind for it. This was suggested to us by Latin scholar Rick LaFleur, Franklin Professor of Classics Emeritus at the University of Georgia, US when we asked him how the Romans would have referred to an eyebrow. Rick suggested the term supercilium, which is Latin for ‘eyebrow’.

It’s been a few years since the Cloud Appreciation Society last argued that the official naming system for clouds should have a new classification of cloud added to it. The chaotic, wavy-looking asperitas cloud was eventually accepted as a new cloud type by the World Meteorological Organisation (WMO) after we’d argued the case for it to be given a name. Asperitas was added to the WMO’s official reference work The International Cloud Atlas back in March 2017. That was exactly five years ago this month. Perhaps it’s time to start a new cloud-classification campaign and raise some eyebrows with the supercilium cloud?

Altocumulus ‘supercilium’ spotted by app user ‘Cielo’ over the Sangre de Cristo Mountains of New Mexico, US.

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Anyone for night Cloudspotting?

A mysterious, rippling veil of gossamer-like clouds hangs high in the sky over the Netherlands. Forming right up at the fringes of Space, these clouds are the very highest in our atmosphere. They’re known as noctilucent clouds, and they form in the mesosphere at altitudes of around 50 miles (85 kilometres), which is far above the troposphere – the part where usual ‘weather’ clouds form, in the lower 10 miles or so (15 kilometers). It is the extreme altitude of noctilucent clouds that ensures they are the only type of cloud you can spot in the middle of the night even when there’s no Moon to light them. Even when the Sun is well below the horizon and rest of the sky is in the Earth’s shadow, these super-high clouds still catch its light, shining out in neon blue hues against the darkened sky. This is why their name means ‘night-shining’ in Latin.

Now is the start of the season for spotting noctilucent clouds in the Northern Hemisphere because they only form during the summer. June always seems to be a good month for them. This is when their ice crystals are more likely to appear because, counterintuitively, summer is when the mesosphere is at its coldest. When temperatures are warmer down on the ground, the lower atmosphere is warmed too and the troposphere expands, causing it to push the upper atmospheric layers upwards, which leads to cooling of the mesosphere. Typically, you can only spot noctilucent clouds from latitudes (whether in the Northern or Southern Hemisphere) that are higher than 50 degrees latitude. This explains their more formal name, polar mesospheric clouds. Any higher than about 70 degrees, however, and the summer sky will never become dark enough for their subtle forms to be visible.

If you are at the right latitude during the right time of year, look out when the lower sky is clear for these ghostly streaks of pale blue shining among the stars, late in the night or in the early hours. Who said cloudspotting needs to end when the Sun goes down?

Noctilucent clouds spotted over Katwijk, Netherlands by Mark McCaughrean (Member 32,672).

Have you spotted a Noctilucent Cloud?

If you’ve managed to photograph one of these extremely high night-shining clouds, why not contribute your image to a new Noctilucent Cloud Showcase that we’re creating in collaboration with our friends at Go Stargazing? We plan to bring together the best noctiulcent cloud images as a new resource for nighttime cloudspotters. 

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