A Mystery in the Clouds
Earlier this month, a peculiar ring appeared in the clouds over Warwickshire, England. James Tromans, who photographed the formation, asked what might have caused it. Clearly, this was time for some cloud detective work.
The ring was actually more of a curve, or a ‘U’, as there was no other half to it. It appeared to consist of regular lumps, or lobes, hanging down from the underside of a cloud layer. We wondered if this layer was an Altostratus, but off in the distance to the right of the image there appeared to be showers, like those produced by a storm cloud. This suggested that the cloud layer might in fact have been part of the huge canopy that spreads out at the top of a Cumulonimbus storm cloud.
Sometimes, to the rear of a storm, lobes of cloud known as mamma can be seen hanging from the underside of the canopy (see image 1). There certainly was a general appearance of mamma in the cloud layer. But could some of these mamma cloud lobes have arranged themselves into this strange, regular curved formation? It seemed unlikely.
There was something decidedly unnatural about the cloud’s appearance – as if it were man-made. Then it occurred to us that the photograph was looking towards Coventry Airport. Might this cloud effect have been in some way caused by an aircraft?
As planes fly through clouds made of ‘supercooled’ water droplets, they can encourage these extremely cold droplets to freeze and fall below, leaving behind a gap, known as a dissipation trail, or ‘distrail’. Such formations are often seen in straight lines where planes ascend or descend through the cloud. But they can also appear in circular shapes when one is flying in a holding pattern as it waits to land (see image 2). James’s cloud wasn’t quite a distrail, but we felt we were getting close.
It was then that we recognised the regular spacing of the lobes. These sometimes appear below aircraft condensation trails (see image 3). They are caused by the interaction between the two swirling vortices produced by the wings. As these rotate in opposite directions in the wake of an aircraft, the two turbulent flows interact and combine to form a periodic pattern of turbulent downdrafts. In the right conditions, they appear as lobes hanging below the condensation trail.
This explained the formation. It was caused by an aircraft, which happened to be flying just above the base of the cloud layer as it turned in a holding patternabove the nearby airport. The plane’s condensation trail was hidden within the cloud layer, but the lobes of cloud descending below it, caused by the turbulence from its wings, appeared extending below the layer.
Phew! With that cleared up, we could finally relax once more.
Turbulence lobes beneath the contrail of an aircraft in holding pattern over Hampton Lucy, Warwickshire, UK, by James Tromans.
A Response from our Poet in Residence
Katharine Towers, poet in residence at the Cloud Appreciation Society, has written the third of her Cloud Studies inspired by our Clouds of the Month. Here is her commentary on it:
It was quite a challenge to write a poem about a condensation trail! I was keen to avoid getting involved with the technicalities of ice droplets and downdrafts, as such poems can easily be side-tracked into just showcasing unusual vocabulary.
What eventually seemed interesting was the idea that we can’t help trying to interpret natural phenomena – or seeing things we don’t understand in terms of things we can describe. The use of the word ‘U-turn’ in connection with the unusual circular contrail gave me a way into the poem.
Cloud Study III
A long-drawn breath that won’t last
or an after-thought spelled out
in ice and heat and fall-streaks
or that hush that follows pain;
what’s left when everything has been said.
Hard to believe they mean nothing –
yes, even this one
shaped like a change of heart.
© Katharine Towers, September 2016