Fluctus is a cloud feature that curls over in a crest, like a breaking wave. A classic example is the one on the left here, which was spotted by Kelvin Turner over the Macalister Range, just north of Cairns, Australia. The feature is formed by wind shear, where winds above the cloud flow at a very different velocity to those within it. Usually, the winds above flow faster than those below. This has the effect of curling the tops of any cloud undulations over into distinctive vortices that resemble ocean breakers. But sometimes, very rarely, the same fluctus features seem, like on the right here, to happen upside down.

Spotted by Violet Ballard (Member 57,105) over Emmaus, Pennsylvania, US, the undulation in the darker cloud layer in the foreground looks to have been sculpted by the winds into an inverted fluctus feature. The trough of the cloud undulation rather than the crest has been curled back upwards – likely, by the shearing effect of winds below the cloud being slower than those within it. Either way around, fluctus features never last long. Violet thought hers looked like the valley between snow-covered mountains (just out of view here). In no time, she said, ‘the mountains were gone, back into the sky’.