Ac undulatus

Formation of altocumulus clouds ahead of a bad weather system that brushed the Maltese Islands. This is evidenced in the fourth thumbnail that showed a cold pocket of air at 500mb height (-20C) just brushing the Maltese Islands and initiating the cloud formation. The third thumbnail shows the surface low pressure and associated fronts passing over the Maltese Islands from Africa but did not produce much rainfall as the sea was at its coolest at this time of year. Despite indicating no clouds, the weather sounding on the second thumbnail showed a lot of wind shear in mid-levels and this is a pre-requisite for undulations as differing wind speed causes the cloud to roll over itself as in the photo. This cloud could have also been higher level stratocumulus undulatus. The first thumbnail shows an advancing mid-level cloud system that followed the pictured cloud formation giving weight to its identification as undulatus. Convective inhibition inhibited low level cloud formations hence only light rain was registered during that upcoming night.

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