Ac undulatus

A spectecular mosaic of various undulations in the altocumulus cloud layer overlooking the Valletta bastions. It is also known as mackarel sky because such clouds resemble the scales of mackerel fish. These clouds preceded the arrival of much colder air for that following weekend represented by a cold front moving southeastwards towards the Maltese Islands as shown by the SLP chart on the fourth thumbnail. The mackerel sky was more spectacular than usual because of the sharp temperature contrast following the cold front (though it did not produce much bad weather) with MAX temperature falling from 18.4C on this day to 12.5C on the following day.

The first thumbnail shows a close-up view of part of the altocumulus. It could also have been Ac perlucidus (meaning small gaps in the cloud cover) especially because as seen in the first thumbnail, the cloud looked more like lumps of cotton wool in the sky. However, I decided to go for undulatus because of two reasons; (i) the first one is that when I looked at it from a wide view undulation were visible and (ii) the second is that a close-up view through satellite imagery of the day (3rd thumbnail) clearly showed that altocumulus undulatus clouds were present around the Maltese Islands. However, the weather sounding (2nd thumbnail) of Friday afternoon indicated little wind shear, if any, at mid-levels hence possibly changing the cloud’s subspecies to perlucidus as it arrived over Malta.

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