Formation of widespread altocumulus stratiformis opacus mammatus on the edges of a thunderstorm that developed over the Gulf of Gabes and was approaching the Maltese Islands whilst weakening. The mammatus formation was very clear of a sinking air motion following the thunderstorm’s spreading out as per the satellite image of the fourth thumbnail.

The first thumbnail shows the initial formations of mammatus cloud feautures on the approaching altocumulus whilst the second thumbnail photo is a more widespread formation of such clouds literally above the observer’s head and very widespread. These clouds were then followed by a sudden increase of wind from the SE gusting up to 82kmh (Force 9) at the Luqa Meteorological Airport. Higher wind gusts of over 100kmh were reported by various private weather stations in the north of Malta and Gozo and seem to be corraborated with the occuring wind damage vide the article https://www.tvm.com.mt/en/news/strong-winds-cause-danger-on-the-roads-and-some-areas-end-up-without-power/ also due to the fact that such wind was not accompanied by rain adding more force to it.

From the weather sounding on the third thumbnail, it seems that the Maltese Islands were hit by a dry microburst as only very isolated little rain showers occured. A subsidence inversion was noticed at an altitude of 963 metres along with a very dry airmass at the same altitude with just 18% humidity. At altitudes of 4km, isolated altocumulus clouds were indicated but the overcast cloud base could have been at very high altitudes of 7km classyfying them as cirrocumulus instead. From such cloud height and with a very dry layer of around 3km or more underneath it, rain falling from such an upper-level thunderstorm would quickly evaporate into the dry air causing the rain-cooled airmass to sink towards the ground like a stone whilst becoming hotter and drier due to adiabatic compression following its spreading outwards away from the rain shaft in severe wind gusts.

One must stress that whilst mammatus cloud formations may be indicators of downdrafts especially when widespread as was in this case, they generally do not harbinger strong winds. In fact for most of the time, such clouds (particularly when isolated) pass harmlessly away without causing changes in the surface wind or weather in general. The fifth thumbnail is the surface pressure chart taken at midnight of 9th April indicating that these clouds were associated with a solid black line of bad weather that had just passed over our Islands in association with a heat low pressure system rising from over North Africa.