Ac opacus

The photographed cloud is “altocumulus stratiformis opacus mamma”. It was classified as altocumulus due to upper-level instability which will be explained below. The panoramic photo shows one whole extensive cloud cover hence the species ‘startiformis’ and of the ‘opacus’ variety because it was thick enough to hide the sun completely and its base was rather uneven. In fact, it had mammatus features showing that some of the air parcles inside the cloud was actually sinking, perhaps a sign that precipitation was falling in the base pushing the air downwards.

The weather sounding on the first thumbnail showed lots of moisture and upper-level instability but that alone is not enough for clouds to form. In this case, the trigger was an upper-level cold front passing over the Maltese Islands in the morning as per second thumbnail showing the surface pressure chart. The following thumbnails show the satellite image linking the clouds with the said front and an unseasonably strong jet stream over the Islands which probably pushed the upper-level cold front towards them. From the weather sounding, the cloud height was estimated at between 5.7km and 7.7km being around 2km thick. Lower-level clouds were inhibited due to thermal inversion leading to an unbreakable surface inhibition and also because the cold front and instability were only present at upper-levels.

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