As opacus

Widespread mid-level altostratus clouds ahead of an approaching early Autumn thunderstorm over the Maltese Islands. Such clouds are very typical just before an early Maltese thunderstorm as clearly observed in the visible satellite image of the second thumbnail. These have a blue-grayish appearance and are normally found in the lower range of mid-level clouds and therefore obscure the sun completely. Observing such clouds does not necessarily mean that the approaching thunderstorm is weakening as altostratus does not interfere with the storm`s uptake of humid airmass but rather the result of widespread convection. The clouds underneath are cumulus mediocris. The first thumbnail shows towers of cumulus clouds (cumulus congestus) just after the thunderstorm had passed which is the result of the main instability area. The shown group of thunderstorms were moving northwards in accordance with the steering mid-level wind at an altitude of 5.5km. The cause was a line of bad weather (solid black line) to the south of Malta as per sea-level pressure chart on the fourth thumbnail.

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