Widespread formation of quite deep altocumulus clouds in a very calm airmass probably due to convection caused by diverging wind fields in the upper levels of the atmosphere as indicated by the jet stream winds. The atmosphere was very humid but only slightly unstable. To the south of Malta, embedded cumulonimbus clouds were observed producing localized heavy rainfall due to the very slow movement but no severe weather. The first thumbnail shows stratus of bad weather due to evaporating rain in a very moist environment. This upper level disturbance was indicated as a trough (solid black line to the east of Malta) in the surface pressure chart.