Cb arcus

This is a rare panoramic photo of a shelf cloud with a fluorescent green background as a MCS brushed over the Maltese Islands. The weather situation is explained in full detail here and actually this was the final result of how the cumulonimbus clouds had developed over our Islands starting from the photographed cloud in the previous page which we are linking to. Elderly people commented that today’s thunderstorm was “one of the most beautiful and colourful storms in recent years”. They added that a similar storm occured in the 1970s when the sky became completely green. From the weather sounding, I am sure that this cloud was at least 7.3km thick with a base of at most 3.8km as indicated by the sounding though it looked like having a lower base.

A 1993 study from Penn State University contends that for such photographed green cloud base, there simply needs to be a strong thunderstorm with a large volume of precipitation and the right alignment of the sun and thunderstorm to turn the sky green, in Malta’s case the low morning sun angle to the SE shining over the thunderstorm cloud that was exactly opposite to the NE. Researchers calculated hail’s contribution to the green color was actually small, althaugh waterspouts and small hail showers were reported around the Maltese Islands.

The arcus formation in the cumulonimbus clouds at the edge of the shelf was because of a cold air stream (downdraft) from heavy precipitation in the background. This cold downdraft sinks and spreads out preventing rising warmer air from being pulled into the storm cloud’s updraft. The storm’s gusty winds buffeted the whole cumulonimbus cloud giving it this rolling effect feature. When such clouds are spotted, it normally means that a severe thunderstorm is lurking somewhere in the vicinity.

The following thumbnails pick up where we left in the previous photo with the cumulonimbus capillatus giving way to the formation of a shelf cloud as seen in the first thumbnail when the storm clouds seemed to have started rotating. The second thumbnail shows that the cumulonimbus cloud have developed further and became heavy acquiring slowly the green colouration and the third thumbnail showing a part of the same cloud later being buffetted by strong downdraft winds as it began producing heavy rainfall with the final being the main photo taken just seconds before the heavy rain started in the area blurring the right side of the photo and closing the opportunity for more storm photos.

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