Formation of cirrus at jet-stream level with an an altitude of around 10km. Such cloud indicate a change in the weather as actually happened on the following morning when a mass of altocumulus clouds passed over the Maltese Islands producing some rain in places. Such cloud is specifically formed when air moves parallel to the main high cloud whereby the `ribs` correspond with rising air and the clear gaps with descending air. In this case, a small trough was approaching Malta from the west as can be seen in the jet-stream chart (3rd thumbnail). The first thumbnail shows a close-up structure of this particular vertebratus cloud.