The photographed cirrus castellanus cloud is the rarest form of cirrus. It developed over Malta along with cumulus clouds due to a shortwave or ripple at 500mb height (refer to the third thumbnail chart). It formed at a height of 8.4km and rounded turrents could be clearly seen at the top of the cloud when zoomed in (first thumbnail) helping the observer to distinguish it from other cloud types. It seems that such clouds form due to a sharp drop in temperature as the cloud rises higher up in the upper atmosphere or due to the release of heat from condensation.