An early evening formation of altocumulus stratiformis perlucidus clouds having regularly arranged cloud elements which were aligned with the mid-level WSW wind. These clouds are clearly differentiated from the same species of cirrocumulus ‘Cc stratiformis‘ or stratocumulus through their apparent width being larger than those of cirrocumulus but smaller than those of stratocumulus. However, all of the cloud types are spread out in an extensive horizontal layers. Distinct spacing between the cloud elements was clearly visible making it of the perlucidus variety. The first thumbnail is a panoramic photo of exactly the same cloud for better illustration of what was described.
Such clouds normally indicate a change in the weather either for better if seen as the last clouds following a weather front or as in this case from good weather to more unstable conditions. In fact, these clouds were the harbingers of an upper-level trough as indicated by the SLP on the third thumbnail and confirmed by the visible satellite image on the fourth thumbnail. Althaugh, the weather sounding did not show any fronts, the upper-level trough depicted by the solid black line might have actually been the diving line between colder air to the NW and warmer air to the SE at the 1.5km altitude trigerring the cloud formations – refer to fifth thumbnail. In fact, the following 24 hours after these clouds, rain showers occured the next day when one of the most beautiful sunsets was photographed here. The weather sounding on the second thumbnail demonstrated that the clouds formed at altitudes of between 3.6km and 4.4km. By sunset, this cloud transformed into altocumulus castellanus as photographed here probably rising further up into the atmosphere up to the altitude of 5.5km.