Alto-stratus does not often, if ever, grow from the fusion of the cloudlets of the maculosus type. But it does come from alto-cumulus glomeratus, and also from a form shown in Plate 32. Here we have alto-stratus in process of growth. Small irregular lumps of cloud forming on the right-hand side of the picture grow larger and more irregular, begin to fuse together towards the centre, and on the left-hand side the fusion is almost complete. Still, although the sky is covered with cloud, the lumpy form is plainly visible. The term “alto-strato-cumulus” is suitable, as it differs from the more frequent and much lower cloud, which will be described further on as strato-cumulus, in little else than altitude and general massiveness of texture. This high strato-cumulus is common enough, too common, indeed, in England, as it produces many a dull grey sky both in summer and in winter. In the latter season it is not unfrequent with the cold east winds of February and March. It is probably the lowest of the alto clouds; the lowest measurement made by the writer being 1828 metres at Exeter, but lower altitudes seem to have been recorded elsewhere. [Illustration: Plate 32. ALTO-STRATO-CUMULUS.] Alto-cumulus castellatus, which is breaking up and disappearing, is shown in Plate 33. It was photographed with a long-focus lens, so that the scale of representation is about eight times as great as that of Plate 27. This view was taken at Exeter while a thunderstorm was in progress at Bristol. [Illustration: Plate 33. SUNSET. (_Alto-cumulus Castellatus Fractus._)] CHAPTER V LOWER CLOUDS THE clouds of the lower portions of the atmosphere are formed in regions where water vapour is abundant, and frequently in easy reach of the strong ascending and descending air currents produced by the varying temperatures and irregular surface of the ground. It is sufficient to recite these conditions to show that these lower clouds will be denser, larger, coarser in texture, and characterized by greater definiteness of form than those we have, so far, considered. In the International system they are classified thus-- Group C. Lower clouds. (_a_) Strato-cumulus. (_b_) Nimbus. Group D. Clouds of diurnal ascending currents. Cumulus and cumulo-nimbus. Group E. High fogs. Stratus.