This is certainly the least satisfactory part of the whole scheme, and it is not at all easy to see upon what grounds it was adopted by the International Committee. Group D--cumulus and cumulo-nimbus--do show important differences from the other groups, though it is often difficult to say whether the sky should be described as covered with strato-cumulus or as covered with numerous small cumulus. It is the separation of stratus--placing it in a group by itself, and making that the lowest--which is the worst point. As a matter of fact, stratus may exist at any altitude from sea-level up to such heights that we should not hesitate to call it alto-stratus. Indeed, there is no essential character of alto-stratus which distinguishes it from some of the lower forms. Whatever its altitude, its thickness and the size of its particles may vary in a precisely similar manner. We may have the particles exceedingly small, when the fog will be dry, and such a stratus may be so thin as hardly to dim the sun; or it may be so thick as to completely hide it. On the other hand, the fog may consist of particles easily visible to the naked eye, forming the so-called Scotch mist, or the “dry” fog of Dartmoor, which will wet things as rapidly and more thoroughly than a smart shower. When such a fog accumulates to a sufficient depth, the particles in their fall pick up others, and the result is a distinct fine rain. This may occur not only near the ground, but at almost any level below that at which the cloud would pass into the region of cirrus. Plate 34 shows three layers of stratus, in each case much broken up. The highest layer is a good example of alto-stratus maculosus. Lower down, by half a mile or more, come parts of a grey sheet considerably denser and thicker. It is a matter of taste whether this should be called high stratus or low alto-stratus. There is no test by which the one can be distinguished from the other. Lower again come the detached darker clouds, which are fragments of a sheet of stratus which is breaking up and disappearing. The photograph was taken in the afternoon, after a wet morning, and all three layers were probably relics of the great rain-cloud system, or nimbus, which produced the rainfall. [Illustration: Plate 34. THREE LAYERS OF STRATIFORM CLOUD AFTER RAIN.]