A low example is given in Plate 40, which represents stratus maculosus, and which has already been described. A higher type is shown in Plate 54, which is a wave-like arrangement of alto-cumulus. Rather higher come the long zig-zag bands of Plate 55, in which the stratiform arrangement is more obvious, and which would be best described as a wave-form of alto-stratus. These two plates form striking contrasts. The clouds shown in the first are distinctly of the cumulus order, and a prominent feature is the way in which the right-hand side of each wave has a clear-cut rounded contour like that of the upper edge of a small cumulus, while the left-hand edge of each band is frayed out into a ragged fringe. The whole cloud was moving slowly in a direction nearly, but not quite, at right angles to the waves, and the fringed edge formed the rear. It is evident that this peculiar structure must be due to a series of narrow waves intersecting a plane in which the air is just on the point of producing alto-cumulus. If there were no such waves, the little uprising currents, with their intervening down currents, would be irregularly distributed, and all the wave disturbances have had to do is to arrange them. The consequence is that as the waves pass along the stratum the air is alternately raised and lowered. Where it is rising condensation takes place, where it is falling evaporation results. [Illustration: Plate 54. CRESTED ALTO WAVES. (_Alto-cumulus Undatus._)] [Illustration: Plate 55. ALTO WAVES. (_Alto-stratus Undatus._)] The cloud, like most other wave clouds, did not retain its features for any length of time, but the gaps closed slowly in as the cloud-bands increased in size, until a sheet of alto-stratus was produced. Since the time of day was the morning, it is almost certain that the plane of saturation was rising in accordance with the general law, which is that the planes of condensation rise steadily, until about two or three o’clock in the afternoon, and then slowly descend. In Plate 55 each band is much flatter and less dense. They are just as evidently formed by wave movements intersecting the plane of condensation; but this was formed in the evening when the sun was nearing the horizon, and at a time when the cloud planes are as a rule rapidly descending. Among the alto clouds wave-forms sometimes persist for a fairly long time, and in this case the bands moved steadily onward in a direction equally inclined to their length and breadth, that is to say, from the bottom left-hand corner of the photograph to the top right-hand corner. As they passed across the sky new bands kept on making their appearance at about the same spot, each band persisting with little change until it had passed out of sight.