(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.
Going much higher up into the region of cirrus, we meet with the most minute and delicate ripple clouds. Some of these have already been referred to. They are connected with either cirro-macula, cirro-cumulus, or cirro-stratus, just as the coarser textured waves we have been considering are connected with alto-cumulus or alto-stratus. In Plate [56] we have an example in which we can see the stages in the process. Nearest to the zenith we have cirro-cumulus, which is here and there irregularly distributed, but is generally arranged in delicate ripples, which are variously curved. Nearer the horizon the troughs of the waves are filled in, and sheets of cirro-stratus are the result. Here, again, the wave-form is evidently not typical. It is an arrangement of either cirro-cumulus or cirro-stratus, produced by the intersection of the plane of condensation by a series of wave movements.
Plate 56.
CIRRO RIPPLES.
(Cirro-cumulus Undatus.)