Strato-cumulus often lasts for hours, with little or no perceptible change, but stratus maculosus rarely persists for more than half an hour. The first is a cloud of fairly stable conditions, the latter is dependent for its existence upon the near approach to critical conditions at one particular level, and, as we have said in other cases, such a critical state is almost always soon passed, with the result that the cloud either masses into a denser form, or else breaks up and disappears. If the up and down currents are strong enough to persist, the result will be strato-cumulus and not stratus maculosus.
A kind of stratus which is frequently seen in the daytime is shown in Plate [41]. This is literally a lifted fog, having been formed about midday, after ground fog in the early morning. It would be called common stratus, or stratus communis. When it appears it is a fairly persistent form, sometimes breaking up or swelling up into strato-cumulus, but more often splitting into long rolls of cloud, with margins like those of cumulus. This phenomenon is shown in Plate [42], which was taken in December at 11 a.m., on a day which opened with a thick ground fog. A precisely similar cloud is frequent in the early hours of a summer morning, as a stage in the dispersal of a radiation ground fog. The fog first lifts from the ground, until it reaches a height of a few hundred metres, when it splits into the long rolls whose axes are at right angles to the direction of drift. The consequence is very strange if you stand on a hilltop close under the drifting mass, and look towards the horizon in the direction of drift. The changing shadows give the impression that the clouds are actually rolling along, though of course no such thing is really taking place. As time goes on the rolls grow larger and the interspaces wider; then transverse fissures appear, and gradually the rolls break up into small detached cumulus. Cumulus radius, from the Latin for a rolling-pin, might be a suitably descriptive name, but it should not be forgotten that it is only an intermediate link between stratus and cumulus, and, indeed, is more nearly related to the former, since it is never produced except on the break up of stratus, while it may dry up and disappear without reaching the cumulus stage at all. Stratus radius would therefore be a better name.
Plate 41.
COMMON STRATUS.
(Stratus Communis.)
Plate 41.
COMMON STRATUS.