STRATUS (5). 'Lifted fog' in a horizontal stratum. When this stratum is torn by the wind or by mountain summits into irregular fragments, they may be called fraeto-stratus.
In general, the cirrus, cirro-stratus, and cirro cumulus are the highest and swiftest; the alto stratus, alto - cumulus, stra to - cumulus, and cumulo-nimbus are median; the nimbus, cumu lus, and stratus are lowest and slowest. These three groups are also generally distinctive as to their appearances and methods of formation.
There are some rarer forms of clouds that have received special or local names, such as the following: Phosphorescent, sometimes called iridescent, opalescent, or luminous night clouds. These are seen even at midnight in Europe as distant, pure white clouds, near the horizon.. Measurements appear to show that they are from 10 to 20 miles above sea-level; they may possibly be self-luminous or plios.phorescent, but it is more likely that they shine by reflecting the light from a distant twilight. Hoods, or false cirri, enveloping a mountain-top, or the summit of a cumulus dome. These are formed in the air that is pushing upward over the obstructing mountain. The tablecloth. of Table Mountain at Cape Town is formed somewhat like the hood, but covers the whole flat top of the mountain and hangs clown a little way over the leeward cliffs with frayed edges as the cloud matter evaporates back into invisible moisture.
The helm-Hand and helm-bar are stationary clouds, formed at the summits of standing waves of air. When an east wind blows over the Crossfell Range, in Cumberland, England, there is formed not merely a cloud or hood or helmet with rain at the summit of the range, but a series of undulations to the leeward, one or more of which may rise high enough as a standing wave to form a cloud at its summit: through this cloud. in fact. the wind is blowing, and the cloud-particles formed on the windward side are carried lip through the cloud and down again on the leeward side, evaporating and disappearing as they descend at about the same level as when they were formed on the windward side. Tornad.o cloud and waterspout cloud are the distinctive, cloudlike form that reaches down nearly to the surface of the earth or ocean from the main cloud-mass above, and marks the central axis of a whirling mass of air. Within such a whirl,
the barometric pressure is reduced by reason of centrifugal force, and any air that is drawn inward expands, cools, and forms cloud, just as it would do if it rose upward into regions of lower pressure. When the whirl ceases, the cloud immediately dishppears. 111obo-rirras, cirrus cloud having a globular form, from which stream downward fihres or filaments. as though the particles of the globular mass were being pulled out by the wind, or were settling down by their own weight into air-currents of a differ ent velocity horizontal. Maminato- cumulus, protuberances or pockets on the under side of an otherwise flat-bottomed cloud, as though the heavier portions of the cloud were settling clown in groups from the main cloud. This formation may also sometimes represent the central por tions of adjacent whirls of air rising up into the clouds: but this latter phenomenon, which has been seen by Abbe, is probably not so fre quent as the former.
The study of the movements of the clouds is our principal source of infonhation relative to the general motion of the air at considerable heights above the sea or land. During the past fifty years an increasing amount of attention has been given to this matter, and the use of the nephoscope and photogram-mater has greatly in creased the accuracy of observation. By inter national agreement, special observations were made in many countries in 1896 and 1897, the results of which were published during the years 1898-1901; of these reports the most important is that by Prof. F. E. Bigelow, published in the Annual Report of Prof. Willis L. Moore, chief of the United States Weather Bureau.
The average area covered by clouds, taking the globe as a whole, i, about one-half of its surface. They, therefore, play a very important part in the distribution of solar heat over the earth's surface and within the atmosphere, and the consideration of this influence alone is a very important but difficult problem in the deter mination of the motions of the atmosphere. See DEW; EVAPORATION; XEPIIOSCOPE; RAIN; SNOW.