CHAPTER X

PREVALENT CLOUD-FORMS OF THE GLOBE

PRACTICAL SUGGESTIONS

167. It would be an almost impossible task to describe in detail the cloud-forms of the globe, because the manifold effects of very varied distribution of land and water, in conjunction with seasonable changes, cause innumerable complications even over a small area such as that of Western Europe. All that can be attempted in this chapter will be to indicate very roughly a few of the general characteristics of cloud distribution. Any further detailed descriptions would serve no useful purpose.

168. We have already seen that for general purposes the globe may be divided into two polar areas, two wide belts of relatively high pressure traversing it in the middle latitudes, and an equatorial belt of reduced pressure. The belts of high pressure are the sources of supply of two great systems of winds the Trades, which blow in a somewhat westerly direction towards the equatorial belt, and the anti-Trades, which blow in a somewhat easterly direction towards the polar areas. The whole system has a periodic variation dependent on the inclination

of the earth's surface to the sun, and it is further extremely interrupted by the relative position of land and sea surfaces during the different seasons. Where the equatorial belt crosses the two great oceans, the fluctuations of atmospheric pressure and the variations in the direction of the winds are comparatively unimportant, and it is here, and still more over the tradewind regions to the north and south of this zone, that the cloud-forms observed are most permanent. Where, however, the seasonal distribution of pressure between the tropics is considerably modified by the seasonal heating and cooling of land areas, causing the Trades to cross the line and to form the periodic winds called monsoons, great alterations in the character of the clouds necessarily result.

169. Within the belts of high pressure which lie outside the region of the Trades the characteristic forms of cloud vary considerably as we proceed towards the poles, and as we advance into the region of the anti-Trades we encounter those great nonperiodic fluctuations of pressure which are accompanied by non-periodic changes in the cloud-forms. These changes are least marked as we advance into the interior of the great continents, and it is in the middle latitudes of the latter that the greatest seasonal variation exists, and in the least interrupted manner, owing to long periods of heat and cold, which are the effect of the continued action of solar and terrestrial radiation. The non-periodic disturbances of pressure extend far into the polar regions, and even, we infer, to the poles themselves; but in these regions the

variability of the cloud-forms is greatly marked by the great diminution of solar heat, also by the diminution of the inequalities of terrestrial radiation due to the ice-covered or snow-clad surface of the soil, and to a still further extent by the small quantity of water-vapour contained in the atmosphere.

170. If we consider the causes which lead to instability in the atmosphere, it will be obvious to us that if the distribution of islands, continents, seas, mountains, lakes, marshes, and plateaux were fairly equable on the surface of our globe, clouds of Inversion would greatly predominate over the equatorial belt and all intra-tropical districts; for here the sun's heat is strongest, the vapour contained in the atmosphere is greatest in amount, and ascensional currents are naturally formed. Moreover, if we imagined that the great ocean currents did not exist, we should find it difficult to suppose that any great amount of vapour could pass poleward beyond the fiftieth or sixtieth parallel, or that Inversion cloudswould occur on the polar sides of such latitudes. We should also be ready to admit that clouds of Inclination would be fairly common all over the globe, for the upper currents move much more uniformly and are much less influenced by geographical irregularities than those nearer to the earth. should expect to find high clouds of Interfret also fairly common, but we should say that low clouds of Interfret could not occur in the equatorial belt or in the regions of the Trade Winds, because here both the middle and lower currents are flowing towards the

We

equator with ascending components of motion. But these latter clouds would occur over the tropical belt of high pressure and over the polar calms. We must now see how far these suppositions apply under the actual conditions prevalent over the globe.

171. It is an undoubted fact that clouds of Inversion increase more and more in frequency as we approach the warmer regions, and as a consequence of this we also notice that those clouds of Interfret which float at high elevations in the atmosphere are abundant over the same regions. For the upper currents, which pour away aloft out of the great areas in which extensive instabilities, and therefore extensive clouds of the Nimbus type, prevail, run off at considerable angles to the winds in the lower layers of the atmosphere, producing high clouds of intense Interfret in large quantity, especially during the nocturnal hours. Nevertheless, we have seen that over oceanic surfaces, especially at a distance from land, there is a remarkable equability both of temperature and humidity; also in the region of the Trades the winds are blowing from colder towards warmer latitudes, a condition adverse to Inversion movements.

172. It thus results that in the actual Trade Winds over the great oceans only two forms of cloud ordinarily prevail, viz. small scattered Cumulus and Cumulo-rudimentum, at no great altitude above the ocean surface. There are occasional strands of Cirrofilum whose form is regulated by the higher air currents flowing over the Trades out of the super

heated belt. These clouds, which predominate in the Trades, both over the Atlantic and Pacific, are so well known as to have gained the title of Trade Cumulus. It may be asked why such clouds exist at all in the middle portion of the Trade-Wind region. The only reply that can be given is, that very slight inequalities of pressure and of wind with very slight inequalities of humidity, due to causes inappreciable to us, give rise here and there to small Cumulus, whose formation gives rise to further inequalities, and therefore to other local and unimportant Inversion

currents.

173. We notice, however, the influence of land surfaces even at a great distance in a distinction between the Cumulus of the N.E. and that of the S.E. Trades over the Atlantic Ocean. In the former, especially in the higher latitudes over which it blows, there is much more cloud, and this cloud is much more irregularly dispersed. In the latter the clouds are comparatively few, and anything approaching a cloud-bank is of uncommon occurrence, this characteristic being obviously related to the relatively fickle character of the N.E. Trade in respect of force as compared with the S.E. Trade. It is noticeable that there is much less cloud as a whole over the Trade-Wind regions of the Pacific than over similar regions of the Atlantic. This absence of cloud is particularly conspicuous in the central part of the South Pacific over those parallels which are covered by the S.E. Trade, the sky in some parts of this region being almost perennially clear.