Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/137

Rh CLOUD.] METEOROLOGY 127 expansion of the air of ascending currents, the mixing of cold air with air that is warm and moist, and the cooling of the air in contact with the surface of the earth when its temperature has been lowered by nocturnal radiation. The forms of clouds are endless. Since clouds are subject to certain distinct modifications from the same causes which produce other atmospheric phenomena, the face of the sky may be regarded as indicating the operation of these causes, just as the face of man indicates his mental and physical states. Hence the importance of the study of clouds, and hence the necessity of a nomenclature of clouds as the basis of accurate and comparable observations. An adequate nomenclature of clouds is still a desideratum. Luke Howard s classification, which continues to hold its ground as a provisional nomenclature, was proposed by him in 1803, and by it clouds are considered as divided into seven kinds. Of these, three are simple forms, the cirrus, the cumulus, and the stratus ; and four intermediate or compound, the cirro-cumulus, the cirro-stratus, the cumulo-stratus, and the cumulo-cirro-stratus, nimbus, or rain cloud. The cirrus cloud consists of wavy, parallel, or divergent filaments, which may increase in any or all directions. It is the cloud of the least density, the greatest elevation, and the greatest variety of figure. It is probable that the particles composing it are minute crystals of ice or snow- flakes. The cirrus is intimately connected with the great movements of the atmosphere ; and it is solely from the movements of the cirrus that we have any direct know ledge of the upper currents of the atmosphere. In recent years much has been done, particularly by Professor Hildebrandsson of Upsala and Clement Ley, in investi gating the relations of this cloud to storms and other changes of weather. The cumulus is the name applied to those convex or conical heaps of clouds which increase upwards from a horizontal base. They are generally of a very dense structure, are formed in the lower regions of the atmo sphere, and are carried along by the aerial current next the earth. They form the tops of the ascending currents which rise from the heated ground, and have a diurnal period so well marked that they are often named the &quot;cloud of the day.&quot; The form of stratus comprehends those mists and fogs which in the calm evening of a warm summer day make their appearance in the bottom of valleys and over low-lying grounds, and sometimes spread upwards over the surrounding country like an inundation ; they have an equally well marked daily period, and are frequently called the &quot;cloud of night.&quot; The cirro- cumulus is made up of small roundish masses, lying near each other, and quite separated by intervals of sky. It may be considered as formed from the cirrus by the fibres of that cloud breaking, as it were, and collapsing into roundish masses, thus destroying the texture but retaining the arrangement of that cloud. This singularly beautiful cloud is commonly known as a mackerel sky, and is of most frequent occurrence during dry warm summer weather. The cirro-stratus consists of horizontal masses thinned towards the circumference, bent downwards or undulat ing, and either separate or in groups. Since this cloud has great extent and continuity of substance, but little perpendicular depth or thickness, it is the cloud which most frequently fulfils the conditions for the phenomena of coronae, solar and lunar halos, parhelia or mock suns, and paraselene or mock moons. The cumulo-stratus is formed by the cirro-stratus blending with the cumulus, or spreading underneath it as a horizontal layer of vapour. The cumulo-cirro-stratus, or nimbus, is the well-known rain-cloud, which consists of a cloud or system of clouds from which rain is falling. At a considerable height a sheet of cirro-stratus cloud is extended, under which cumulus clouds drift from windward ; these rapidly in creasing unite and appear to form one continuous grey mass from which the rain falls. The breaking up of the lower grey mass indicates that the rain will soon cease. When a rain-cloud is seen at a distance, cirri appear to shoot out from its top in all directions ; and it is observed that the more copious the rainfall the greater is the dis play of cirri. The cirrus, cirro-cumulus, cirro-stratus, cumulo-stratus, and nimbus are connected more or less closely with the great atmospheric movements of the cyclone and anticyclone. In what follows here only the amount of sky covered will be taken into account, and not the species of cloud covering. The diurnal variation in the amount of cloud in the sky on the open sea is very small. The following are the means of two hundred and seventy-seven days observations on board the &quot;Challenger,&quot; stated in percentages of sky covered : 2 A.M. 59 10 A.M. 58 6 P.M. 57 4 ,, 59 Noon 56 8 ,, 57 6 ,, 62 2P.M. 58 10 57 8 ,, 62 4 59 Midnight 57 Two maxima are here indicated, the one about or shortly after sunrise and the other in the early part of the afternoon ; and two minima, the one at noon and the other from sunset to midnight. The difference between the extremes is only 6 per cent, of the sky. At Batavia the daily maximum is from 6 to 11 P.M., and the minimum from 8 to 11 A M., the extremes being 52 per cent, at 9 A.M. and 69 per cent, at 7 P.M., a difference of 17 per cent. Of four daily observations at Mauritius, the maximum is 50 per cent. at 1 P.M. and the minimum 38 per cent, at 6 A.M. At Coimbra, observations of clouds have been made live times daily, and six years results give the maximum 63 per cent, at 9 P.M. and the minimum 52 per cent, at 9 A.M. At this place, during July and August, the greatest amount of cloud occurs at 6 P.M., and in these months the rainfall at Coimbra is very small. The minimum is more pronounced at 9 A.M. than at any other period ; in winter this phase occurs about four hours later. At the continental situation of Vienna, during the warm months of the year the maxi mum is at 2 P.M., with a secondary maximum about 6 A.M., and the minimum from 10 P.M. to 2 A.M. ; but during the cold months the maximum is at 6 A.M. and the minimum during the evening and night. In the Rocky Mountains, the chief maximum, 57 per cent., is at 3 P.M., with a secondary one 30 per cent, at 5 A.M. ; and the chief minimum 20 at 3 A.M. and a secondary one 29 at 11 P.M. At Helsingfors the maximum of cloud occurs from 10 A.M. to 2 P.M., and the minimum from 10 P.M. to 2 A.M. Much yet remains to be done with regard to the determination of the diurnal variation of cloud, but from the above one or two deductions of a general character may be drawn. A maximum occurs in the morning and continues till shortly after the sun has risen, and this maximum is more decidedly pronounced over the open sea than over land. Its appearance is without a doubt due to the general cooling of the atmosphere through its whole height by terrestrial radiation, and its disappearance to the heating of the air, which commences about sunrise. Then follows one of the diurnal minima, which continues till midday, or a little later ; in other words, it continues till, owing to the diurnal heating of the air by the sun, the ascending current has fairly set in. The period of this ascending current marks the second maximum, which during the warmer months is larger than the morning maximum over land. The cumulus is the characteristic cloud of this maximum. These clouds are merely the summits of the ascending currents which rise from the heated land, where the aqueous vapour is condensed in cloud by the expansion which takes place with increase of height. These cumulus clouds throw a not unimportant light on the behaviour of the ascending currents which rise from the surface when heated by the sun, inasmuch as they