104 



KNOWLEDGE. 



[May 1, 1899. 



These heavy masses of cloud, rising up like mountains, 

 towers, or anvils, suggest the terms "anvil," and "turret," 

 which have been applied to them. They are generally 

 surrounded at the top by a veil or screen of fibrous texture, 

 known as false cirrus, and below by nimbus-like masses of 

 cloud. 



The next is of the alto-cumulus type ; a dense, fleecy 

 cloud, composed of large whitish or greyish balls, with 

 shaded portions grouped in flocks or rows. Finally 

 come those isolated, feathery clouds — sometimes like thin 

 veils — belonging to the cirrus variety, as depicted iu the 

 fourth photograph. 



Eeproductions of original photographs of both cumulus 

 and stratus varieties appeared in Knowledge for August, 

 18f>4, from Mr. H. C. Eussell, of the Sydney Observatory. 

 Particularly fine and interesting are the two alto-cumulus 

 or " mackerel " sky pictures, while the shades upon the 

 other compounds are exceedingly characteristic. 



In the main, as will be seen from the preceding classifi- 

 cation, the system of Howard has remained unchanged 

 except by natural growth. Even the International system 

 is professedly only an adaptation, for general use, of the 

 cloud names which had already come into use in difi'erent 

 places. It must, moreover, be remembered that a general 

 classification of clouds becomes possible only in view of the 

 fact that cloud-forms are identical in all parts of the world ; 

 in other words, that condensation processes take place 

 everywhere in the same way. Nevertheless, it must not 

 be inferred from this that the same cloud-forms are every- 

 where associated with the same kind of weather. The 

 conditions producing any one particular weather phenom- 

 enon vary greatly in different parts of the globe. 



It will be seen from the International list above that the 

 lower and middle clouds comprise the first three types and 

 sub-divisions of Ley's system, viz., radiation, interfret, 

 and inversion, and the higher clouds the fourth of these, 

 viz., cirrus. They are sub-divided according to the alti- 

 tudes at which they occur, thus presenting a more uniform 

 and convenient method for classification. 



Considering now, more in detail, the actual mode of 

 formation of clouds and the physical changes involved, 

 radiation clouds are produced by the radiation of the heat 

 from any particular locality and by the consequent cooling 

 and precipitation of the water particles. These clouds, 

 only dependent upon radiation and not upon the motion 

 of the air, are reduced as a matter of fact to one only — fog. 

 Speaking generally, however, of the two terms, fog and 

 cloud, there is no real physical dift'erence between them. 

 A cloud may be defined as a fog viewed from the outside, 

 and a fog as a cloud viewed from the inside. 



This formation of fog is, of course, iu accordance 

 with the general physical law, that if a mass of air 

 containing water vapour is cooled below its saturation 

 or dew-point, condensation of the vapour will set in. 

 This can easily be shown experimentally by means 

 of the apparatus depicted in Fig. 1. A is an air-pump 

 connected to one of the vessels, C, D, which are large, 

 round bottom, glass globes — the larger the better. C and 

 D are connected together with a stop-cock, B, inserted. The 

 bore of this connecting tube, as well as the orifice of B, 

 should be as large as possible. G is a screw-clamp cutting 

 ofl' communication with the outside air. A little water is 

 poured into D to saturate the air, and the stop-cock B 

 turned off. C is then exhausted by means of the air-pump, 

 and B again turned on. Air immediately rushes from 

 D to C, and in doing so expands and takes up the necessary 

 heat from the remainder, thus cooling the latter below 

 saturation point. A thick fog is produced in D, and may 

 be shown in a more striking manner by arranging a lime- 



light jet behind D, when brilliantly coloured halos will be 

 seen, varying according to the number and size of the 

 water particles. 



Under certain circumstances, two currents, the one 

 colder than the other, may be formed up in the air, moving 

 more or less horizontally and in opposite directions. The 



Fig. 1. — Fog-producing apparatus. 



one rubbing over the other will produce ripples in the air, 

 such as does a wind blowing over the surface of a lake, 

 but ripples of greater magnitude. In the hollows of these 

 waves the vapour will be condensed and other forms of the 

 stratus type of clouds are thus produced. One variety of 

 these is the well-recognized stratus maculosus or 

 sky, which, compared by some to a flock of sheep lying 

 down, always claims an observer's attention by its quiet 

 beauty. 



The next type of clouds are those produced by the ascent 

 of heated columns of water-laden air, which expand and 

 become colder, as has just been explained. Condensation 

 of the vapour takes place, therefore, as it rises into higher 

 and colder regions. In this division occur those massive, 

 woolpack, or heap clouds, the cumuli, together with their 

 compound forms, such as cumulo-stratus or the "anvil" 

 cloud ; cumulo-nimbus or the shower cloud ; and, finally, 

 the easily recognized and generally disagreeable form of 

 nimbus or rain cloud. 



These flat-bottom, extensive, cumulus-clouds, sometimes 

 measuring twenty-five thousand feet from base to summit, 

 present different aspects according to their positions when 

 looked at. Seen, when fairly low down, by the reflected 

 light of the sun, they give to an observer a beautifully 

 brilliant picture to gaze at. When, however, the sun is 

 not shining on the lower portion of the cloud, the bright 

 picture changes generally to one of approaching gloom, 

 and one thinks sometimes of a coming storm. As a 

 matter of fact, however, simple cumulus is by no means a 

 usual cloud of unsettled weather. It is when it commences 

 to unite to form one of its compound forms — say cumulo- 

 nimbus, that rain may be looked for. 



The final principal type of clouds, the cirrus, presents 

 a different appearance to the preceding varieties, and 

 offers a somewhat different consideration. When from 

 any cause a layer of air containing water vapour has 

 been carried to a high altitude into layers of rarefied air, 

 which, moreover, are at a very low temperature, condensa- 

 tion will commence as soon as this layer has been 

 sufficiently cooled. The minute water particles thus 

 formed are immediately frozen into ice-dust, producing 



