METEOROLOGY. 



105 



they condense it* vapour and render it visible. The clouds 



appear to be stationary around their summit*; bat the fact 



appears to be that, as tho air passes the top of the mountain, 



ita Yapour becomes visible ; aa soon as it has passed on, this 



iissolved, while another cloud is formed from the fresh 



particle* of air which are now in contact with the peak. In this 



A remains permanent, though the particles of which 



it is composed are continually changing. 



Fogs are also very prevalent in places where there is a con- 

 siderable difference in tho temperature of adjoining regions, or 

 whore in the open sea a sand- 

 bank causes the colder currents 

 from below to rise to tho surface. 

 The dense fogs which are so com- 

 mon on the banks of Newfound- 

 land are illustrations of the latter 

 fact. 



The watery vapour of the air ia 

 precipitated to tho surface of the 

 earth in several different forms. 

 The first we notice is that of (?<. 

 The common opinion that the dew 

 falls is a fallacy; the true theory 

 of it was for a long time unknown, 

 and was tir-t published by Dr. 

 Wells in 1814. The simplest 

 illustration of the phenomenon is 



Fig. 5. CIRBUS CLOUD 



the humidity of the air ia generaDy greater there. In Great 

 Britain, tho annual amount is estimated at an average depth 

 of about five inches all over the surface. The freateat amooat 

 falls in the spring and autumn, as in these season* the difference 

 of temperature between day and night is at its n^Timnny 



When the objects upon which the dew is deposited are at a 

 lower temperature than 32, the moisture U frozen a* it is de- 

 posited, and then we have tho hoar-frost, which U in reality 

 merely frozen dew. 



Clouds are masses of visible vapour, somewhat resembling 

 mists, but suspended in the air 

 at a greater or less height. They 

 are produced by the partial con- 

 densation, in the higher regions of 

 the atmosphere, of the vapours 

 that rise from the earth. 



During the day evaporation 

 goes on most rapidly, and warm 

 currents of moist air are then 

 constantly ascending from the 

 surface of the earth, heated by the 

 solar rays. As these ascend higher 

 and higher, they lose heat by ex- 

 pansion and by mingling with the 

 colder air around, until their 

 vapour is condensed and assumes 

 the visible form. 



Fig. 6. cum; 



seen by bringing a decanter or 

 gloss, containing ice-cold water, 

 into a warm room. The surface 

 of the glass will almost imme- 

 diately be covered with moisture. 



The reason of this is that the 

 air, by contact with tho cold glass, 

 is chilled so much that it can no 

 longer retain all ita vapour in 

 solution, and accordingly deposits 

 a portion on the substance that 

 chills it. A similar process goes 

 on in Nature. After tho sun has 

 set, many bodies which have been 

 warmed by his rays begin to 

 radiate their heat into space, and 

 in this way become rapidly cooled, 



so as to be several degrees lower in temperature than the sur- 

 rounding nir, some of tho moisture of which accordingly con- 

 denses upon them. 



This 'deposition of dew is much more plentiful on a clear and 

 oolm night, ca then radiation goes on much more rapidly, and 

 consequently the temperature is lowered to a greater extent than 

 when tho sky is covered with clouds. A brisk wind, however, 

 considerably interferes with its formation, as it does not allow 

 the air to remain long enough in contact with the earth to 

 part with its moisture. 



Those bodies which radiate heat most rapidly receive, of 

 course, tho largest amount of dew ; and, by comparing various 

 objects in this way, we find that plants receive much more than 

 bare soil, and that again more than the solid rock ; so that it is 

 usually most plentifully deposited in those places where it is 

 most needed. It is most abundant in maritime countries, since 



Fig. 8. NIMBUS OR RAIN CLOUD. 



Fig. 7. STRATUS CLOUD. 



HB^k There is some little difficulty 



in understanding clearly why the 

 clouds remain suspended in the 

 air, instead of falling to the gar- 

 face of the earth. It may, how- 

 ever, be partly accounted for by 

 these ascending currents of heated 

 air which bear them up. Where 

 these cease, the clouds appear to 

 fall slowly towards the earth's 

 surface, the lower portion being 

 slowly dissolved away by the 

 warmer air below, while fresh 

 additions are frequently being 

 mode by the condensation of more 

 vapour above. Small clouds may 

 not unfrequently be observed to 

 fall slowly, and gradually vanish, much in the same way aa 

 tho stoam from a railway engine melts away in the air. 



Tho elevation of the clouds varies very greatly. Many of the 

 smaller ones are believed to be at an elevation considerably 

 greater than five miles. A series of observations as to this 

 point was carried on in the neighbourhood of Skiddaw for a 

 long period. From these it appeared that on nearly half the 

 occasions of observation tho clouds were above 1,000 yards in 

 height. Only ten times in five years were they seen at e less 

 height than 100 yards. 



The varieties of form which the clouds assume are almost 

 endless ; the causes which produce these are, however, partly 

 known, and from the earliest times the weather-wise have drawn 

 many of their signs from the appearance they present. As 

 this is frequently done, it is necessary to have some general 

 terms indicative of the forms they more commonly assume. A 



