May, 1902.] 



KNOWLEDGE, 



105 



and Cumulus-so distinct in actual appearance and in 

 i.l.vsical formation that thev may be taken as the basis of 

 rlissiti.atien. Sometimes b.>th types appear to merge into 

 each other, in xvhieh ease no variety of cUissification sufcees 

 to describe them satisfactorily, as anyone who has studied 

 doud-forms must allow. -'Stratus" is a sheel-like for- 

 mation of cloud. ■•Cumulus" is recognizable by its 

 heaped-up api.earance and vertical thickness. Numerous 



varieties of cloud-forms may be observed graduating from 

 one of these types tr. the other, but when an observer can 



chanwe, the gradation of Stratus into Cumulus foreboding 

 worse weather, and of Cumulus into Stratus heralding good. 

 Again, as we shall show later on, the vertical thickening of 

 the stratiform clouds is a distinctly bad indication. 



Up to quite rcc'Mitlv, Luke Howard's division of clouds, 

 formulated in 1802, held first place ; even now it is in 

 constant use, for though attempts have been made at a 

 more scientific classification, all of them, with the single 

 exception of that proposed by the late Rev. Clement Ley, 

 can onlv be termed make-shifts. Mr. Ley's classification 

 uufdrtiiuately is long, and not well adapted to the use of 

 anv but i.rofJssional investigators, or enthusiasts with ample 

 time on their hands. There exists a so-called "inter- 

 national " system of cloud nomenclature, but, for all that, 

 each country has its own especial system, with the result 

 that vast collections of cloud statistics are of little value 

 as helps to a classification, and are useful only as records 

 of clouds present at certain times. 



Clouds owe their existence to two causes : 



1. Through the passing of warm moist air into colder, 

 when, owing to condensation, a certain proportion of the 

 moisture becomes visible in the form of a cloud. 



2. Through changes occurring in the atmosphere as it 

 rises into higher regions of atmosphere, where decrease in 

 pressure and expansion and consequent loss of heat take 

 place and cause condensation of moisture. 



The first process may be described as the condensation 

 formation of clouds, and the second as the adiabatic 

 formation of clouds. As a matter of fact, no hard and 

 fast line separates these two operations; they act in 

 unison, and the combination of vertical and horizontal 

 currents go to make up the diversity of forms which clouds 

 assume. 



In settled states of the atmosphere. Stratus clouds are 

 common, or the sky may be perfectly clear. In unsettled 

 conditions. Cumulus or Heap clouds are formed. 



We shall now describe a few familiar forms of cloud, 



F:g. :).— Cuiuulu?. 



251 (a) 



dearly distinguish Stratus from Cumulus he has already 

 acquired valuable knowledge. 



The presence of either type of cloud alone indicates a 

 more or less set condition of the atmosphere, and generally 

 foretells a continuance of the existing weather. The 

 simultaneous presence of both types indicates a coming 



giving them simple names and endeavouring to compare 

 Them with other nomenclatures. „ , ^ , • ■• 



Of Cumulus clouds there are five well-defined varieties 



as follows : — / . 1 



(1) Fine weather variety ... ... Lumulus 



Clement Ley ... " 



