ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 221 



vanish at sunset, and stratiform cloud by itself is commonly the expres- 

 sion of moderate condensation under quiet conditions insufficient to 

 precipitate vapor rapidly. 



A cloud layer may continue for some days with strong wind, being 

 caused by (1) a gradual ascending movement of the lower air so as to 

 precipitate a small quantity of vapor continuously by expansion ; (2) 

 by contact of the upper surface of the lower current with a colder cur- 

 rant at a higher level; (3) by radiation from a rather moist stratum 

 through dry upper air; or (4) by a warm, moist wind arriving, after a 

 long passage, in cooler latitudes, and gradually becoming cooler by 

 radiation and mixture. 



In showery weather cumulus clouds are very often seen to consist 

 of two or more masses at levels wide apart, and the upper mass, 

 which is harder and firmer-looking than the lower, seems to move 

 much less fast. Such clouds, even though heavy-looking, may pass 

 over without rain, and it is generally, only by the appearance of rain 

 in the air and landscape under them that they may be distinguished 

 as actually shower-laden. Eain is, however, far more probable in 

 these cases when the clouds are in tiers or separate layers; indeed, a 

 single cumulus mass, simple and uncillifled, seldom precipitates at all. 



What, then, are the causes of rain ; and why does it fall from some 

 clouds more than from others '? 



The simplest and a very common cause of rain is the sudden eleva- 

 tion of moist air to a higher level, with the consequent chill by expan- 

 sion. Standing on a mountain between the west and east ends of a 

 loch in Perthshire, when a west wind is blowing, one may see showers 

 frequently falling among the mountains westward, and failing to reach 

 the flatter ground toward the east. The wind, even before it reaches 

 the mountains, is tilted upward by the pressure of air in front of it, is 

 consequently cooled, and precipitates moisture upon their western slopes. 

 When the air descends in a drier and warmer condition toward the 

 lower ground, the clouds quickly dissolve and thin out. The cloud- 

 forming and the shower-forming effect is in general roughly propor- 

 tional, between certain limits, to the height and steepness of the 

 mountains. The great cliff called Slieve League, on the coast of 

 Donegal, and the cliffs of Hoy, in the Orkneys, both about 1,800 to 

 2,000 feet high, cause clouds to be thickly formed sometimes fully half 

 a mile to windward. Whether rain falls, and how heavily, depends 

 chiefly on the moisture of the air and the coldness of the stratum into 

 which it is forced. 



A similar but little recognized effect is caused by opposing masses 

 of air. Thus, let a moist warm southwest wind meet a cold northeast 

 wind; the southwest wind is forced upward, especially over certain 

 localities, and flows over the northeast wind, expanding very largely 

 and rapidly and precipitating moisture heavily. The production of 

 heavy thunderstorms may be fully accounted for by the local eddies 



