HOW RxVIN IS FORMED 29,-) 



is clearly subject to the pressure of all those that rest ou it. This is 

 equal to about 14i| pounds on every scpuire inch of surface. Another 

 layer, say 1,000 feet above the ground, will clearly be under a less press- 

 ure, since 1,000 feet of air are below it; and this 3,000 feet of air weighs 

 sliglitly less than half a pound for every square inch of horizontal surface. 

 At 2,000 feet the pressure will be less by nearly 1 pound per square inch, 

 and so on. If, then, any mass of air begins to ascend tbrougb the at- 

 mosphere it will be continually subject to less and less pressure as it 

 ascends, and therefore, as we have already seen, it expands and becomes 

 cooler by expansion. Cooling from this cause is termed dynami(! cool- 

 ing. Its rate may be accurately computed from the work it h-as to do 

 in expanding. 



It amounts to 1° for every 183 feet of ascent if tbe air be dry or free 

 from vapor, and if, as is always the case, it contains some vapor, the 

 beight will not be very much greater so long as there is no condensa- 

 tion. But so soon as this point is passed, and the vapor begins to con- 

 dense as cloud, the latent beat set free retards tbe cooling, and the 

 height through which this cloud laden air must ascend to cool 1^ is 

 considerably greater and varies with the temperature and pressure. 

 When the barometer stands at 30 inches, and at the temperature of 

 freezing, the air must rise 277 feet to lose 1'^, and if the temperature is 

 G(P nearly 400 feet. 



Conversely, dry air descending through the atmosphere and becoming 

 denser as it descends, since it is continually becoming subject to an in- 

 creased pressure, is heated 1° for every 183 feet of descent ; and fog 

 and cloud-laden air at 30 inches of pressure and the freezing point Mill 

 be warmed 1° in 277 feet only, or if at 60^^ nearly 400 feet of descent, 

 owing to the re evaporation of the fog or cloud and the absorption of 

 latent heat. 



Now, let us see how these facts explain the formation of cloud, and 

 first I will take the case of the common cumulus or heap-cloud, which 

 is the commonest cloud of the day-time in line weather. 



When after sunrise the air begins to be warmed, the lowest stratum 

 of the atmosphere, which rests immediately on the ground, is warmed 

 more rapidly than the higher strata. This is because the greater part 

 of the sun's heat passes freely through a clear atmosphere without 

 warming it, and is absorbed by the ground, which gives it out again to 

 the air immediately in contact with it. So soon as the vertical decrease 

 of temperature exceeds 1° in 183 feet the warm air below begins to as- 

 cend, and the cooler air above to descend, and this interchange gradu- 

 ally extends higher and higher, the ascending air being gradually 

 cooled by exi)ansion, and {;easing to rise when it has fallen to the same 

 temperature as the air around it. This ascending air is more highly 

 charged with vapor than that which descends to re])lace it, since, as 

 was mentioned before, most land surfaces furnish a large amount of 

 moisture, which evaporates when they are heated by the sun. This 



