WATER IN THE ATMOSPHERE. 



215 



density or some error in experiment. Still the agreement is sufficiently 

 close to justify the statement that the relative density of the water- 

 vapour present in the atmosphere is practically the same as if the 

 vapour alone were present at the same pressure. 



Cloud. When a mass of air is cooled below the saturation point 

 the dew-point the excess of water- vapour is deposited as cloud, which 

 consists of minute drops of water forming in general on " dust-particles " 

 as nuclei (see p. 168). The cooling may sometimes result from a 

 mixture of warm and cold air, and it is quite possible that a mixture 

 of two quantities of air at different temperatures, neither saturated, 

 may form a supersaturated mass. This may easily be seen from 

 Fig. 126. Let PRQ represent the vapour-pressure curve, PM, QN being 

 the saturation pres- 

 sures at ^ and t 2 . 

 Let equal masses at ^ 

 and t 2 contain vapour 

 at pressures AM and 

 BN respectively. 

 When mixed, the 

 temperature will be 



1 2 , and the vapour 

 a 



will be sufficient to 

 produce pressure OL. 

 But this is in excess 

 of the saturation 

 pressure RL, and 

 OR represents the 

 amount which can be 

 deposited as cloud. 

 Clouds appear to be 

 formed in this way 

 on changes of the 

 wind. When a south- 

 west wind comes on in 



the upper regions over a north-east wind in the lower regions of the air, 

 clouds are formed where the two streams mingle. 



But a much more obvious mode of cloud formation is that in which a 

 large mass of air expands and, in doing work, cools below its saturation 

 point, depositing the excess of vapour as cloud. Such formation may be 

 often watched on mountain sides and summits. A stream of nearly 

 saturated air may be blowing against a mountain side, and being 

 deflected upwards it comes into regions of less pressure, where it expands 

 and cools, and the hitherto transparent vapour is converted into visible 

 cloud. Sometimes beyond the mountain top the air descends again, and 

 the clouds trailing off are gradually dissolved in the air which is now 

 becoming warmer. 



Expansion may also result from ascent of masses of air due to 

 destruction of vertical equilibrium of the atmosphere. The limiting 

 condition of equilibrium of the atmosphere, if it is uniform in composi- 

 tion, is that known as a condition of convective equilibrium. 



lemperaturt 



FlG. 126. Mixture of Equal Masses of Unsaturated 

 Air at Different Temperatures. 



