14 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



But so far as the formation of rain clouds is concerned we need only 

 consider the dynamical cooling of the air caused by its expansion, because 

 it is in that way that nearly all such clouds are formed. 



Air, like all gases, expands when it is relieved of pressure, and in 

 doing so it becomes cooled in accordance with definite and well understood 

 laws ; and a body of air which from any cause is made to ascend to a 

 higher level in the atmosphere must necessarily become cooled in this 

 way, because of its expansion as it passes into air whose density is less 

 than its own. 



Now let us imagine a body of warm moist air blowing across the 

 surface of the earth and meeting in its path a range of high hills. 

 The horizontal movement of the air in front becomes checked by the 

 mountains, but being pressed on by the moving mass behind it is forced 

 up the mountain slopes to flow over their summits. At that higher level 

 the pressure may be two or three pounds per square foot less than it was 

 at the level of the sea, and owing to its being subject to this reduction 

 in pressure the air would expand considerably, the expansion would be 

 accompanied by a large fall of temperature to below the dew point, and a 

 dense cloud would necessarily be formed. 



Those who live in the neighbourhood of high hills are perfectly 

 familiar with the result of this dynamical cooling, although possibly 

 they do not always attribute it to its real cause, but they see it in the 

 covering of cloud with which the flanks and summits of the hills are 

 frequently hid. 



But away from mountains the process of cloud-making may often 

 be watched on a warm summer day when the ground has become heated 

 by the sun. The warm ground radiates its heat into the layers of air 

 immediately above it, and the air thus warmed rises in a stream to higher 

 levels, causing the process of expansion, and consequent cooling, to take 

 place in the same way as when the air is forced up the slope of a 

 mountain. When the temperature of the air has fallen to that of the 

 dew point, or, in other words, to the temperature at which it can no 

 longer carry all the vapour it then contains, condensation will begin, and 

 will be shown by the formation of a tiny cloudlet, which will continue to 

 grow in size as fresh supplies of air arrive from below, and will presently 

 form one of those large cumulus clouds which, when lighted by the sun's 

 rays, are often objects of surpassing grandeur and beauty as they slowly 

 sail across the sky. Very often the bases of these clouds appear as if 

 they had all been cut off at a given level, which marks, indeed, the point 

 at which the condensation took place and the formation of the cloud 

 began. 



As the evening comes on the reverse of this process may sometimes 

 be seen. With the declining sun the earth's surface begins to cool, and 

 consequently the supply of warm ascending air is first of all checked, and 

 then entirely ceases. The clouds then sink slowly to a lower level, where 

 the air is warmer, and there they again dissolve into vapour and gradually 

 disjippear, till soon the sky again becomes cloudless. 



Very often, however, instead of dispersing in this way condensation 

 proceeds, and the cloud particles increase in number and size till they 

 fall to the earth as rain. 



