526 



NATURE 



[Jan. 2, 18S9 



happens that up to the end of the last century very much 

 the same explanation was popularly held to account for 

 dew. It was supposed to be a kind of perspiration 

 emitted from the earth, and no satisfactory exphnation of 

 the pheno'.nenon had been arrived at by the ohysical 

 philosophers of the day. It remained for Dr. Wells to 

 prcve, by a long series of observations and experiments, 

 which have been quoted by Sir John Herschel and Mr. 

 John Stewart Mill as a typical instance of philosophical 

 inquiry, that the cold surface of grass and shrubs con- 

 denses the vapour previously held in suspension in the air, 

 these surfaces being cooler than the air, and below its 

 point of condensation. And such, of course, is also the 

 case of the glass tumbler containing ice. Anyone may 

 try the experiment for himself. To produce hoar-frost, it 

 is only necessary to cool the condensing surface below the 

 freezing point, which may be done by crushing some ice 

 and mixing it with salt. A tin pot is better than a glass 

 to make this experiment. 



When not only the ground, but also the air to a 

 considerable height above it, is cooled in like minner, we 

 have the production of fog, fog being the form in which 

 the vapour is first condensed, and consisting of water in 

 drops too minute to be separately visible. The formation 

 of fog is very much aided if the air be laden with smoke. 

 Smoke consists of extremely minute particles of unburnt 

 coal or other fuel, and these cool faster than the air at 

 night, and so cool the air in contact with them. Each one 

 of them, too, condenses water on its surface, and being 

 thus weighted they sink and form that dense fog that 

 Londoners know so well. 



Clouds are essentially the same as fog, but formed high 

 up in the air. But in their case, and that of rain, snow, 

 and hail, another and different cooling agency comes into 

 play, and this will require some preliminary explanation. 



I dare say that some of you may at some time or other 

 have charged an air-gun. And if so, you will be aware 

 that when so charged the reservoir becomes pretty warm. 

 Now this heat is produced, not, as might be supposed, by 

 the friction of the piston in charging, but is due to the tact 

 that work has been done upon the air by compressing it 

 into a very small space ; in other words, work has been 

 converted into heat. If the compressed air be allowed to 

 escape at once, its heat is re-converted into work. It has 

 to make room for itself by thrusting aside the atmosphere 

 into which it escapes, and when thus expanded it is no 

 warmer than before it was compressed. Indeed, not so 

 warm, for it will already have parted with some of its heat 

 to the metal chamber which contained it. And if when 

 compressed it is allowed to cool down to the ordinary 

 temperature, and then to escape, it will be cooled below 

 that temperature just as much as it was heated by 

 compression. Thus, if in being compressed it had been 

 heated ico°, say from 60° to 160°, and then allowed to 

 cool to 60"^, on escaping it will be cooled 100° below 60'', 

 or to 40"" below zero, which is the temperature at which 

 mercury freezes. This is the principle of the cold air 

 chambers now so extensively employed on ship-board for 

 the transport of frozen provisions from New Zealand and 

 Australia. 



Bearing in mind, then, this fact— that air in expanding 

 and driving aside the air into which it expands is always 

 cooled— let us see how this applies to the case before us, 

 the production of cloud and rain. 



The volume of a given weight of air— in other words, the 

 apace it occupies— depends on the pressure to which it is 

 subject : the less this pressure the greater its volume. If 

 we suppose the atmosphere divided into a number of 

 layers superimposed on each other, the bottom layer is 

 clearly subject to the pressure of all those that rest'on it. 

 This is equal to about 14! pounds on every square inch of 

 surface. Another layer, say 1000 feet above the ground, 

 will clearly be under a less pressure, since icoo feet of 

 air are below it; and this 1000 feet of air weighs slightly less 



than half a pound for every square inch of horizontal 

 surface. At 2000 feet the pressure will be less by nearly 

 one pound per square inch, and so on. If, then, any mass 

 of air begins to ascend through the atmosphere, 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 dynamic cooling. Its rate may be accurately 

 computed fi-om the work it has to. do in expanding. 



It amounts to i'' for every 183 feet of ascent if the air be 

 dry or free from vapour-, and if, as is always the case, it 

 contains some vapour, the height will not be very much 

 greater so long as there is no condensation. But so soon 

 as this point is passed, and the vapour begins to condense 

 as cloud, the latent heat set free retards the cooling, and 

 the height through which this cloud-laden air must ascend 

 to cool i'^ 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 r, and if the temperature is 60' 

 nearly 400 feet. 



Conversely, dry air descending through the atmosphere 

 and becoming denser as it descends, since it is continually 

 becoming subject to an increased pressure, is heated i" 

 for every 183 feet of descent ; and fog and cloud-laden air 

 at 30 inches of pressure and the freezing point will 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 fine 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 K 

 in 183 feet, the warm air below begins to ascend, and the 

 cooler air above t6 descend, and this interchange gradually 

 extends higher and higher, the ascending air being 

 gradually cooled by expansion, and ceasing to rise when 

 it has fallen to the same temperature as the air around it. 

 This ascending air is more highly chai-ged with vapour 

 than that which descends to replace it, since, as was 

 mentioned before, most land surfaces furnish a large 

 amount of moisture, which evaporates «hen they are 

 heated by the sun. This process goes on until some 

 portion of the ascending air has become cooled to the 

 point of condensation. No sooner does it attain this, 

 than a small tuft of cumulus cloud appears on the top of 

 the ascending current, and the movement which was 

 invisible before r:ovv becomes visible. In a calm atmo- 

 sphere each tuft of cloud has a flat base, which marks the 

 height at which condensation begins, but it is really only 

 the top of an ascending column of air. No sooner is this 

 cloud formed than the ascent becomes more rapid, because 

 the cooling which checked its further ascent now takes 

 place at a much slower rate, and therefore the cloud grows 

 rapidly. 



On a summer afternoon when the air is warm and very 

 damp, such cumulus cloud ascends sometimes to very 

 great heights, and develops into a thunder-cloud, con- 

 densing into i-ain. Rain differs from fog and cloud only 

 in the size of the water drops. In fog and cloud these are 

 so minute that they remain suspended in the air. But as 

 the cloud becomes denser, a number of them coalesce to 

 form a rain-drop, which is large enough to overcome t.ie 

 friction of the air. It then begins to fall, and having to 

 traverse an enormous thickness of cloud below, it grows 

 larger and larger by taking up more and more of the 



