925 



RAIN. 



above the earth. The filaments becoming by an accession of heat 

 partially liquefied, it may happen that many of them will adhere 

 together, and thus form flakes of snow, which, at length acquiring 

 sufficient weight to overcome the resistance of the air, descend to the 

 ground. In order to explain the origin of rain and hail, he supposes 

 that the flakes, on arriving near the surface of the earth, may pass 

 through a wanner region than that in which they were formed, and 

 there dissolving, they assume the figure of spherical or spheroidal drops 

 of water. Again, if in the descent the latter should meet a current of 

 cold air, they become globules of ice. ('Meteora,' cap. v. vi.) The 

 absence of observation in all this, however, is evinced by the theory of 

 the formation of snow, to which it ig evident that Descartes never 

 applied even such a moderate magnifier as he might readily have 

 obtained, or he could not have failed to see the true nature of its 

 aggregation. 



The diffusion of electricity through the earth and atmosphere has 

 led some meteorologists to believe that the variations in its quantity or 

 intensity in particular regions may be the cause of the formation of 

 snow, rain, and hail. The electrical particles, being endowed with a 

 great repulsive power, are supposed to keep in general the particles of 

 vapour asunder; and when, from any cause, some given volume of air 

 ia deprived of its natural quantity of electricity, these particles unite 

 by their mutual attractions, and thus form drops of rain or ice. From 

 the showers which accompany a thunder-storm, there is no doubt that 

 electricity co-operates in some measure in the production of rain ; and 

 it may be remarked in support of the above hypothesis that rain is 

 most abundant among mountains, their elevated summits being favour- 

 able for receiving and discharging electricity ; while in some regions 

 where thunder is little known there is also little rain. But the idea 

 that repulsive electrical particles keep asunder the particles of aqueous 

 vapour is itself purely hypothetical. 



The theory first proposed by Dr. Button of Edinburgh (' Trans. Roy. 

 Soc. Edin.,' 1784), is that which appears to correspond most satis- 

 factorily to the observed phenomena of the atmosphere ; and accord- 

 ingly it has been adopted by nearly every distinguished meteorologist 

 since that time. This theory will be briefly described. 



The atmosphere surrounding the earth is known to consist of air 

 and aqueous gas or vapour, both of which are elastic ; and, according 

 to the experiments of M. Qay-Lussac, the elasticity of the vapour is 

 equal to that of the air at an equal temperature, both when the vapour 

 exists alone, and when it is diffused through the air ; hence it 

 is inferred that in the atmosphere the vapour and air are in 

 mechanical mixture only, and also that the particles of the former have 

 the power of moving freely in the intervals between those of the latter. 

 The atmosphere is supplied with humidity by evaporation from the 

 waters of the earth, and its power to hold the water in suspension 

 i Is on its temperature, an increase of the latter augmenting that 

 power, and a decrease diminishing it : but in the theory of Hutton,the 

 diminution of the power takes place in a higher ratio than the diminu- 

 tion of the temperature. 



Now the quantity of moisture in the atmosphere will at all times be 

 nearly equal to the greatest quantity that can be maintained in it in a 

 state of vapour at the existing temperature. Therefore if two volumes 

 of air thus saturated with moisture, but of different temperatures, 

 become by any means mixed together, a mean degree of heat results 

 iV. .M the union ; but the whole quantity of moisture in the sum of 

 the volumes of air will, agreeably to the theory, be greater than that 

 which is due to the mean temperature, and the excess will of course 

 be condensed or precipitated. The vapour so condensed forms a 

 cloud [CLOUD] ; and if this be specifically heavier than the air in 

 which it is formed, it will begin to sink : the internal friction of the 

 air, as Professor Stokes has recently shown, will for a time retard its 

 descent, allowing it to remain suspended until the resistance from this 

 cause is overcome by the increasing magnitude of the drops. Should 

 the atmosphere near the earth be less dense than the cloud, the 

 latter will continue to descend till it touches the ground, when the 

 aqueous particles, if small, will form what is called a mist ; or if large, 

 and particularly if the condensation of the vapour has been rapid and 

 copious, they will descend by their gravity in rain, snow, or hail, 

 according to the temperature of the region in which they have been 

 formed or through which they pass. It may happen, however, in the 

 descent that a cloud arrives in a wanner region than that in which 

 it was formed ; in this case the condensed moisture may again become 

 vapour, which may re-ascend to a region at which a new condensa- 

 tion takes place. But though it be true that some precipitation 

 must follow, whatever be the difference between the temperatures of 

 the two volumes of air, yet unless the mean of the two quantities 

 of vapour should be greater than the quantity necessary for com- 

 plete saturation at the mean of the two temperatures, the precipitation 

 will not be perceptible in the form of rain. 



In order to illustrate the general subject of clouds and rain, the late 

 Professor Daniell, in his ' Essays on the Constitution of the Atmo- 

 sphere' and ' Meteorology/ supposes, first, that the earth is a sphere of 

 uniform temperature, and surrounded by an atmosphere of dry and 

 permanently elastic fluid ; and he shows that on this supposition the 

 density of the air would diminish in a geometrical progression at eleva- 

 tion* increasing by equal increments. He observes also that the tem- 

 perattm would decrease with the densities, and th.it the atmosphere 



RAIN. 926 



would be constantly in equilibrio. This would continue to be the case 

 if the general temperature of the sphere were to be increased, provided 

 that increase were uniform at all points on its surface. Now, if the 

 temperature of the sphere, instead of being uniform, were supposed to 

 increase from the poles towards the equator, the unequal densities pro- 

 duced in vertical columns of the air by the differences of temperature 

 at equal heights above the surface of the sphere, would give rise to 

 lateral pressures which, in the lower strata, would produce currents 

 tending from the poles towards the equator ; but the elasticity of the 

 air, which is constant near the surface of the sphere, varies with the 

 height above that surface, according to such a law that, beyond a cer- 

 tain elevation, it would produce lateral pressures exceeding those which 

 arise from the density in the neighbouring columns at equal altitudes, 

 and thus there would arise a current in the upper regions flowing con- 

 tinually from the equator towards the poles. . 



He supposes next that the sphere is covered everywhere with water 

 of equal temperature, and is surrounded by an atmosphere of pure 

 aqueous vapour; and he shows not only that the density of this 

 vapour would diminish upwards, according to the law before men- 

 tioned, but that the atmosphere would in this case also be in equilibrio 

 and transparent even when the general temperature of the sphere 

 experiences a uniform increase. But if the temperature of the sphere 

 were to increase as before, from the poles towards the equator, the 

 density and elasticity of the vapour varying also with the temperature, 

 there would arise by evaporation at the equator a current tending from 

 thence to the poles, and this, being condensed in its course, would 

 return from the poles towards the equator in the form of water. The 

 condensation thus going on would cause the atmosphere to be con- 

 stantly charged with clouds and fain. Unless, however, the excess of 

 temperature at the equator were maintained by some foreign power, as 

 solar radiation, the temperature over the whole sphere would by 

 degrees become equalised; the equatorial parts becoming cooled by 

 evaporation, while the polar regions would become warmed by the 

 condensation. 



Mr. Daniell afterwards contemplates an atmosphere consisting of a 

 permanently elastic fluid mixed with aqueous vapour, and surrounding 

 a sphere of water of uniform temperature ; and he observes that, since 

 the evaporation would be slow, the small quantity of water precipitated 

 would be almost immediately dissolved by the superior temperature of 

 the stratum below, into which it would tend to fall ; therefore this 

 atmosphere would be free from clouds. But in the event of the tem- 

 perature of the sphere increasing from the poles to the equator, the 

 evaporation in the latter region would destroy the regular gradation of 

 temperature in the atmosphere from the surface of the sphere upwards ; 

 the evaporated water rising to the middle regions would there, in con- 

 sequence of the diminished temperature, give out its latent heat, and 

 become condensed ; then descending, it would acquire from below a 

 new portion of heat, with which it would rise till it was again forced 

 to part with its caloric. This process may be supposed to continue till 

 those regions of the atmosphere become saturated with vapour, and at 

 the same time rarefied by the heat. The rarefaction of the air would 

 diminish its resistance to the general movement of the vapour towards 

 the poles, and thus the vapour would rush with force in those directions; 

 but on arriving in latitudes at which the temperature is too low to 

 allow the air to hold it in solution, condensation would take place, and 

 clouds would be formed. 



The circumstances just mentioned correspond nearly to those which 

 would take place about the earth if local and other accidental circum- 

 stances did not interfere with the general process. In its actual con- 

 dition, when a column of air vertically over any place is from any 

 cause heated more than the neighbouring columns, it begins to ascend 

 by its diminished specific gravity, the colder air of the vicinity flows in 

 to fill up the void, and thus the relation between the temperature and 

 humidity at the place is deranged. Then, agreeably to the general 

 theory of Dr. Hutton, a precipitation of the vapour takes place. 



In proportion to the density of the vapour, the magnitudes of the 

 condensed particles of water are greater (this, however, does not appear 

 to be proved, and the magnitude of the particles must be in some 

 ratio to the temperature of condensation) ; in the upper regions of the 

 ah- the cloud assumes a light appearance, but below it is more dense. 

 After their formation, the clouds are driven about by the winds, 

 receiving new accessions of precipitated vapour till the air is no longer 

 capable of supporting them, and then their substance descends in rain, 

 snow, or hail. 



On the supposition that the surface of the earth is without inequalities, 

 and that the temperature gradually diminishes from the equator towards 

 either pole, it should follow that the rarefaction of the air and the 

 evaporation of the water, and consequently tljc quantity of rain, must 

 diminish according to some law with the distances of places from the 

 equator. Now the mean temperature in any latitude being known, the 

 quantity of moisture in the atmospherical column at that latitude can 

 be found, since it depends on the temperature : hence, knowing also 

 the variations to which the temperature of the atmosphere at the place 

 is subject in the course of the year, the mean annual depth of rain in 

 that latitude may be computed. On such principles, Humboldt deter- 

 mined that the mean annual depth of rain should be, at the equator, 

 96 inches; in lat. 45, 29 inches; and in lat. 60, 17 inches. The 

 circumstances, however, which render the temperatures in different 



