Rain
RAIN The antients appear to have been very imperfectly acquainted with the constitution of the atmosphcrr . and Descartes is probably the fust who, in attempting t.i refer meteorological phenomena to their causes, ha* j,proached near the hypotheses now generally received; (•* lie asciibes the formation of clouds, snow, rain, and hail •■ variations of temperature in the upper regions of the ait. He supposes that when the coldness in any portion of lho-v regions becomes intense, the subtle mutter dis&eminuttnl among the particles of vapour becoming too weak to kr. •> those particles at a distance from one another, the Utter must rush together, and either form small spicular litaimiii or spherical drops of ice. The superficies of these filament; or drops being supposed to he considerable when rcmparvtl with their volumes, he conceives that the resistance of tlic air may he great enough to prevent them from descendii,^ by their weight, and that thus a great assemblage of tin in may remain suspended in the form of a cloud above the earth. The filaments becoming by an accession of boat partially liquefied, it may happen that many of them will adhere together, and thus form (lakes of snow, whick, at length acquiring sufficient weight to overcome the resitlance of tho air, descend to the ground. In order to explain the origin of rain and hail, he supposes that the Hakes, in arriving near the surface of the earth, may pass through i warmer region than that in which they were formed, and there dissolving, they assume the figure of spherical nr 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 diffusion of the electric fluid through the earth and atmosphere has led some meteorologists to believe that the variations in its quantity or intensity in particular region* may be the cause of the to mation of snow, rain, and bail The electrical particles, being endowed with a great rcpulme power, are supposed to keep in general the particles of vapour asunder: and when, from any cause, some given volume of air is deprived of its natural quantity of electricity. the»e particles unite by their mutual attractions, and thus fotm drops of rain or ice. From the showers which aeronipam a thunderstorm, there is no doubt that electricity co-opciafcs in some measure in the production of rain; and it may he remarked in support of the above hypothesis that lain u most abundant among mountains, their elevated summit* being favourable for receiving and discharging electncity. while in some regions where thunder is little known there is al-o little rain.
But the theory first proposed by Dr. Hutton of Edinburgh (Phil. Trans., Kdin., 1784) is that which appears to correspond most satisfactorily to the observed phenomena of the atmosphere; and accordingly it has been adopittl by ceariy every distinguished meteorologist since that time. Th» theory will be briefly described.
The atmosphere surrounding the earth is known to consist of air and aqueous gas or vapour, both of which arc elastic; and, according to the experiments of M. Gay-Lussae, the elasticity of the vapour is equal to that of the air at an equal temperaluic, both when the vapour exists abne, and when it is combined with the air; hence it is inferred that in the atmosphere the vapour and air are in mechanical union 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 solution depends 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 diminution 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 from 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; and if this be specifically heavier than the air in which it is formed, it will begin to sink. 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 de>cetid by their gravity in rain, snow, or hail, according to the temperature of the region through which they pass. It may happen however in the descent that a cloud arrives in a warmer region than that in which it was formed; in this case the condensed moisture may again become v aponr, and the cloud may re-ascend to a region at which a now condensation 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 complete 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, Mr. Daniell. in his ' Essays on the Constitution of the Atmosphere,'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 elevations increasing by equal increments. He observes also that the temperature would decrease with the densities, and that the atmosphere would be constantly in equiiibrio. 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 produced 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 sin face, according to such a law that, beyond a certain 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 continually from the equator towards the poles. He supposes next that the Sphere is covered with water everywhere of equal temperature, and is surrounded by an atmosphere of pure aqueous vapour; and be shows not only that the density of this vapour would diminish upwards according to the law before mentioned, but that the atmosphere would in this case also be in equiiibrio ami 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 constantly charged with clouds and niin. Unless how ever the excess of temperature at the equator were maintained by some foreign power, as solar radiation, the temperature over the whole Sphere would by degress 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 temperature 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 consequence 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 rarified 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 circumstances did not interfere with the general process. In its actual condition, 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 vapours takes place. In proportion to the density of the vapour, the magnitudes of the condensed particles of water are greater; in the upper regions of the air the cloud assumes a lig'ht appearance, but below it is mote dark. 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 the 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 t.lso 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 has determined that the mean annual depth of rain should be, at 'he equator, 96 inches; in lat. 45°, 29 inches; and in lat. 60°, 17 inches. The circumstances however which render the temperatures in different latitudes, and even on the circumference of the same parallel, irregular, must produce irregularities in the quantities of rain which fall at different places; yet the results of observation show that, in preceding from the equator towards the north.pole, there is in reality a diminution in the mean annual quantities of rain.
From an average of the observations made during fourteen years (1810 to 1823 inclusive), the mean annual depth of rain on the Malabar coast is 1235 inches, and the mean annual temperature is 804° (Fahrenheit), hut the annual depths of rain are very irregular and differ considerably. From a mean of observations for seven years (1817 lo 1823 inclusive), the mean annual depth of rain at Bombay was only 85-24 inches. From a mean of observations during seventeen years (1802 to 1818 inclusive), Mr. Dalton makes the mean annual depth of rain at Manchester equal to 33*596 inches, the mean annual temperature being 47-6°; iind here also the annual quantities of ruin vary very irregularly. The same meteorologist estimates the average of the annual quantity of rain in England to be 31*3 inches; the greatest quantity being at Keswick in Cumberland ( = 67*j inches), and the least at U pminster in Essex ( = 195 inches); but il is supposed that this estimate of the mean quantity is higher than the truth, because too many of the observations were made in the maritime counties, where the atmosphere may be expected lo be the most humid. In regions where the trade-winds blow constantly, rain seldom falls; and the reason may be, that both the temperature and the currents of air being there nearly uniform, the vapours raised from the ocean are carried about the earth without suffering those partial accumulations by which condensation and precipitation might be produced. But elsewhere the irregular distribution of land and water, the existence of mountain-chains, and even the various capacities of different parts of the earth's surface for absorbing or communicating beat, independently of variations in the electricity of the air, are to be considered as the most frequent causes of perturbation in the general currents of the atmosphere, and consequently of the fall of rain.
The douse mists which rest on the ocean near Newfoundland are precipitations caused by inequalities in the temperature of the ocean in the line of the Gulf stream. In the year 1821, in consequence of very strong winds between the tropics during the summer having caused an extraordinary difference between the levels of the waters in the Gulf of Mexico and those of the Atlantic Ocean, the stream of ■warm water was found to extend eastward of the Azores; and it deserves to be remarked that this unusual circumstance was attended, both in France and in England, by a very hot and damp winter, together with an excessive fall of rain. (Sabine, Experiments on the Figure of the Earth, 1825.) The rains which frequently deluge the tropical islands are in part produced by the volumes of air which are intermingled by the sea and land breezes; and those which fall at tlie time of the summer solstice in Africa may be ascribed to the immediate precipitation of the vapours which How from the seas to supply the place of the rarefied air above the heated lands: while the drought which prevails in the sandy deserts of that quarter of the earth is explained by the level character of those deserts, over which the currents of air may he supposed to flow nearly without interruption.
From April to October, the winds blowing from the southwest towards the coast of Malabar are accompanied by heavy rains, and the circumstance may be accounted for by the vapours of the ocean being brought from a warm region to one which is less so, and consequently becoming there condensed and precipitated. On the other hand, the prevailing winds on the coast of Peru, being from the south and southwest, come from a cold to a warmer region; consequently a diminution of the degree of saturation must there take place, and the vapours remain suspended; accordingly it is found that rain seldom falls on that coast. The clouds which overhang the coast of Malabar during the monsoon above-mentioned are arrested by the chain of the Ghauts, and while it rams on the western side the fair season is enjoyed on the coast of Coromandel. Again, the currents of air which pass over Peru, in crossing the chain of theAndes, where the temperature is lower,become condensed by the cold, and the rain is there precipitated in abundance. The vapours which come from the Atlantic ocean, and pass over the south-western counties of England, must be more abandant than those which arrive there from the continent of Europe; and from observations made at Penzance, the rains which accompany the westerly winds at that place exceed those produced by the easteily winds in the ratio of about three to one.
In tropical regions the quantities of rain which fall in different months of the same year are very unequal: at Bombay the mean monthly depth in June was found to be 24 inches, and in October, P26 inches. In temperate climates the quantities differ much less, but more rain falls during the second half year than during the first. The means of observations continued during 40 years at Londou give, for the depth of rain from January to July inclusive, 8*539 inches, and from July to December inclusive, 12*147 inches.
In general the lowest stratum of air about the earth contains the greatest quantity of water in solution; and hence il might bo expected that more rain should fall on low level plains than in elevated countries. The contrary however is the fact: and this may be accounted for by the variety of currents among mountains, and by clouds resting frequently on the summits of hills without descending to the plains. While the average annual depth of rain at Keswick is 67*5 inches, in the interior of the country and on thesea-coast it is hut 25 inches: and while the average depth on the St. Bernard is 63 13 inches, that at Paris is 20 inches only. Yet, from the observations of Dr. Heberden, Mr. Howard. and M. Arago, it appears that the depth of rain on the level of the ground is greater than at the top of a building. The first of these philosophers found that the annual depth at the top of Westminster Abbey was 12*099 inches, while at a lower level, on the top of a house in the neighbourhood, it was 18*139 inches; and on the ground, in the garden of the house, it was 22608 inches. M. Arago observed, from observations during twelve year*, that on the terrace of the Observatory at Paris the annual depth was 50*471 centimetres (19*88 inches), while in the court of that building. which is 28 metres (30 yards) lower, the annual depth was 56 371 centimetres (2221 inches).
Mr. Howard has observed that, in this country, when the Moon has south declination there falls but a moderate quantity of rain, and that the quantity increases till she has attained the greatest northern declination; and on some such results of observation the popular opinion that there M a connection between the alternations of rain and fnir weather and the changes of the Moon maybe founded. Oar knowledge of the variations to which the temperature of tl>e air is subject, is however yet too imperfect to allow murk dependence to be placed on predictions relating to the weather which are formed from the moon's phases, or even from variations in the state of the barometer or hygrometer
Note - this article incorporates content from The Penny Cyclopaedia of the Society for the Diffusion of Useful Knowledge (1840)
