Page:The New International Encyclopædia 1st ed. v. 16.djvu/765

KAIN. 673 RAIN. jostled together by cunents of air, or that the larger ones fall fast enough to overtake the smaller ones, so that in either case larger par- ticles are formed which, as they descend, grow by the accretion of small particles that lie in their path. (6) That some of the smaller particles are positively and others negatively electrified, and consequently by attraction are made to coalesce, (c) That sonie particles, being larger than others, have different surface ten- sions, and that the lai'ger ones are thereby enabled to grow at the expense of the smaller ones. {d) That the original cloud particles consist of vapor that has condensed upon par- ticles of dust or foreign substances in the air, and that this condensation takes place more readily upon some nuclei than others, as is known to be the case from the observations of ■Vilson. Aitken, Bai-us, and others, (e) Tliat the atmosphere within a cloud, being saturated, has no remaining nuclei upon whieli condensation fan take place, and as the air continues to rise and cool it comes to a state of supersaturation and intense molecular strain, which is finallj' iclieved by a violent condensation upon groups of cloudy particles already existing: this violent condensation takes place in such a way as to sweep many of the smaller cloud particles to- <»ether into one large drop : C. T. R. Wilson has shown that these larger drops can only be fonned when the air in the dustless cloud has been expanded and cooled at least one-third more than is required for ordinary dusty air. (f) Wil- son has latel.y shown the plausibility of a slight modification of the preceding method: he finds that dustless air virtually acquires new nuclei on which condensation takes place when a beam of ultra-violet light or of the Roentgen rays, the radiation from uranium, or even ordinary sun- light, passes through the moist air, that in fact such nuclei are being formed in it all the time. Professor J. J. Thomson's observations on the formations of 'ions' — namely, the breaking away from a molecule of some one of its integral com- ponents which he calls 'corpuscles' — suggest that flic atmospheric 'ions' thus formed are active in producing cloudy condensation, and that the negative 'ions' attract moisture to themselves more readily than the positive, therefore they grow to be larger drops, and descending to the farth with their negative charges, give it nega- tive electricity, while the atmosphere is left essentially positive. Geogkaphical Distribution of Rain. As all animal and vegetable life on the earth's surface depends more or less directly upon the quantity of rainfall and its distribution throughoiit the months of the year, therefore the geographical and seasonal distribution of rain claims our first attention. From the preceding section it is seen readily that the distribution of rain over the earth's surface must depend upon the influences tliat force air to ascend rapidly. Thus on warm clear days, when the surface of the ground or water is highly heated, the lower stratum of air acquires a decided upwaid motion by reason of its buoyancy. Masses of hot air are rising while the cooler air near by is descending. Thunder storms are usually formed in this ■way, and nearly every station in the torrid and temperate zones has a preponderance of local rains in the afternoon. Whei^ever an ocean breeze or a monsoon wind rises high enough on a mountain side it gives rise to cloud and rain, so that the ocean winds bring more rain than the land breezes. The finest illustrations of this jjrinciple are seen in the rains of the southwest monsoon in India, in the rains that fall with southwest winds on the coast of Europe, or in the southerly winds with rain on the Gulf coasts of the United States. Again, when a moist warm wind meets a cold dry wind, the latter generally flows under and lifts up the former, because of the greater density of the cold air compared with the warm. Therefore above the cold air is formed a layer of cloud and oftentimes of rain due to the rapid elevation of the warm air. Illustrations of this are to be found on the south- east and southwest sides of the areas of low pressure that pass eastward over the United States throughout the year, and especially in the winter season. From the preceding it fol- lows ( 1 ) that every rising slope, whether of mountain ranges or interior plains, should, other things being equal, show a greater rain- fall as we proceed up the slope, and this distribu- tion of rainfall with altitude has been found to agree with observations in Great Britain, Germany. India, the East Indies, and the United States; (2) considering the world at large, the heavier rainfalls should occur in regions where warm moist winds steadily impinge upon the mountain slopes. The above principles are exem- plified in the rainfall charts contained in Bar- tholomew's Physical Alias (London, 1809), and especially in the twenty-six maps showing the monthly and annual distribution of the rainfall on the land surface throughout the globe com- piled by A. J. Herbertson and published by the Royal Geographical Society of London in 1900. Herbertson's charts of annual rainfall on the land surfaces are reproduced herewith. Seasonal Distribution of Rain. This is in many respects more imjiortiint than the total annual quantity. It is the combination of rain- fall, temperature, and smishine that determines the character of the vegetation in each part of the globe. In the Xorthern Hemisphere some stations have a large rainfall in June and July, when the sun is nearly overhead; but other sta- tions have the minimum rainfall at this season. In the tro])ics most stations have two minima and two maxima of rainfall during the year. In the Southern Hemisphere, where the sun has its maximum power in December or January, even greater diversities appear. The growth of the perennial vegetation, and especially the annual plants and the important crops, is entirely, con- trolled b}'. these relations of sunshine and rain. In regions where the rainfall is insufficient to perfect the important grain crops, recourse mu^t be had to irrigation, the success of which again depends upon the annual distribution of snow and rain. Various types of rain prevail over the ocean as well as over the land, although, unfortunately, we have but verv few measurements of the actual rainfall at sea, and must, therefore, speak onl.y of the frequencv of the rain. Thus on the Atlantic and Pacific oceans, under or near the equator, is a rainy belt, where the pressure is always low. the winds exceedingly light, varia- ble or calm ; here the sun almost invariabl.y rises in a clear sky. but about midday clouds begin to gather, and in a short time the whole