CONDITIONS AFFECTING PRECIPITATION. 197 



As long as the trade-winds are blowing in their normal course, and be- 

 fore they reach the line where their progress is checked by meeting with the 

 current coming from the opposite direction, they are necessarily dry winds, 

 because they are blowing toward a region of higher temperature, and thus 

 have their capacity for holding moisture increased. This is a point of the 

 greatest importance, not only with reference to the disti'ibution of the rain- 

 fdl on the ocean surfoce, but also on that of the land, as will be more fully 

 noticed farther on, the precipitation on the continental masses being the 

 matter with which we are esjoecially concerned in the present inquiry. 



An inspection of a rain-chart of the earth will show at once how distinctly 

 marked are the areas of small precipitation on each side of the equatorial 

 belt of continuous precipitation. This is especially conspicuous on the ocean, 

 and particularly on the Pacific, where we naturally find the phenomena of 

 normal rain-fall, independent of tlie effect of the land masses, best displayed. 

 To quote from an excellent authority in this branch of meteorological in- 

 quir_y, A. Wojeikof: "An understanding of the nature of the trade-winds 

 shows us at once the necessity of admitting the existence of zones of rainless 

 trade-winds, for these are winds constantly blowing from colder to warmer 

 regions ; that is, away from the point of saturation. Therefore, in oceanic 

 regions, where the trade-wind blows steadily the whole year through, there 

 can be no precipitation."* 



From the rainless trade-wind belts, going north or south, we meet next 

 with the regions over Avhich the rain-fall has a character intermediate be- 

 tween that of the tropical rain-belt and that of the temperate zones. These 

 sub-tropical zones of pi'ecipitation are somewhat marked features on the 

 ocean ; but in reference to the object of the present inquiry they are of 

 comparatively small importance. 



Beyond the sub-tropical rain-belts we have the temperate and polar 



• See A. Wojeikof, Die atmosphaiisclie Circulation. Verbreitung des Luftdruclces, dcr Winde und der Kegen 

 auf der Oberflache del- Erde. Ergan/ungsheft , No. 3S, to Petennann's Mittheilungen, 1874. The raiii-cliart 

 accompanying this vahialile memoir may be referred to in connection with what is said in the present section in 

 regard to the rain-fall of the earth. So also may that which accompanies Professor Loomis's "Contribution to 

 Meteorology," No. 16, in the American Journal of Science for January, 1882 ([3] XXIll. 1). The latter, however, 

 has on it only the distribution of the rain-fall upon the land, while Wojeikofs map gives it for the entire surface 

 of the earth. Professor Loomis's paper bristles with facts ; that of Wojeikof, on the other hand, has more the 

 character of a theoretical resuvie of this department of meteorology. In view of the existence of these valuable 

 papers, it is not necessary for the present writer to dwell on the subject of tlie causes influencing the distribution 

 of precipitation on the earth. The main object of the present section is to impress on the mind of the reader the 

 simplicity of the causes which influence precipitation in general as to quantity, and the complexity of the condi- 

 tions which determine its local distribution. 



