LOCAL VARIATIONS IN RAINFALL 305 



grees north of the equator, the climate of the region is tropical, fj'ost being 

 entirely unknown. Furthermore, since it forms a narrow belt between 

 two oceans, its climate is also insular, the annual range of temperature 

 being very much smaller than all continental areas experience. 



AMOUNT AND DISTRIBUTION OF RAINFALL 



Tliroughout the greater part of the year the trade winds prevail with 

 fairly constant direction and force. These winds are probabl}^ deflected 

 slightly to the north by the high volcanic range of Costa Rica, and to 

 the south by the mountains of central and northern Nicaragua. The 

 low gap across the isthmus constituting the Nicaraguan depression thus 

 receives considerably more wind than would be due to the normal trades. 

 It is probably this congestion of the air currents that causes the excep- 

 tional precipitation of this region. Coming from the warm Caribbean 

 sea, the trade winds are saturated with moisture, and, as they strike the 

 slightly elevated land forming the isthmus, the precipitation is there 

 very abundant. Within the zone of maximum precipitation, which 

 embraces the coastal plain and the adjacent hills, forming a belt from 

 50 to 100 miles wide, the annual rainfall reaches nearly 300 inches. 

 Beyond this belt, at increasing distances from the Caribbean coast, it de- 

 creases very rapidly, and in the western part of the region the annual 

 rainfall is less than a third of that on the eastern coast. 



More important, however, than the absolute amount of rainfall is its 

 distribution throughout the year. The isthmus may be divided into 

 two distinct and well marked subdivisions by a line coinciding approx- 

 imately with the present divide between lake and Caribbean drainage 

 and crossing the San Juan near the point where that river leaves the 

 lake. In the eastern division the rain is distributed with tolerable uni- 

 formity throughout the entire year. There are some years in wbich 

 little rain falls for a period of three or four weeks in August and Sep- 

 tember, but this scarcely constitutes a dry season. In the western divis- 

 ion, on the other hand, there is a distinct dry season of five or six months, 

 in which there is practically no rainfall. The rain begins about the 

 middle of May, when the trade winds become less constant, and an oc- 

 casional storm comes from the northwest. 



PHYSIOGRAPHIC EFFECTS 

 I 



Eastern division. — These climatic differences between the eastern and 

 western portions of the region give rise directly to very striking differ- 

 ences in vegetation, and, either directly or indirectly, to differences in the 

 appearance and structure of the soils, in th6 topographic forms of the land 

 surface, and in the effectiveness of various physiographic processes. 



