22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



is accompanied by comparatively light, but not infrequent, precipita- 

 tion along most parts of the Atlantic slope. At this season rather 

 light cloud formations discharge their moisture along the northern 

 side of the Isthmus, the rainfall of the coast depending in a measure 

 on the height and proximity of the mountains. At the higher eleva- 

 tions fogs are very prevalent, and are often so dense that one's 

 vision penetrates only a few feet, and the dimly lighted forest be- 

 comes still darker as the cloud mass settles down ; a fine spray drifts 

 through the trees and soon the leaves are dripping steadily. The 

 Pacific coast, in marked contrast, has a true dry season, during 

 which little or no rain falls. During the wet season, beginning 

 usually about the latter part of May and ending about the first of 

 December, southerly winds become dominant and rains are more 

 general throughout the Isthmus. At the Canal Zone, which is a 

 cross-section of the Isthmus about 50 miles in extent, the annual 

 rainfall on the Atlantic coast is about double that on the Pacific coast. 

 Official records for 1909 show a total rainfall of 93.06 inches at 

 Balboa, and 183.41 inches at Cristobal; but the average for 13 years 

 at the former station is 71.67, and for 40 years at the latter station 

 130.03. This relative humidity of the two sides probably obtains as 

 far west as the Costa Rican frontier, but in eastern Panama the 

 difference is less marked. In much of the Darien region the total 

 rainfall is increased to an annual precipitation of perhaps more than 

 200 inches ' which renders this area one of the wettest in America. 



Excepting at the Canal Zone and limited areas in western Panama 

 the republic is sparsely populated by man; clearings are few, and 

 aside from the rather extensive, open, grassy savannas near the 

 Pacific coast and smaller grass areas in the Chagres Valley, the 

 Isthmus is a practically unbroken expanse of forest. Under the 

 stimulating influence of frequently recurring showers and con- 

 tinuously moist conditions throughout the year, the Atlantic water- 

 shed maintains a much more exuberant growth of vegetation than 

 the Pacific watershed, where long periods of drought check vegetative 

 vigor. At the height of the dry season these climatic differences 

 are manifested in the contrasting aspect of the forests on the two 

 slopes. While the trees of the Atlantic forest are clothed with 

 brilliant evergreen foliage, those of the Pacific forest, truly deciduous 

 for the most part, present bare stems, and the landscape has an 



1 During the construction of the Panama Canal 237.28 inches was recorded 

 in a single year, and 58 inches in a single month at Porto Bello, Panama ; the 

 annual average, however, was 178.67 during three years of record. 



