28 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



RaiiifaU records — Continued. 



The rainfall map (PL V) is based mainly on the above records, 

 but over mountainous regions where no records have been made 

 the lines drawn are based on topography. Owing to the great vari- 

 ation in rainfall caused by mountains and to a less extent by deep 

 canyons, which often make great differences within small areas, the 

 rainfall map is drawn on broad lines. Enough is known of these 

 local differences in rainfall caused by topography to make it certain 

 that an accurate map of Washington to show these smaller differ- 

 ences would be a very complex affair. 



The larger differences in the normal annual precipitation are 

 clearly shown by the map. As a whole, western Washington has a 

 heavy rainfall w^hile that of eastern Washington is light. The coast 

 region has the greatest precipitation, heaviest on the western slope 

 of the Olympic Mountains. The region immediately to the north- 

 east of these mountains has a correspondingly light rainfall. No 

 satisfactory records are available for the precipitation in the higher 

 parts of the Cascade Mountains. Presumably it is between 70 and 

 J)0 inches. 



In eastern Washington perhaps the most striking feature is the 

 suddenness with which the precipitation decreases east of the crest of 

 the Cascades. 



The area over which the rainfall is less than 10 inches is practically 

 identical with the Upper Sonoran life area. 



As compared with the rest of the United States the narrow strip 

 immediately along the ocean has the maximum amount of rainfall, 



