32 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



As regards temperature, eastern Washington while having a normal 

 annual mean but slightly lower than that of Avestern Washington, 

 has much greater extremes, being decidedly colder in winter and 

 warmer in summer. The Upper Sonoran area as compared with the 

 Arid Transition is several degrees warmer. 



A notable feature of the temperature of Washington, in conformity 

 with much of the region west of the Eocky Mountains, is the great 

 variation of temperature between day and night, especially in sum- 

 mer. It is emphatically a region of cool nights, where one can ap- 

 preciate blankets at night throughout the year. These cool nights 

 are least marked in the Upper Sonoran area, and it nuiy, indee(l,'-be 

 found that this factor is an important one in limiting the range of 

 Upper Sonoran plants. 



These cool nights naturally indicate late frosts in spring and early 

 ones in autumn. In western Washington such killing frosts are not 

 unusual up to April 15, and rarely a month later. Except in the 

 warmest portions of eastern Washington killing frosts occur not 

 uncommonly up to June 1, and infrequently three weeks later. 



The first autumnal frosts in western Washington occur as early as 

 the middle of September or as late as the middle of November; 

 rarely they may be delayed until December. The dates are much the 

 same for eastern Washington. 



WINDS. 



The most prevalent winds are from the southeast, and are ivsually 

 accompanied by rain. East of the Cascade Mountains they are 

 known as chinooks. This term is usually applied only to the winds 

 that blow in the winter months, but there seems no proper reason to 

 distinguish such from similar winds during the remainder of the 

 year. The winter chinooks are commonly warm winds, accompanied 

 or immediately followed by heavy rains ; rarely they are cold and dry. 



The wet chinooks are in eastern Washington remarkable chiefly 

 for the rapid rising of the temperature which they occasion. 



These winds are often quite severe. This is particularly true of 

 the occasional southwest winds which occur in summer, which in 

 eastern Washington usually assutne the form of dust storms. Such 

 storms may cause much destruction. 



The only other winds deserving of special mention are those which 

 sometimes occur in eastern Washington during the summer months, 

 blow^ing from the north or northeast. These winds are gentle but 

 exceedingly dry, and are therefore capable of causing great damage 

 to growing crops. For this reason they are much feared in the agri- 

 cultural districts. 



