32 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
As regards temperature, eastern Washington while having a normal 
annual mean but shghtly lower than that of western 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 Rocky Mountains, is the great 
rariation 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 may, indeed, 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 usually 
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 assume 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, 
blowing 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. 
