December, '22] severin and basinger: beet leafhopper 409 



In the Sacramento Valley, the beet leafhopper was bred from 

 Orchard Morning Glory (Convolvulus arvensis) which is the most trouble- 

 some weed in the beet fields of California. 



VII. Climate 



It is evident that the food and breeding plants on the foothills of the 

 Coast Range and in the cultivated region of the Sacramento Valley are 

 not the limiting factors which prevent the beet leafhopper from es- 

 tablishing itself. Climatic barriers determine whether the foothill 

 breeding grounds are unfavorable to the beet leafhopper in this valley. 

 A comparison of the humidity, sunshine and temperattire in the mi- 

 gratory and natural breeding areas will be given. 



Humidity. — The winter himiidity is high on account of the rains and 

 fogs in the Sacramento Valley. A low atmospheric himiidity accom- 

 panied by cloudless skies is usual throughout the summer. In the 

 southern portion of the valley the relative humidity is about 10 per 

 cent higher than at the northern part. 



Precipitation. — ^A comparison of the average rainfall in the Sacra- 

 mento and San Joaquin Valleys shows some striking differences. The 

 rainfall increases northward in the Sacramento Valley and varies from 

 19.28 inches at the city of Sacramento near the southern boundary of the 

 valley to 24.9 inches at Red Bluff in the northern extremity. The pre- 

 cipitation is considerably less upon the west side of. the valley than in 

 corresponding localities upon the east side. The rainfall along the 

 west side decreases from the south to about the central part of the 

 valley and then increases to Red Bluff. The rainfall along the east 

 side increases from south to north throughout the valley. The following 

 figures show the average rainfall from south to north at the weather 

 bureau stations situated in the western and eastern halves of the valley: 



Table 3. Average Rainfall from South to North in Sacramento Valley 

 Western Half 



Vacaville 



Woodland 



Colusa 



Willows 



Corning 



In the San Joaquin Valley the rainfall decreases from north to south, 

 and with minor exceptions is considerably less on the west side of the 

 valley than on the eastern side. Stockton, in the northern part of the 

 San Joaquin Valley has an annual rainfall of 14.57 inches while Bakers- 



