114 BULLETIN 12.33, U. S. DEPARTMENT OF AGRICULTURE. 



Table 33. — Distribution, by hours, of sunshine in representative decades at Statu 



F-l. 



[Average, 1910-1918, inclusive.] 



Period. 



Sunshine duration in minutes. 



For hour ending (a. m.) — 



First decade, 

 March 



Last-, June I l. 



Middle, August...). . . 



Third decade, I 

 April.. I... 



2.5 

 42.7 

 18,9 



19 



2S.9 

 52.1 

 44.6 



37.6 



46.7 

 52.5 

 51.8 



53.7 

 52.9 

 49.5 



42.3 



53.8 

 51.8 

 41.9 



12 

 noon. 



53.3 



51 



33.7 



42.3 



For hour ending (p. m.] 



54.8 

 48.6 

 28.6 



40.5 



51.4 

 42.5 

 26.6 



40.4 



47.7 

 38.8 

 21.9 



36 



43.7 

 37.8 

 17.1 



IS. 9 

 35.4 

 16.8 



27.1 i 22. 



30.3 

 11.6 



13.2 



To show more clearly the immediate causes for high and low sun- 

 shine percentages, there are given in Table 33 and Figure 9 the dis- 

 tribution of sunlight by hours for representative decades. 



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i to. 9.— Normal sunshine distribution at control to July, 1921. 



The first three months of the year, when anticyelonic winds pre- 

 vail, are characterized by high percentages of sunshine, which reach 

 a climax at the middle of March. After this, as storm centers move 

 eastward in a more northerly path, the humidity of the air is greatly 

 increased, and this makes itself felt in a great deal of foggy weather 

 in April or May, accompanied by much snow or rain. In general, a 

 tendency is noted for the fog to be dissipated in the middle of the 

 day. The control station is at such an elevation as to be frequently 

 engulf ed by the clouds which hang over the plains country, while 

 above this elevation it may be clear. Or, again, it may be clear over 

 the plains, but the rising air currents during the day induce the for- 

 mation of clouds over the mountains. 



As the storm centers move farther north, this locality is character- 

 ized again l>\ ilry weather, which reaches a culmination at the end 

 of June, after which time the summer rains begin. Although the 



