DISCUSSION AND ANALYSIS OF WINDS. 



685 



when the heated and rarefied air of the plains draws in that of the surrounding 

 regions. 



On the west these plateaus are walled in by ranges of mountains, and the 

 indraught of air from the Pacific slope is thus prevented. 



We know that there is a depression of the barometer in summer over the pla- 

 teaus of the interior, but there are yet too few observations to decide as to the 

 region where this depression is greatest. It is, however, most probable that it is 

 in Utah. 



There is also a low region, where pressure must be low in summer, that is the 

 valley of the Gila and lower Colorado. The heat is extreme there, Fort Yuma 

 and vicinity having the warmest summer in America, and the ascending current 

 must be very powerful. Air is drawn in towards this hot region, and, owing to its 

 geographical position, principally from the south, from the Gulf of California. (See 

 also Map of Isobars, PI. U, and of Winds, PI. 8 and 11.) 



The following table gives the percentage of winds of the region east of the 

 coast: — 







Summer. 



Winter. 



Percentages. 



fe 



p4 



« 



w 



03 



m 



o5 



^ 



^ 



fc 



fc 



a 

 fc 



7 

 8 

 4 

 3 

 11 

 18 

 9 

 5 

 6 



3 

 12 



m 



6 

 8 



11 

 5 

 9 

 6 

 8 

 3 

 8 



14 

 3 

 6 



m to 



^ 





Fort Yuma, Cal. . . . 

 N W. Arizona .... 





5 

 9 

 8 



11 

 4 

 2 



10 

 3 



19 

 5 



18 



9 

 6 

 4 

 7 

 4 

 4 

 9 

 1 

 11 

 7 

 6 



10 



7 

 3 

 6 



4 

 11 



12 

 3 



11 



12 

 7 



11 



21 



19 



24 



7 



17 



22 



10 



7 



8 



9 

 13 



23 

 32 

 25 



17 

 36 

 22 

 20 



21 



16 



5 



5 



24 



17 

 13 

 16 

 15 

 21 

 20 

 14 

 31 

 10 



11 



6 

 7 



12 

 7 

 14 

 26 

 6 

 13 

 19 

 23 

 15 

 39 

 35 

 19 



4 



6 



5 ; 

 11 



8 



5 



6 

 12 

 10 

 16 

 14 

 11 



21 

 35 



18 

 13 

 22 

 11 

 14 

 25 

 25 

 7 

 21 

 10 



14 

 12 

 10 



5 

 15 

 17. 

 12 



9 

 14 



8 



7 



8 



7 

 9 

 15 

 13 

 13 

 5 

 15 

 10 

 14 

 20 

 10 

 17 



9 



8 



18 



18 



9 



10 



12 



15 



8 



30 



13 



6 



15 

 9 



14 



28 



7 



22 



16 

 20 



10 



19 

 27 

 25 



20 

 11 

 10 

 15 

 15 

 11 

 14 

 13 

 14 

 16 

 16 

 16 



Ceutra' Arizoua . . . 



Central New Mexico . . 

 S. New Mexico .... 

 N. W. and N. New Mexico 



S. W. Utah 



N Central Utah . . . 

 W. and S. W. Montana . 

 N W. Montana . . . 

 N. Central Dakota . . 



The predominance of southerly windg in summer, as shown by this table, is 

 very great, and it must be remembered that the greatest part of this region is 

 mountainous, and thence great local discrepancies should be expected. The period 

 of observation was short in nearly all cases. Considering this, the agreement 

 between the different regions is very satisfactory. (See Plates 8 and 11.) In Utah 

 there are less southerly winds in summer, and still less in Montana. But this is 

 easily explained. As Montana lies north of 44° N. latitude where there is no 

 extensive plateau, and the mean height of the Rocky Mountains is less than to 

 the south — the westerly winds from the Pacific can therefore readily reach Montana. 



We should also expect to see southwesterly winds in winter in Montana, as in 

 California and Oregon. This is really the case. In Arizona and New Mexico, on 

 the contrary, the winds are much more northerly in winter than in summer. I give 

 below the mean direction of the wind in some of the regions here considered. (See 

 also maps, Plates 5, 6, 8, and 11). 



