66 b 



seasonal swing north-»7ard and southward. And this even 

 distrioution of humidity and teinperature (except near ths^ 

 continents) favors rather uniform belts of pressure and of 

 winds, with their fair weather where the pressure is high, 

 in latitudes about 20® to 40°, and their showery or stor/iy 

 weather where the pressure is low, near the equator and fron 

 high middle to sub-polar latitudes. Furthermore, the flat « 

 nees of the surface of the ocean permits the maximum devel" 

 opnent of rotary storm movements, such as the winds of a 

 West Indian hurricane, 



^{tiexe lands lie athwart these wind and storm belts 

 they receive a full measure of oceanic weather on their 

 windward margins, as on our ITorth Pacific coa.st. If, how- 

 ever no high mountains, like our western ones, form a 

 barrier, marine influences are felt hundreds, even thousands 

 of milss inland, as in Europe and the eastern United States, 

 Winds and storms from the Gulf of Mexico and other tropical 

 waters of the ;vestern Atlantic thus traverse eastern North 

 America and provide the rainfall for this vast agricultural 

 region. 



The continents throw a diverse land surface across 

 the lacitudinal belts of moisture, temperature, pressure, 

 winds and storniness fostered by the oceans, and thereby 

 interrupt the continuity of these belts. The lower hum- 

 idities of the air over land, the high temperatures in 

 sur.-urer and the low ones in winter favor strongly contrasted 

 pressure conditions in the warm and cold seasons. In sumr- 

 mer the continental air is expanded and a considerable 

 quantity is forced to overflow into the cooler oceans; in 

 winter the air over land is chilled and contracted so much 

 that groat masses of -^ir return aloft from the sea. Thus 

 continental air pressures tend to be lovi in summor and high 

 in winter, while oceanic air pressures tend to be high in 

 s^oiraer and low in winter. The major areas of high and low 

 pressure, whi^h are essentially the oceanic and continental 

 sections of the planetary pressure belts modified, as just 

 outlined, by the contrasted humidity and temperature coi>- 

 ditions, have long been known as the grand centers of 

 action, Th^iy are the large areas of high or low pressure 

 around a.nd from which or around and into ?;hich the prevail- 

 ing winds blow. 



Recalling that onl^y one of the half dosen centers of action 

 by which our Atlantic seaboard, in fact, the eastern half of 

 the United States, is dominated either in winter, or in 

 surxier, is continental, the importance of the oceanic centers 

 i s at once apparent. 



If these centers of action went through their seasonal 

 transformations v;ith consistent regularity year after year, 

 their nature and underlying causes would not give us much 

 of a challenge; but such is not the case. 



It is, of course, easy to surmise that if appreciable 



