THE WEATHER BUREAU 393 



tendency of cool, clear air is to fall. Being heavy, gravity 

 pulls it down from the regions above the clouds to the earth. 

 When it strikes the ground, having no other course left open 

 to it, it flows out over the surface of the earth from the center 

 of each clear weather region in all directions. As the air 

 travels outward it picks up moisture and becomes warm, 

 and the barometers show that it is becoming lighter and 

 lighter as it flows away from the fair weather centers. It 

 is inevitable that part of the air which traveled west from 

 the Atlantic fair weather section should come into contact 

 with air traveling east from the Pacific fair weather section. 

 The contact at once causes a mixup, which takes on a gyra- 

 tory motion. The two currents of air rush together and 

 begin rotating in a great circle. Having become heated 

 in travehng, the whirling column of hot air ascends, dropping 

 moisture in the form of rain, until it is dissipated far above 

 the earth. 



So we have the low pressure or storm area situated be- 

 tween two high pressure or fair weather areas and receiving 

 its energy from them. While this relation may be constant 

 as regards the areas themselves, j^et the whole system moves 

 slowly across country. That is, there is a constant succession 

 of fair and stormy areas traveling eastward at intervals of 

 three days, and at the rate of twenty two miles an hour in 

 summer and thirty seven miles an hour in winter. Storms 

 always move to the eastward, and no matter where in the 

 United States they form, they always curve in the direction 

 of New England. It is an invariable law. 



This is the background of the panorama that flits across 

 the forecaster's mind. On this background he sees the 

 shadowy paths of monthly storms; for each month of the 

 year has its own class of storms. These storms enter the 

 country at certain places, and follow well defined courses. 

 United States storms, according to Professor Bigelow, have 

 nine average places of generation. The great majority form 

 in Alberta, north of Montana, and after coming into the 

 United States, travel eastward. A few come in over the 

 North Pacific coast. A third group forms on the northern 

 Rocky Mountain plateau. A fourth forms in Colorado, being 



