138 TEE POPULAR SCIENCE MONTHLY 



heat energy from the solar rays, the lower atmosphere is warmer than 

 the higher atmosphere. 



The source of the water which falls in the form of rain or snow 

 in the United States is erroneously stated in several geographical text- 

 books to be the Pacific Ocean. Such a statement is doubtless based 

 upon the delusion that the United States, located in a region of pre- 

 vailing west winds, naturally should receive precipitation from the air 

 which has been moving for thousands of miles across the Pacific, and 

 therefore must have accumulated as much moisture as its temperature 

 will allow it to carry. As a matter of fact, by far the greater propor- 

 tion (one authority says 90 per cent.) of our precipitation has its source 

 in the Gulf of Mexico and in that part of the Atlantic Ocean lying 

 directly east and southeast of the continent. West of the Pocky Moun- 

 tains the precipitation comes ultimately from the Pacific, but as the 

 rainfall throughout this large area is deficient, except in western Wash- 

 ington and Oregon, the sum total is small compared with that of the 

 nation as a whole. General and widespread precipitation accompanies 

 the passage of a barometric depression, where the winds in its front, 

 blowing from points between northeast and south, discharge a part of 

 their load of water vapor in the form of rain. The condensation is 

 brought about primarily through the cooling air compressing some of its 

 moisture from it, the lowering temperature being caused by a passage of 

 the air from the relatively warm Atlantic or Gulf to the relatively cold 

 continental interior in winter, or from ascending, expanding, and there- 

 fore cooling air during summer. Even so large a water surface as that 

 of the Great Lakes contributes but little to the total rainfall of the 

 United States. 



Northeast storms, a characteristic feature of the winters of the 

 Middle and the North Atlantic States, do not come from the northeast, 

 as many infer. The strong, northeast, rainbearing winds do, it is true, 

 bring their loads of moisture from the Atlantic Ocean, but they are 

 simply the indraught of a barometric depression which the weather map 

 shows has come from the west or southwest, usually along a well recog- 

 nized track. Only upon rare occasions does a storm travel from east to 

 west in these latitudes, and storms of this type, called " flarebacks," are 

 still a stumbling-block in weather-forecasting. In general, a storm or 

 barometric depression is accompanied by winds blowing in a counter- 

 clockwise direction and spirally inward toward the center. An exami- 

 nation of the weather map when a northeast storm is in progress will 

 show that the center of the disturbance is southwest or west of the ob- 

 server, the winds backing to northwest when the center subsequently 

 passes close by or south of the point of observation in its easterly or 

 northeasterly movement. 



The tradition that the climate of a city is very different from 



