Page:Popular Science Monthly Volume 86.djvu/142

138 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 textbooks to be the Pacific Ocean. Such a statement is doubtless based upon the delusion that the United States, located in a region of prevailing 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 proportion (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 Rocky Mountains the precipitation comes ultimately from the Pacific, but as the rainfall throughout this large area is deficient, except in western Washington 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 therefore 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 recognized 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 counterclockwise direction and spirally inward toward the center. An examination 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 observer, 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