IN RAIN AND SNOW, IN THE UNITED STATES. 121 



and more on the Pacific Coast in Washington Territory ; on the Gulf Coast 61 

 inches appears to be the maximum amount, and 48 on the Atlantic Coast. 



The principal supply of rain over the United States comes from the Gulf of 

 Mexico ; its diffused vapor can be traced from the eastern slope of the Rocky 

 Mountains to the Great Lakes : while the supply of vapor from the Atlantic Ocean 

 is. distinctly traceable over that area lying north and east of Virginia. All States 

 and Territories west of the Rocky Mountains receive their supply of rain from the 

 condensed vapors of the Pacific Ocean. 



There are distinct localities of entry of maximum rain from each of these basins 

 of supply ; the vapors from the Pacific are deposited within a remarkably well- 

 defined coast region between latitude 41° and our boundary at the Straits of Juan de 

 Fuca ; the rain pours down with great intensity on the coast between the mouth of 

 Columbia River and Cape Flattery. It is surprising how little rain falls on the 

 Pacific coast between San Diego and Cape Mendocino, and how quickly the atmo- 

 sphere becomes drained of its vapor as we leave the coast and proceed inland in 

 latitudes north of 41°. The coast range of mountains here act powerfully as 

 condensers by forcing the air up their western slopes. 



The densest part of the Gulf vapor is thrown over the delta of the Mississippi 

 River, and as far east as longitude 86° its axis of diffusion can be traced distinctly 

 to the west end of Lake Erie ; it is inclined towards the northeast for two reasons — 

 the effect of the earth's rotation on a flow from the south, and the influence gene- 

 rally of the prevailing westerly winds. A second sweep over the country occurs in 

 southern Florida, most likely due to the immediate proximity of the Gulf Stream ; 

 and there is a third, as yet undefined, influx, passing through Georgia and South 

 Carolina. 



The condensation of vapor from the Atlantic is most apparent a short distance 

 inland at the following localities : Along the coast of Maine near Eastport and 

 near Portland, in central Connecticut, western Massachusetts, and extending to 

 southern Vermont, and near the entrance of Chesapeake Bay. Upon the whole, 

 hills and mountain ranges appear to have a comparatively small directive influence 

 upon the distribution of the rain. Florida, which may be considered as almost 

 perfectly flat, exhibits well-defined bounding lines of rain distribution. River courses 

 also seem to influence the amount of rain, as along the Rio Grande. At the mouth 

 of the Hudson River, the curves become suddenly contracted ; and some similar 

 feature can perhaps be traced out on the Mississippi delta near New Orleans. 

 Beyond furnishing by their evaporation a supply to the general fund of moisture, 

 the Great Lakes do not appear to exercise any direct influence ; on the yearly average 

 the rains along their borders are not increased. There is even a remarkably small 

 amount of rain-fall in northern New York, close to Lake Ontario. The effect of 

 equalizing the temperature produced by all large bodies of water has no doubt a 

 direct influence upon the distribution of rain ; the greater and more sudden the 

 variations in temperature, the greater, comparatively, the rain-fall. 



The laws of the distribution of the rain-fall, as far as they depend upon the 

 changes of temperature and direction of wind, can be studied to better advantage 

 by means of the two charts showing the distribution in summer and in winter, than 



16 June, 1871. 



