AND CONSTRUCTION OF RAIN-FALL CHARTS OF THE UNITED STATES. 189 



amount. It at once discloses the broad feature of two nearly equal parts, one of 

 which, or the eastern area, is well sui3i:)lied with rain, and the other, or the western, 

 is deficient in amount ; the meridian of about 99° west of G-reeiiwich (one or two 

 degrees more or less) separating these areas. We further recognize the fact that the 

 main source of vapor by whose condensation rain is produced is the Gulf of Mexico, 

 and by it nearly the whole of the area lying east of the above meridian, and com- 

 prising the most fertile region of the United States, is supplied, and that but a narrow 

 strip of coast along the Atlantic hardly fifty miles (more or less) broad receives an 

 additional supply from the evaporation of that ocean. Similarly there is a narrow 

 strip of coast in the western ai-ea, in Oregon and Washington Territory, which is 

 supplied with abundant moisture from the Pacific Ocean. Over the eastern area the 

 amount is nowhere less than 20 inches ; it reaches about 50 inches along the Atlantic 

 coast, and on the Gulf coast in Louisiana it attains its maximum of about 62 inches. 

 Over the western area the amount is generally less than 20, sinking even below 8 

 inches over a great part of it, excej)t in the extreme northwest, on the immediate sea 

 coast, where it rises to more than 70 inches, thus surpassing the greatest amount in 

 Louisiana. To understand this distribution and to explain its immediate cause, we 

 shall find much light thrown upon it by collocation with the general motion of the 

 atmosphere over the United States as shown by the prevailing winds, charted in 

 direction and frequency (and velocity) by the late Professor James H. Cofiin.* The 

 general direction of the surface winds may be summarized as follows : The normal 

 or average dii-ection of the winds over nearly the whole of this surface is from the 

 west, — in fact, they sweep from the Pacific to the Atlantic coast, — but are impinged 

 upon by a southerly current derived from the trade winds, which, as an east wind, 

 crosses the j)eninsula of Florida up to the limit of about 30° of latitude, where the 

 mean annual temperature is about 68° Fahrenheit, passes the northern part of the 

 Gulf of Mexico, and blows across its shores in Louisiana and Texas as a south or 

 southeast wind, turning more southerly as it passes up the country (Indian Terri- 

 tory) and southwestly (in Arkansas and Missouri,) and finally merging into the gen- 

 eral westerly drift. The winds, as they emerge from the Atlantic coast between 

 North Carolina and Maine, assume an average direction of about west by north. 

 These winds coming from the Gulf fully explain where the supply of vapor is ob- 

 tained which is sftread in rain over Texas, Louisiana, and Alabama, in that singular 

 fan-like shape or in streaks as shown on the chart. As they pass up into Tennessee 

 and North Carolina they tend more to the westward — a consequence of the earth's 

 rotation. The great lakes do not appear to influence perceptibly the distribution of 

 rain-fall in their vicinity, unless the detached, quite circumscribed, areas of greater 



* Smithsonian Contributions to Knowledge, No. 26S, " The Winds of the Globe, or the Laws of Atmospheric Circulation 

 over the Surface of the Earth," Washington, 1876. 



