Climate of United States. 145 
the rainfall is, in some months, above 6 inches. The first is in the extreme 
northwest. It begins at Tatoosh Island, Washington in October. It grad- 
ually increases and reaches its greatest extent in December. At this time 
the line of six inches of rainfall, entering the United States on the north, 
south of Vancouver Island, extends down along Puget Sound; thence south- 
ward to Mount Rainier; thence eastward along the Columbia River up to the 
Cascade Mountains and back to the Willamette; thence south, not far from 
the coast, disappearing on the coast not far to the north of San Francisco, 
California. The highest monthly rainfall at this time is 14.5 inches in the 
vicinity of Tatoosh Island, Washington. There is a small outlier in the Sierra 
Nevada Mountains. The second area is a very variable one in Alabama and 
Mississippi and extending to some extent into adjoining states. It begins 
in January and continues until March. The highest rainfall is 9 inches in March. 
Within it is small area of unusually high rainfall at the southern extremity of 
the Appalachian range, with an annual precipitation of over 76 inches. The 
third area is one in northeastern Texas extending slightly into Indian Terri- 
tory. It is found there in April and May, the highest rainfali being 9 inches. 
The fourth area is a small patch to be found in northern Missouri in May 
and June, the highest rainfall here being 9 inches. The fifth area beginning 
properly in March and continuing to October, starts at Cape Hatteras, 
extends gradually inward, never reaching far from the coast, and attains its 
extreme limit in July and August. At this time, it covers extreme south- 
eastern Virginia, the eastern third of North Carolina and a considerable 
part of the coast of South Carolina. The sixth area is the region of the 
subtropic rains in Florida. It begins in June and ends in October. Its 
greatest expansion is in July, and also the greatest rainfall, which amounts to 
about ro inches. It extends into southern Georgia, thence westward along 
the Gulf coast to the middle of the coast of Louisiana. 
Topography and its effects on rainfall. From the great plains westward 
the lines of equal rainfall are, approximately, north and south. They are 
approximately parallel with the Atlantic coast. In the lake region, while they 
approach parallelism to the parallels of latitude, yet there are some variations, 
evidently due to the effects of these great bodies of fresh water. In general, 
the rainfall decreases also with the elevation above sea-level. Certain peaks 
and mountain ranges on the other hand are outlined by the mean rainfall, 
This is true of the Sierra Nevadas and to a less extent in the Cascades at 
the Dalles, where the Columbia River breaks through. In spring, this area is 
distinctly outlined with a rainfall of ı5 inches, while in the Willamette Valley, 
it is only ıo inches and immediately to the east of the mountains only about 
5 inches. In winter, the whole range is outlined by the lines of greater rain- 
fall, about 38 inches or more. 
The rainfall to the leeward and windward side of mountain ranges is also 
marked. On the Pacific coast, where the prevailing winds are distinctiy from 
the west, the western slopes of the Sierra Nevadas show on the annual map 
Harshberger, Survey N.-America, 10 
