144 Part II. Chapter 2. 
Moisture in the Air. In January, the least moisture (0,5 grain per cubic foot) R 
is found: in the extreme northwest; this increases quite regularly to 4 gr. on 
the Gulf, 6.5 gr. at Key West, nearly 3 gr. on the Pacific, 2.1 gr. at Yuma, 
1.3 gr. at Chicago, 1.6 gr. at New York City and ı.7 gr. at Washington. In 
July, the northwest has 5.5 gr. per cubic foot, increasing to 9 gr. on the Gulf 
coast. The lowest, 1.5 gr. is found at Winnemucca. All through the region 
from the Rockies westward, except on the Pacific coast the air is very dry, 
hardly reaching 3 gr. per cubic foot. Chicago has 5.7 gr.; New York City, 
6.7 gr.; Washington 7.0 gr.; and Key West, 9.1 gr. Corpus Christi has 9.3 gr. 
the largest in the country. 
Precipitation. The annual precipitation, unlike the temperature varies inan 
east and west direction more than from north to south. The Atlantic coast 
receives over 40 inches in the north and over 5o inches in the south. The 
lake regions have from 33 in. to 49 in., the Gulf coast has about 60 in., but 
this diminishes toward the south. The extreme point of Texas has but 32 in., 
and Key West 4ı in., while the Dry Tortugas in the Gulf have even less. 
The upper Missouri has about ı5 in., as also the north and the middle Rocky 
Mountain region. Yuma has 3 in.; San Diego, ı0 in.; San Francisco, 23 in.; 
Portland, Oregon, 5o in. and the rainiest part of the whole country in no 0 
western Oregon. | Si 
Least Rainfall. The least rainfall in the United States is found in the we: 
and southwest. The limit of least rainfall for the purpose of this discussion 
will be taken at ı inch per month. The area of least rainfall, as defined above, 
is smallest in April. At this time it occupies New Mexico, the most of Utah 
and Nevada and parts of the adjoining states, By May, the western boundary 
of this area has traveled westward and strikes the Pacific coast, and progresses 
northward until in August, it leaves the coast and the line crosses the area 
of the United States approximately from north to south. Meantime the hea- 
vier rainfall has begun in the extreme northwest, and a western boundary of 
the area appears in this month for the first time. The area in August, there- 
fore, is bounded on the east by a line which is, approximately, the meridia ; 
of 115°. It is bounded on the west by a line which crosses the extreme 
northwestern angle of the United States. With the succeeding months, this. 
band travels gradually eastward and reaches its extreme eastern position i 
the winter months. With the beginning of spring, the area travels rapi 
westward, and by April it has taken up the position with which we beg 
The area of least rainfall is not confined to the United States. It extends Ä 
the southward over Mexico, and this extension continues during the whole 
year. There is an extension northward which begins in July and lasts v 
March. In the period of greatest extension in the United States, namely; 
winter, the band extends northward and slightiy eastward, and probably oCeNT 
pies the plains to the north of us, finally joining the Arctic area of "least rail 
Greatest Rainfall. Six inches of rain per month is taken in this discus‘ 
as a maximum. It appears that there are six different regions W 
