A moment’s inspection will show that the middle group is intermediate between the northern and southern in all its characters. The spring quota of rainfall progressively diminishes from north to south, and so does the summer, while the fall and winter quotas increase. What is lost in summer is gained in winter, and thereby the inequality of rainfall from season to season is diminished, so that a rainy season is not so well defined in Texas as in Dakota. What is lost in spring is gained in autumn, and thereby the place of the rainy season in the year is shifted. In Dakota the maximum of rain is earlier than in Texas, and corresponds more nearly with the maximum of temperature.

Table VIII.—Seasonal precipitation in the San Francisco Region.

Total extent of record = 130 years.
Mean of yearly rainfalls = 15.90.

Second District.—In the San Francisco Region a rainy season is still more definitely marked, but occurs at a different time of year. It will be seen by Tables III and VIII that no rain falls in summer, while the winter months receive 60 per cent. of the annual precipitation, and the spring 25 per cent. The general yearly rainfall of the district is only about 16 inches, but by this remarkable concentration a period of five months is made to receive 13 inches. The winter temperature of the district is no less remarkable, and supplies the remaining condition essential to agriculture. Frosts are rare, and in the valleys all the precipitation has the form of rain. The nine stations which afford the rainfall records given above show a mean spring temperature of 57° (see Table IX). Thirteen inches of rain coming in a frostless winter and spring have been found sufficient for remunerative agriculture.

Table IX.—Mean temperatures, by seasons, for the San Francisco Region.

The same winter maximum of rainfall is characteristic of the whole Pacific coast. The Region of the Lower Columbia, with an average rainfall of 46 inches, receives 47 per cent. of it in winter and 24 per cent. in spring. Southward on the coast, Drum Barracks (near Los Angeles) and San Diego receive more than half their rain in winter, but as the whole amount is only 9 inches agriculture is not benefited. The eastern bases of the Sierra Nevada and Cascade Range exhibit the winter maximum of rainfall, and this feature can be traced eastward in Idaho and Nevada, but in these districts it is accompanied by no amelioration of winter temperature. (See Table X.)

Table X.—Seasonal precipitation and temperatures on the Pacific coast, etc.

Third District.—In Arizona and New Mexico there is a general maximum of rainfall in summer, and a restricted maximum in winter. The principal minimum is in spring. In Table XI the stations are arranged according to longitudes, a disposition well suited to exhibit their relations. In eastern New Mexico the distribution of rainfall has the same character as in adjacent Texas, but with a more decided maximum. Half of the total rainfall is in summer and half of the remainder in autumn. Westward the maximum diminishes slightly, but it appears in every station of the two territories. In western Arizona the winter maximum of the Pacific coast asserts itself, and it can be traced eastward as far as Fort Wingate, New Mexico. Except at Camp Mohave, on the western border of Arizona, it is inferior in amount to the summer maximum.

Table XI.—Seasonal precipitation in Arizona and New Mexico.

In all this region the daily range of temperature is great, and frosts occur so early in autumn that no use can be made of the autumnal rainfall. The yearly precipitation is very small, and the summer quota rarely exceeds seven or eight inches. Nevertheless the Pueblo Indians have succeeded, in a few localities, and by a unique method, in raising maize without irrigation. The yield is too meagre to tempt the white man to follow their example, and for his use the region is agricultural only where it can be watered artificially.