CLIMATE. The greater part of the area of the United States proper has a climate pertaining to the temperate zone. The average annual tem peratures vary from somewhat less than 50° on the northern border to 75° in the extreme south east. The average temperature for July is about 00° on the northern border and for -January it is about 20°. The whole country is exposed to much greater annual oscillations of temperature than occur in Europe. The average of the abso lute maximum temperatures is as high as 115' to 120° in the drier portions of Texas and Arizona, and the average minimum falls as low as —10° in northern Minnesota. The climate of the United States is controlled very largely by its characteristic winds, In January these are from the west, hut in :111/y southeast winds prevail in the Southern States, which penetrate up the eastern slope of the Rocky Mountain range, hut finally turn eastward with the wester ly winds of the northern portion of this region.
The storm centres that pass across the country originate almost equally on the Pacific Ocean and on the Atlantic. Those that approach from the Pacific move sontheastwardly into Kan sas and Nebraska and then turn eastward over the lake region: those that come from the At lantic move westward among the West Indies and turn northward toward the Lower Lake Region; a certain number originate on the southeastern slope of the Rocky Mountains and move north eastward to the lake region. The lake region is, therefore, on the average the stormiest portion of the continent, and indeed of the whole North ern Hemisphere. The passage of every storm centre is accompanied by a shifting of the winds from warm southerly to cold northwesterly, and this, consequently. gives to the interior of the United States its extreme variability of climate, so that even the southern extremity of the coun try scarcely belongs to the tropical zone. The cold air that flows in behind the storm centres is very dry and the sky is quite clear. Thus originate the so-called anti-cyclones or cold waves which follow each other rapidly. moving south eastwardly toward the Gulf of 'Mexico, while the areas of low pressure or cyclones move northeast over and beyond the lake region. The absolute annual range of temperature is therefore great est in the interior of the country, being about 150° in the upper portion of the \lissonri Val ley and diminishing to 60° in the southern part of Florida and the northwestern part of Oregon.
The suddenness with which the air temperature falls is an important consideration from many points of view. Thus changes of 20° in twenty four hours occur far more frequently in the lake region than in the Ohio Valley, and there again oftener than on the southern coasts.
The average date of killing frosts in the spring and autumn determines the average length of the growing season for most of the important crops. The autumnal (late is September 1st for the region from North Dakota to Lake Huron, and October 1st for the region from Colorado to Pennsylvania and northeastward along the New England coast. The latest spring frosts occur on May 15th from Idaho to Lake Superior and February 15th along the south Atlantic and east Gulf coasts. The growing season may be con sidered as the interval between the last frost of spring and the first of autumn, or it may also be defined as the season within which the average daily temperature does not fall below 40°. (See Maps accompanying article FROST.) From this point of view the growing season diminishes as one goes northward and amounts to about 120 (lays at the northern border of the United States. By a natural process of selection, stimulated by culture, plants that formerly required this length of time for maturity are now pushing north ward beyond the borders into Canadian regions where the growing period is as much as twenty days shorter. Mr. Peunywitt of the Weather Bureau has published, in the Monthly Weather Review for February. 1901, three charts showing the dates on which the normal daily temperature at any place equals the annual mean at that place. There are two such dates for each sta tion, dividing the year into the warmer half and the colder half. These are not quite the dates of frost, hut generally come earlier than the frost in spring and later in autumn. His chart reveals the interesting fact that in general over the United States the warmer portion of the year is decidedly longer in duration than the colder season. This difference is greatest on the east ern slope of the Rocky :Mountain range, and in creases from thirty days in Texas and Kansas to forty-eight clays on the northern horder of Mon tana and probably increases somewhat as one goes north into Canada.