the United States of America

The Psychozoic age is the present and has little geologic signifi cance. As Charles Schuchert, the doyen of American palaeontolo gists and stratigraphers, states: "We are now living in a time of rugged lands, obliteration of old peneplains, cold polar climates, and marked temperature belts. . . . Human mentality now domi nates the organic world, and to it all creation will soon be more or less subservient." The climates which are found within the boundaries of the continental United States are extremely diverse, yet it is possible to deduce certain facts of broad application concerning them. The country lies wholly in the temperate zone. Hence its mean annual temperatures are intermediate between those of the Arctic regions on the one hand and those of the torrid zone on the other ; a fact not inconsistent with the occurrence, in some parts of the country, of summer maximum temperatures surpassing any found within the tropics, and in other parts of winter minimum temperatures rivalling those of the Arctic. The country also lies, for the most part, within a so-called "belt of prevailing westerly winds." This expression implies a general drift of the atmosphere from west to east within the region concerned, but it does not imply anything like constant wind directions in particular localities. The winds of this belt are much modified locally by topography, they are sub ject to certain seasonal variations, and they vary markedly, es pecially in the northern part of the country, with the constant pas sage of the disturbances known as cyclones and anticyclones. These disturbances, which, with some exceptions, cross the country in a general west-to-east direction at speeds averaging from 500 to 600 m. a day, bring with them frequent weather changes—ups and downs of temperature, alternations of clouds and sunshine—which are a striking feature of the climate in most parts of the United States.

The eastward atmospheric drift from the Pacific ocean gives the western coast of the United States a marine climate ; which means essentially a small range of temperature, annual and diurnal. This is, however, confined to the immediate border of the Pacific, on account of the mountain barriers that prevent the in fluence of the ocean from extending far inland. Even the rela tively low coast range gives the valleys east of it a quasi-conti nental climate, and "continentality" becomes pronounced east of the lofty Sierra Nevada-Cascade system. Wide ranges of tem perature are found thence eastward all the way to the Atlantic coast. The mountain barriers above mentioned, besides their striking effects on temperatures to the eastward, precipitate most of the moisture brought by winds from the Pacific on their windward slopes, and are thus responsible for the great region of arid and semi-arid climates, embracing about one-third of the country, which extends roughly from the I2oth to the moth meridian. Farther east a moderate to ample rainfall is supplied mainly by intermittent tropical air currents, which import mois ture from the Atlantic and the Gulf. Another factor of major

importance controlling climates east of the Rockies is the absence of mountain ranges trending east and west. A broad open low land stretches from the Arctic ocean to the Gulf of Mexico. Thus polar air masses moving southward from the interior of western Canada and masses of tropical origin moving northward have no topographic barriers and they exercise profound effect on the dis tribution of both temperature and precipitation. In winter, south ward moving, dense, cold air sometimes causes abrupt changes from mild to severely cold weather and carries freezing tempera tures as far south as the Gulf coast. In summer warm, muggy, tropical air coming from over southern waters often affects the comfort of people living far inland.

Temperature.

In the United States, January is normally the coldest month of the year; the mean monthly temperature ranges from about zero in the extreme central-north to above 50° in more southern sections. Thus the winter north-south temperature range is large. The lowest temperature of record is —66° in Yellow stone National park, Feb. 1933. In the memorable freeze of 1899 subzero temperatures were carried as far south as extreme north western Florida. July, with a mean temperature ranging from 65° locally in the extreme north to slightly above 80° in Gulf sections, is usually the warmest month of the year. Every State in the Union has experienced maximum temperatures of oo° or higher. The highest of record is 134° at Death Valley, Calif., observed on July to, 1913 ; this is the second highest shade temperature of record for the world.

Precipitation.

On a basis of average annual precipitation the United States may be divided into an eastern and a western part, the dividing line roughly coinciding with the tooth meridian, in the vicinity of which the amount is about 20 inches. To the east of this the climate is sub-humid to humid, but to the west much of the land is semi-arid to arid, primarily suited for grazing purposes rather than for growing crops. About 40% of the land area receives, on the average, less than 20 inches of precipitation annually; about 35%, 20 to 4o inches; and 40 to 6o inches. The heaviest precipitation, more than too inches a year, occurs near the coast of the Pacific North-west, and the lightest, less than 5 inches a year, in south-eastern California and some ad joining localities.

Frost and the Growing Season.

The crop growing season, or period between the average date of last killing frost in the spring and the first in the fall, ranges from Too to 120 days locally in the extreme north to about 26o days in Gulf sections. In some elevated western sections the growing season is less than 3 months in length. On the other hand, in southern Florida kill ing frost usually occurs in less than half the years.