continental climate with much greater extremes of temperature; the Cordilleran region, which is dependent upon the Pacific as its source of precipitation, has an arid climate; but in the east, where the Gulf of Mexico and the Atlantic serve as sources of supply, the rainfall is ample.
Central America is within the region of the trade winds, and has measurably an insular climate, owing to the narrowness of the land. That portion of South America which lies in the tropics, over which the trade winds blow continuously from the east, has a warm, moist climate and a heavy rainfall. This region is limited on the west by the Andes, over whose wall none of the moisture from the Atlantic can pass. Hence, most of the Pacific coast of South America within the tropics is a desert. In Chile and Argentina the conditions prevailing in North America are duplicated. Here in the south temperate zone the prevailing westerly winds bring to the western coast the mild, saturated atmosphere of the Pacific. The temperature is uniform throughout the year and the rainfall heavy; while east of the Andes the westerly winds, deprived of their moisture in crossing the mountains, blow dry over the land, and the semi-desert pampas are the result.
North America.—The mean annual temperature ranges from 80° F. in Central America down to 5° on the Arctic coast, and except on the Pacific coast the temperature decreases quite regularly with the latitude. On the Pacific coast the reduction in temperature with increase in latitude is much less rapid. At San Diego, on the southern boundary of the United States, the mean annual temperature is about 70°, while the Alaska coast, even as far north as Prince William Sound, has a temperature only 30° lower, and 20° higher than in the same latitude on the Labrador coast. This measures the effect of the ocean in ameliorating the mean annual temperature.
In midwinter (January) the temperature ranges from 80° in the south to -25° on the Arctic coast. Here again the reduction with increasing latitude is much less on the Pacific coast than in the interior or on the Atlantic coast. The coast of southern Alaska is 30° warmer than that of Labrador in approximately the same latitude. The midsummer (July) temperature is highest on the arid plateau of northern Mexico and in southern Arizona, where it reaches 95°. Thence it diminishes in all directions, sharply to the west as the Pacific coast is neared, and much more gradually northward and eastward. The range of summer temperature between San Diego and the Aleutian Islands is but 20°, from 70° to 50°, while in the eastern part of the continent its range is from 80° to 40°, and in the Cordilleran region from 95° to 40°. In this latter region extreme heat as well as extreme cold is frequently encountered; in southern Arizona temperatures of 120° have been recorded, and 100° as far north as latitude 60°. On the Pacific coast the range of temperature between midsummer and midwinter (July and January) seldom exceeds 20°, while upon the Atlantic coast the corresponding range is nearly twice as great, and in the Cordilleran region it is in many places three times as great.
The distribution of rainfall over North America depends upon the configuration and relief of the land and on the direction of the winds. In the region of the trade winds the rainfall is very heavy, 200 inches at Panama, and diminishing northward. In the region of the anti-trades, the Pacific coast receives nearly all the moisture brought by these winds from the Pacific, and here the amount and distribution of the rainfall are radically affected by the relative temperatures of land and sea. Where and when the land is colder than the sea, moisture is condensed from the air currents and falls in rain; the rainfall is therefore heavy on the northern part of this coast and light on the southern part, and is heavy in winter and light or entirely absent in summer.
At San Diego the rainfall, even in winter, is very light, while at Puget Sound it has increased to from 75 to 100 inches, and has an average along the Alaska Pacific coast of about 90 inches annually, most of which falls in winter. Air currents from the Pacific, deprived of most of their moisture in passing over the mountain ranges near the Pacific coast, flow over the Cordilleran region during most of the year as dry winds. In the summer, however, they retain a little moisture, which they give up to the high ranges of the interior. Hence, this region, which is upon the whole desert, or semi-desert, receives most of its scanty supply of rain, 20 inches or less, in the summer time.
Moving eastward, this general air movement from west to east, which commonly takes the form of great cyclones or anti-cyclones, draws air currents from all directions. These, coming off the Gulf of Mexico, are saturated with moisture, and cooling as they go northward, give rain to the land. Thus the great depression of the continent is watered in the main from the Gulf of Mexico, the rainfall ranging from 60 inches on the coast to 30 inches in the region of the Great Lakes and Hudson Bay. These cyclonic disturbances, as they approach the Atlantic, draw saturated air currents in from that ocean, and from that source of moisture the Atlantic coast is watered, the amount of rainfall ranging from 50 to 40 inches.
South America.—The southern continent has no such range of temperature as North America, since it lies on both sides of the equator. The annual temperature ranges from 80° to 40°, the highest temperature being in the northern part. The midwinter (July) temperature ranges from about 80° in the north to 35° in the south, and the midsummer temperature from 85° to 50°, the highest being in the interior, in northern Argentina. On the southern part of the west coast of South America, where the prevailing winds are from the west, the temperature is moderated by them as on the western coast of North America, making the winter temperature higher and the summer temperature lower. The greatest range between summer and winter is found in northern Argentina, a region corresponding in situation to the Cordilleran region in North America. Here the range between the hottest and coldest months is from 25° to 30°.
The great Amazon basin, lying within the tropics, is abundantly watered by the trade winds which come to it saturated from the Atlantic. The rainfall over this great area is estimated at from 50 to 75 inches, and in some parts is 150 to 200 inches. This heavy rainfall extends to the foot of the Andes, and even up its abrupt eastern slopes. The air currents, thus deprived of their moisture, descend the western slope as dry winds, and the narrow western base of the
