Facts About the Climate of North America
The climates of North America range from tropical in southern Mexico to polar in Greenland and the Canadian Archipelago. Except in the areas of high mountains, certain well-defined climatic types may be recognized. Within high mountains, local site conditions cause extreme variety in relatively small areas. For this reason, the high mountains are grouped together to be discussed at the end of this section.
A huge area of arid and semiarid climate occurs through and adjacent to the Western Highlands. The western edge of this dry climatic belt starts on the west coast just south of Los Angeles and follows inland and northward along the eastern face of the Sierra Nevada and the Cascades. The margin continues around the central Rockies and still northward into southern Alberta in Canada. There it leaves the Rockies to swing in an eastward and southward arc across the Great Plains of the United States into southern Texas.
In Mexico, the dry belt includes most of the Mexican plateau, the land between the Western Sierra Madre and the Gulf of California, and the whole of Baja California. Within the bounds thus described, the dominant climatic feature is insufficiency of moisture. Baja California, the southern part of the Basin and Range country in the United States, and the central portion of the Mexican plateau are truly arid with precipitation averaging less than 10 inches (25.4 cm) per year. Throughout the dry belt, though moisture is generally deficient, there is between 10 and 20 inches of precipitation annually. But rains are infrequent and unreliable.
On the Great Plains, reliability and frequency both increase eastward until the lands with ample moisture are reached, roughly along the 100th meridian which cuts through the Dakotas, Nebraska, Kansas, western Oklahoma, and central Texas. Within the dry belt, temperatures fluctuate markedly with the seasons and diurnally. At Yuma, Arizona, for example, a maximum shade temperature of 120° F. has been reached and the minimum of record is 22° F. January averages 55° F. and July, 91° F. The highest air temperature (in shade) ever recorded in North America was noted in Death Valley (134° F.). Northward and northeastward from southwestern United States, temperatures are lower both in summer and in winter. The northern part of the dry belt in Alberta can be bitterly cold in winter. Likewise, the elevation of the Mexican plateau modifies the temperatures so that Mexico City's warmest month averages only 65° F., 10 degrees lower than that of New York City.
West of the dry interior and along the Pacific coast, California enjoys a mild winter—warm summer climate in which the rains are concentrated in winter and the summers are droughty. The moderating influence of the Pacific continues only a short distance inland so that the summers of the Central Valley of California are hot rather than warm. For example, San Francisco's warmest month averages a scant 60 ° F. whereas Sacramento's 90 miles (145 km) inland, averages 72 ° F. Through northern California and into Oregon, Washington, and British Columbia, summer drought is less pronounced until it disappears altogether. Rainfall increases to the largest amount experienced anywhere on the continent. The moist winds from off the Pacific annually drop over 100 inches (254 cm) of water on some sections of the western mountain face. The moderating effect of the ocean upon temperature continues well northward; Sitka, Alaska, despite its far north location, is warmer in January than New York City.
East of the dry interior, the climatic pattern is one of east-west belts. In Yucatan and on the narrow coastal plains of southern Mexico, temperatures are high the year round and rainfall is seasonal. The southern tip of Florida has a truly tropical climate with ample rainfall (nearly 60 inches, 152 cm) spread quite evenly through the year and no real winter season. From southern Florida northward roughly to the line of the Ohio and Missouri rivers, a humid, mild winter climate reaches from the Atlantic coast to the dry Great Plains. Along the southern side of this belt, freezing temperatures are not unknown in winter, but they are infrequent. Winter temperatures average lower northward so that along the northern margin of the belt the coldest month averages 32 ° F. Over the whole zone, summers are hot and days with temperatures of 100 ° F. are not uncommon. Rainfall decreases in amount and becomes less regular from the coast inland.
Northward still lies a belt of humid, severe winter climate which stretches almost to the shores of the Arctic. In the Interior Plains of the United States, roughly within the triangle formed by lines from southeastern North Dakota to eastern Ohio, eastern Ohio to northeastern Kansas, and northeastern Kansas back to southeastern North Dakota, the summers are hot with high relative humidity. Summer rainfall comes mostly in heavy thunderstorm downpours and that of winter comes largely in the form of snow. Through the Lake states, New England, and southern Canada, the severity of the winter increases and the summers are less hot and humid. Near the coast, precipitation is heavier and more evenly spread through the year; in the interior, skies are less cloudy and the precipitation tends to be concentrated in summer.
Through central Alaska and Canada north of the St. Lawrence-Great Lakes line, summers are short and only warm while winters are long and excessively cold. Longer hours of summer sunlight (Fairbanks, Alaska, has no real night on June 21st) somewhat offset the comparatively few days of the summer, but the long hours of darkness of winter intensify the severity of that season. In the upper Yukon, at an airport called Snag, the lowest air temperature ever recorded in North America, 82 ° F. below zero, was observed. In the coastal extremities of this belt, snowfall is very heavy, but in the interior it is relatively light. The Labrador coast, for example, receives annually over 100 inches (254 cm) of snow whereas, in Saskatchewan, the fall is less than 40 inches (102 cm).
Along the Arctic coast and over the Canadian Archipelago and Greenland, there are no true summers. Warm month averages rise barely above freezing over most of the area. Interior Greenland and most of Ellesmere Island are so continuously cold as to permit the existence of a permanent snow and ice cover.
Within the high mountain areas, climatic types are spottily distributed. In general, the lower slopes of the mountains have climates similar to those of adjacent plains, plateaus, and hills. Increase in elevation brings with it decrease in temperature, but, since the north side of mountains in North America is less directly exposed to the sun, temperatures are usually lower, elevation for elevation, on the northern than on the southern sides. Up to moderate levels (approximately 8,000 feet, 2,438 meters), increase in precipitation is likewise a feature of mountain climates, but the sides exposed to rain-bearing winds, usually the western side in North America, are wetter than the sides in the lee. Beyond these general changes, each mountain range, really each small section of any one range, is a rule to itself. Site and exposure determine the details within any mountain zone. The variety thus developed is almost infinite.