The ice which occupied Puget Sound was the extreme southern portion of a great but irregular Piedmont glacier which fringed the rough and ragged coast of the continent all the way to southern Alaska. A remnant of this former ice body still exists near Mount St. Elias, and constitutes the very instructive Malaspina glacier.
In a preceding chapter the rugged topography of the western margin of the continent has been briefly described, and a general explanation given of the contrasts which it presents to the coastal plains and plateaus on the Atlantic border.
The long, narrow peninsula known as Lower California, as yet unstudied in the light of modern geography, is known to be mountainous throughout. Although nearly surrounded by the waters of the ocean, the climate of the peninsula is hot and arid and its surface desert-like. The Gulf of California, which separates such a large portion of the Pacific border of Mexico from the main body of the republic, has the characteristics of a drowned intermontane or orogenic valley. But whether the great depression was ever dry land or not is unknown. The waters of the gulf are shallow, however, and a moderate upward movement of the earth's crust in that region would transform it into a great valley similar in its general features to the central basin of the State of California.
What are frequently designated collectively as the Coast Mountains begin at the south and adjacent to the shore of the Pacific, in the vicinity of Santa Barbara, Cal., and extend northward along the immediate seaboard far into British Columbia. A continuation or branch of this series of elevations follows the south coast of Alaska, and is prolonged so as to form the Aleutian Islands. The length of the mountain system or succession of ranges referred to is between 3,500 and 4,000 miles. The detailed study of this long, narrow, and in many parts excessively rugged region is as yet in its infancy, and only a brief account of its salient features can be attempted at present.
In southern California the structure of the mountains and the deep stream-deposited gravel, etc., in the intervening valleys, as well as the aridity of the climate and character of the vegetation, correspond closely with the similar conditions in the Great Basin. In fact, the Great Basin region, as the term has been used on a previous page, there meets the Pacific, and the islands rising from the adjacent portion of the ocean seem to be the summits of mountains of the Basin Range type. Owing to the dryness of the climate in southern California and adjacent portions of Mexico, the deeply alluvial-filled valleys are treeless, and agriculture is only possible with the aid of irrigation. Where water can be had, however, there are wonderfully productive orchards, vineyards, and gardens, in which the fruits and flowers of both the temperate and torrid zones flourish side by side with marvellous luxuriance. The palm there casts its shadow on fragrant bowers of the most superb roses. The grass-clothed mountain slopes are either bare of trees or but scantily forested, while the upland valleys produce a dense jungle of native trees and shrubs.
To the north of the irregular and diversified portion of southern California, where the Great Basin region extends southwestward to the Pacific, rises the southern Coast Range of California. The indefinite beginning of this range is in the neighbourhood of Point Conception, to the north of Santa Barbara, and its northern terminus is at the Golden Gate. The same belt of mountains extends northward, however, and forms the northern Coast Range, which extends to the Klamath Mountains in northern California. The coast ranges of California as a whole are about 500 miles long and from 30 to 40 miles broad, and comprise several seemingly distinct uplifts, some of which have culminating peaks from 4,000 to about 7,000 feet high. In general this elevated region is conspicuously sculptured, and in part at least has the characteristics of an eroded plateau. The suggestion has been offered that the northern portion of the Coast Range is a dissected peneplain.
The Coast Ranges, although generally bare of trees to the south of the Golden Gate, become more and more densely forested when followed northward. It is in this northern division that the great forests of redwood occur, now so largely used for lumber. Reference is here made not to the "big trees," which grow in certain restricted areas on the west slope of the Sierra Nevada, but to the far more extensive forests of a related species.
Considering mountain forms simply, it is difficult if not impossible to determine where the Coast Mountains of California terminate at the north, but, as has been shown especially by J. S. Diller, there are reasons based on geological structure for separating them from the irregular group of ranges and peaks in northern California and southern Oregon recently named the Klamath Mountains. The coast system is continued north of the Klamath Mountains by the Coast Mountains of Oregon, which extend to the Columbia River, and consist of irregular ridges or series of ridges, with bold lateral spurs, especially on the ocean side. It varies conspicuously in height from place to place, yet nowhere attains a great altitude. The elevations of the bolder summits, although not accurately measured, seldom exceed 3,000 feet.
The Coast Mountains of Oregon are considered as terminating at the northern boundary of the State, there defined by the Columbia River, but no reason is apparent, however, for not including in the same group the elevated land lying in southwestern Washington and adjacent to the Pacific coast. Between the Columbia and Chehalis River in Washington there is a rugged region which attains an elevation of over 4,000 feet, and is separated from the Olympic Mountains to the northward by Chehalis Valley. Although the geology of this group of ridges and peaks is entirely unknown, its position and general appearance, when seen from a distance, suggest that it might properly be considered as a direct extension of the Coast Mountains of Oregon.
Following the general belt of the Coast Ranges still farther northward, we come to the splendid group of forest-clothed mountains, with usually snow-covered summits, situated to the west of Puget Sound, and known as the Olympic Mountains. This magnificent range is in full view from Victoria, Seattle, and Tacoma, and would be far-famed for its grandeur were it not for its near rival, the still more lofty Cascade Range.
There are several fine, sharp peaks in the Olympics that have never been scaled, the highest of which, Mount Olympus, rises 8,150 feet above the sea. Owing to the excessive humidity and other favourable climatic conditions, these mountains are clothed with magnificent forests up to an elevation of about 7,000 feet. On account of the ruggedness of the country, the extreme density of the tangled undergrowth, and the obstructions formed by the fallen moss- and lichen-covered trees, this region is extremely difficult to traverse, and to-day is the least known of the continental portion of the United States. On the north the excessively rugged Olympic peninsula is bordered by a deep, broad fiord known as the Strait of Fuca. To the north of this formerly ice-filled channel lies Vancouver Island, the central and northern portion of which is mountainous. The highest summit on the island rises about 7,500 feet above the sea, and a considerable area in its central part has an elevation of over 2,000 feet.
The Olympics, together with the mountains of Vancouver and Queen Charlotte Islands, and the northern extension of the same belt, embraced in part within the mainland of British Columbia and southeastern Alaska, have been termed the "Vancouver Mountains" by Canadian geographers. The northern boundary of this mountain system, justly named in honour of the celebrated English explorer who mapped large portions of the northwest coast about a century since, remains indefinite, and cannot be determined until geologists have made more thorough explorations of the land it occupies. The leading geographical features of this region, as remarked in a preceding chapter, are due to the deep dissection, by streams and glaciers, of an elevated table-land. When the ice-streams melted, the sea was permitted to enter the valleys, so as to form numerous deep, narrow, steep-walled fiords (Fig. 11). The coast is, in fact, the most ragged of any portion of the border of the continent. All but the higher summits are clothed with a dense mantle of vegetation, the upper limit of which decreases in elevation when followed northward, from about 7,000 feet in the Olympics to approximately 2,500 feet in southern Alaska. Perennial snow exists in the higher valleys and amphitheatres of the Olympics, but the presence of true glaciers in that group of peaks has not been demonstrated. When followed northward the snow-line becomes lower and lower, and well-defined alpine glaciers are known to exist in many of the valleys, more especially on the mainland of British Columbia and southeastern Alaska. There streams of ice descend lower and lower with increase in latitude, and to the north of Stickeen River, in a number of instances, enter the fiords which connect with the ocean and become tide-water glaciers.
To the west of Lynn Canal, and extending to beyond Copper River, is the most rugged portion of North America, and contains also some of the highest mountain-peaks on the continent thus far measured.
The region of high mountains in Alaska and the adjacent portion of Canada begins on the east in the group of magnificent peaks which cluster about Mount Fairweather as a centre and extend westward, with a breadth of some 80 miles, to beyond Mount St. Elias. Farther westward, beyond Copper River, other great mountains are known to exist. One of these, Mount McKinley, has an elevation of 20,400 feet, and so far as now known is the highest peak in North America.
The highest summit to the east of Copper River is Mount Logan, 19,500 feet. This superb ice-sheathed peak is situated in Canada about 40 miles from the coast and 12 miles east of the one hundred and forty-first meridian. Second in rank is Mount St. Elias, 18,070 feet, situated close to the one hundred and forty-first meridian, and within the territory belonging to the United States. These two summits are the highest in a land of lofty snow-covered mountains, and for this reason have claimed a large share of attention. There are many neighbouring peaks, however, that are wonderfully magnificent, but only a few of them have been measured and many of them are still unnamed. Only one of the high mountains of Alaska, namely, Mount St. Elias, has been climbed. This splendid feat of mountaineering was accomplished by Prince Luigi, of Savoy, in 1899.
In southern Alaska the snow-line is only about 2,500 feet above tide, and a large number of magnificent glaciers descend to sea-level, and many of them actually enter the ocean. All of the valleys and basins among the higher summits are occupied by snow-fields and glaciers. The general covering of ice and snow as well as the ruggedness of the land makes this the most difficult of all the mountainous portions of North America to traverse.
In the St. Elias region the mountains have been produced by upheaval, and are not volcanic in their origin. The frequently repeated statement that Mount St. Elias is a volcano is incorrect. Although igneous rocks occur near its summit, they are of the nature of dikes or intrusions, probably of ancient date, and not lava-flows. The principal volcanic mountains of Alaska are farther west in the region of the Alaskan peninsula and the Aleutian Islands. This western extension of the continent is excessively rugged, but the mountains rise directly from the ocean and in part form a chain of precipitous islands with irregular topographic forms.
There are mountain ranges also in the central and northern portions of Alaska and the adjacent part of Canada, but this region awaits exploration, and but little accurate information concerning its topography is on record.
The Mountains of Western Canada.—Reference has already been made to the differences in the nomenclature applied to the portions of the Pacific mountains on opposite sides of the United States-Canadian boundary, and at present this lack of harmony cannot be adjusted. As is well known, the great Pacific cordillera crosses the boundary nearly at right angles, and there is no abrupt change in the topography of the land. From the western border of the Great plateaus to the Pacific, between the forty-fifth and fifty-sixth parallels, as stated by the Geological Survey of Canada, the cordillera has an average breadth of about 400 miles, and is composed of four great mountain chains, named in their order from east to west, the Rocky, Gold, Coast, and Vancouver Mountains. These four great chains are nearly parallel and have irregular northwest and southeast trends.
The Canadian Rockies rise abruptly from Great plateaus in which the rocks are nearly horizontal, and have a complex structure, due to the folding and other disturbances that have affected the strata. Deep dissection by stream erosion has occurred, as is the case generally throughout the Pacific cordillera, and the peaks and ridges remaining are remarkable for their grandeur. Although less elevated than the higher portions of the same great belt in the United States, many of the summits are from 8,000 to 10,000, and, as reported, in a few instances reach 13,000 feet in height, while the passes range is elevated from about 4,000 to 7,000 feet. The western border of the Rocky Mountain range is well defined for a distance of some 700 miles to the northward of the international boundary by a remarkably straight, wide valley, which is occupied by the head waters of several large rivers, namely, the Kootenay, Columbia, Fraser, Parsnip, and Findley. To the west of the great valley just referred to rises the Gold system, composed principally of the Selkirk, Purcell, Columbia, and Caribou Ranges. It is in this rugged region that some of the most remarkable of the splendid scenery of western Canada occurs.
To the west of the Gold system is a broad region of valleys and lesser mountains, known as the interior plateau of British Columbia, which is a northward extension of the Great Basin region of the United States. The breadth of this belt of comparatively low country is about 100 miles. Like the similar region in Washington and Oregon, it is without forests, but favourable as a grazing country. In part it is occupied by extensive lava-flows, similar to the Columbia River lava of the northwestern part of the United States.
The Coast Mountains of Canada, although stated by geologists to be distinct from the Cascade Mountains, are in part at least, as determined by the present writer, a direct northward extension of that range. The average elevation of the higher peaks in the Canadian Coast Range, as it is termed, is between 6,000 and 7,000 feet, while the culminating points reach an elevation of about 9,000 feet. How far northward the nomenclature applied to the Pacific mountains in southwestern Canada will be found applicable can not be stated, as the region to the north of the fifty-sixth parallel is almost wholly unknown.
As has been clearly pointed out by R. T. Hill, the Pacific cordillera ends at the south in south-central Mexico, while the Andean cordillera at its northern end terminates in the rugged mountains of Venezuela to the south of the Caribbean Sea. These two great cordilleras do not overlap, but there is a difference of about 10 degrees of latitude between them, and if extended they would pass each other at a distance of nearly 1,000 miles. In the space thus indicated, measuring some 600,000 square miles, is included the southern portion of Mexico, Central America, and the West Indies. The rocks in these countries present a great series of folds which trend in an easterly and westerly direction, and thus present a conspicuous exception to the major structural features of both North and South America. To this newly recognised division of the larger geological and geographical characteristics of the New World the name Antillean mountains has been given.
The folds or corrugations in the rocks of the Central American and Caribbean region extend in an east and west direction along the seaward margin of Venezuela and Colombia from the Orinoco westward to the Isthmus of Panama, and thence continue westward through Costa Rica, the eastern portions of Nicaragua, Guatemala, and Honduras, and reach southern Oaxaca in Mexico. The same system of plications is revealed also on the larger West India islands. The rocks of this great region include granite and allied metamorphosed terranes, old lavas, and sedimentary beds.
One of the most conspicuous features of this region with a structure and relief commonly found in mountains is that to a great extent it is depressed beneath the sea, and only the higher summits are in view. Some of the larger inequalities of the rock surface have been discovered by means of the sounding-line. By referring to Fig. 3, it will be seen that two submarine ridges extend in an east and west direction beneath the Caribbean Sea, from the West Indies to the Central American coast, and are separated by Bartlett Deep. These ridges correspond in trend with the longer axes of the folds in the Antillean mountains, and suggest a common origin for the leading geographical features of the land and of the still more remarkable topography of the sea-floor.
In addition to mountains produced by corrugation and upheaval, there are also in the middle American region numerous volcanic mountains. Of these there are two well-defined belts, each trending in general north and south, or directly across the longer axes of the folds of the Antillean mountains. One of these belts of volcanic cones and craters is situated on the Pacific coast of Central America and Mexico, and includes some 25 active volcanoes, and the other is defined by the numerous volcanic islands of the Lesser Antilles. The association of these belts of fracture through which molten rock has been extruded and where earthquakes are of common occurrence, with the junction of the east and west belt of plication to which the Antillean mountains are due, with the north and south belts of mountains forming the Pacific and Andean cordilleras, is significant in connection with the study of the origin of the larger features of the relief of the solid earth.
Varied as is the relief of North America when studied in detail, an outline sketch of its major features may be readily retained in mind. On the east side of the main continental area are the Atlantic mountains, extending from near the Gulf coast northward to beyond Hudson Strait; in the central part is the broad continental basin, a vast region of low relief reaching from the Gulf of Mexico to the Arctic Ocean; west of the continental basin are the Pacific mountains, the greatest of all the elevations on the continent, which begin abruptly in south-central Mexico and extend northward, expanding to a width of about 1,000 miles in the United States and reach the Arctic Ocean and Bering Sea. The movements in the earth's crust, which blocked out these major physiographic features, were produced by forces acting in east and west directions, and gave origin to folds and faults with their longer axes trending north and south. To the south of the main body of the continent, in middle America, are situated the Antillean mountains, also a cordillera comparable with the Atlantic and Pacific cordilleras, in which the longer axes of the folds and faults trend east and west, and are due to forces acting in north and south directions. The Antillean mountains in a general way connect or intervene between the Pacific and the Andean cordilleras. Where the Antillean mountains cross the axes of the Pacific and Andean cordilleras are situated the volcanoes of southern Mexico and Central America, and those of the Lesser Antilles.
Geographers will recognise that this outline is drawn boldly, but although it will no doubt have to be modified as detailed studies progress, it should serve to emphasize the leading geographic divisions of the North American continent when viewed as a whole.
The following list of publications relating to the physiography of North America is here presented largely because the books mentioned contain bibliographies or references which indicate sources of more special information:
- American Geographical Society, Bulletin. Published annually.
- Canadian Geological Survey, Ottawa, Canada. Index to reports from 1863 to 1884, published in 1900.
- Darton, N. H. Catalogue and Index of Contributions to North American Geology, 1732 to 1891. United States Geological Survey, Bulletin No. 127, Washington, 1896.
- Davis, W. M. Physical Geography. Ginn & Co., Boston, 1898. Contains a valuable bibliography.
- Dryer, C. R. Lessons in Physical Geography. American Book Company, New York, 1901. Contains a valuable bibliography.
- Geological Society of America, Bulletin. Published annually since 1890 at Rochester, N. Y. Index to vols. i-x, published in 1900.
- International Geography. Edited by H. R. Mill. D. Appleton and Company, New York, 1900. Contains several chapters on North America by various authors.
- Journal of Geology. Edited by T. C. Chamberlin and published at the University of Chicago, Chicago, Ill.
- Merrill, G. P. Rocks, Rock-Weathering, and Soils. The Macmillan Company, New York, 1897.
- National Geographic Magazine. Washington, D. C.
- National Geographic Monographs. American Book Company, New York, 1895. One volume published.
- Powell, J. W. Exploration of the Colorado River of the West. Published by the Smithsonian Institution, Washington, 1875.
- Powell, J. W. Cañons of the Colorado. Flood & Vincent, Meadeville, Pa., 1895.
- Russell, I. C. Lakes of North America, Ginn & Co., Boston, 1895; Glaciers of North America, Ginn & Co., Boston, 1897; Volcanoes of North America, The Macmillan Company, New York, 1897; Rivers of North America, Putnam's Sons, New York, 1898; The Names of the Larger Geographical Features of North America, in Bulletin of the Geographical Society of Philadelphia, vol. ii, 1899, pp. 55-68.
- Shaler, N. S. Nature and Man in America. Scribner's Sons, New York, 1891.
- Stanford's Compendium of Geography and Travel. North America: Canada and Newfoundland, by S. E. Dawson; United States, by H. Gannett; Central America and West Indies, by A. H. Keane. Published by Edward Stanford, London.
- Tarr, R. S. Elementary Physical Geography. Macmillan & Co., 1895.
- United States Geological Survey. Bibliography and Index, contained in Bulletins No. 100, 127, 177, 188, and 189, issued by the Survey. The reader is referred especially to the Geological Atlas and the Topographic Atlas, published by the Survey.
- Warman, P. C. Bibliography and Index to the Publications of the United States Geological Survey. United States Geological Survey, Bulletin No. 100, Washington, 1893. (Relates to publications of the United States Geological Survey issued previous to 1892; continued in Bulletin No. 177 by the same author.)
- Warman, P. C. Catalogue and Index of the Publications of the United States Geological Survey, 1880 to 1901. United States Geological Survey, Bulletin No. 177, Washington, 1901.
- Weeks, F. B. Bibliography of North American Geology, Paleontology, Petrology, and Mineralogy for the Years 1892-1900, inclusive. United States Geological Survey, Bulletins No. 188 and 189, Washington, 1902.
- Whitney, J. D. The United States. 2 vols. Little, Brown & Co., Boston, 1889 and 1894.
CHAPTER III
North America, embracing as it does essentially a quadrant of the earth's surface, presents a variety of climatic conditions ranging from those characteristic of the equatorial belt to those normal to polar regions, as well as every gradation due to variations in elevation from sea-level and even below that horizon in Death Valley, California, to the summits of high plateaus and lofty mountains.
The principal elements of the weather which go to make up the conditions of the atmosphere embraced in the broader term climate are temperature, precipitation, and the winds. On the accompanying map, Plate II, the mean annual temperature of the continent is represented by isotherms, or lines drawn through localities having the same average temperature for the year. On the same map is also shown in blue the average depth of precipitation, including both rains and melted snow. On Fig. 24 lines are drawn through points having the same average barometrical pressure (isobars) for the months of January and July, together with arrows indicating the general direction of the surface winds during each of these months, which may be considered as representative of the summer and winter seasons. The data shown on these maps have been compiled mainly from the reports of the weather bureaus of Canada, the United States, and Mexico, and indicate, at least in a general way, a summary of what is known concerning the main meteorological elements which determine the climatic conditions in North America. An examination of these maps will suggest certain general conclusions in reference to the leading characteristics of the climate in various portions of the continent and the changes they undergo from season to season.
Click image to enlarge.
Distribution of Heat and Light.—The distribution over the earth's surface of the heat and light received from the sun is not only of fundamental importance as respects climate, but furnishes a part of the essential conditions on which depend the presence and distribution of living organisms. The heat and light, or more accurately, the radiant energy of the sun, the full significance of which is probably not thoroughly understood, we term, for convenience, insolation. The intensity and seasonal distribution of insolation are prime factors on which many important results hinge.
Owing to the inclination of the axis about which the earth rotates (23° 27') to the plane in which the earth travels about the sun, or the plane of the ecliptic, the northern end of the axis is turned towards the sun in summer and away from it in winter—that is, the axis of rotation of the earth at all times is parallel to the same imaginary straight line. As a result, the sun appears to migrate northward in the heavens during the spring-time of each year, being vertical over the equator on March 21st, and to an observer in north latitude 23° 27' rises higher and higher each moon, until on June 21st it is vertically overhead; and then returns southward. The latitude in which the sun is in the zenith at the time of its greatest northward migration determines the position of an imaginary line on the earth's surface, named the Tropic of Cancer. This line, as shown on the accompanying maps, crosses the Bahama Islands, passes about 40 miles to the northward of Havana, divides Mexico into two approximately equal parts, and cuts the peninsula of Lower California near its southern end. The portion of the continent to the south of the Tropic of Cancer lies within the torrid zone.
When the sun is vertical over the equator, as it is about March 21st and September 23d each year, its rays, not allowing for refraction, are tangent to the earth's surface at the poles, and the hours of light and darkness are equal the world over. During the winter season the sun appears to migrate southward of the equator until December 21st, when it is vertical at noon at all points situated in south latitude 23° 27', which is termed the Tropic of Capricorn. Its rays are then tangent to the earth's surface in the northern hemisphere in latitude 66° 33', which defines the position of the arctic circle. This imaginary line on the earth's surface, as is indicated on the accompanying maps, crosses Canada to the north of Hudson Bay, and passes through Alaska near where the Porcupine River joins the Yukon. To the north of the arctic circle lies the frigid zone. Between the torrid and frigid zones is situated the temperate zone, within which is included about seven-eighths of North America, exclusive of Greenland. The relation of the continent to the three great zones of climate into which the northern hemisphere is divided is thus most fortunate so far as man's activities are concerned.
The climatic zones just referred to, while based on precise astronomical data and representing important facts, are not separated one from another by tangible lines, and might easily pass undiscovered by one who studied only the surface characteristics of the earth. Each summer a wave of heat and light sweeps northward over the continent and reaches beyond the pole; and each winter a counteracting wave of cold and darkness moves southward, the influence of which is marked even well within the torrid zone. A comparison of the isothermal lines drawn on the map forming Plate II with the parallels of latitude shows at a glance that there is only a general relationship between the two. In order to understand this discrepancy between what might be expected from astronomical considerations in reference to the distribution of solar energy and the actual conditions as learned by observation, it is necessary to take a more critical view of the manner in which insolation is received by the continent, and also to consider secondary conditions which exert far-reaching influences on its distribution.
The amount of heat, or to avoid objections, the distribution of insolation over North America, depends on three primary conditions: First, the angle at which the sun's rays strike the earth, the range being from zero to 90°; second, the length of time a particular locality is exposed to sunlight; and third, variations in the distance of the earth from the sun. Each of these conditions varies from day to day for every locality throughout the continent. The sun is highest in the heavens in the torrid zone, being twice vertically overhead each year at every locality, and the hours of light and darkness each day are approximately equal throughout the year. North of the torrid zone, however, the rays of the sun become more and more oblique to the earth's surface, and hence insolation becomes weaker and weaker for a given period of sunshine as one travels from south to north. But the hours of sunlight each day undergo marked variations, lengthening from December 21st to June 21st, and shortening as the sun makes its southward migration. At the north pole, as all know, there are six months of light and six months of darkness each year. The amount of insolation reaching the northern portion of the continent each day increases with the lengthening of the hours of light, and during midsummer is greater for a given area in a single day (twenty-four hours) than the amount received by a similar area in the torrid zone. The almost magical springing into life and bloom of the vegetation over the northern portion of the continent with the lengthening of the hours of sunshine each summer is thus explained. In the portion of the continent within the temperate zone, more especially within the continental basin, the large number of hours of sunshine during a summer's day is frequently accompanied by a temperature as great as is usually experienced in the torrid zone. It is the high summer temperature of this region, together with the lengthened duration of sunshine in the growing season, that makes the Mississippi basin and the adjacent region on the east and north so favourable for agriculture when the requisite amount of moisture is present.
The distribution of heat over the earth's surface depends not only on the direct influence of insolation, but on its transfer from one locality to another through the agency of the winds and ocean currents. The movements of the waters of the ocean, it will be remembered, are largely under the control of the winds, so that the essential factor in the transfer of heat from place to place is atmospheric circulation. The primary causes of movements in the air, as is a matter of current knowledge, are the differences that arise in temperature at various localities. In regions where the air becomes more highly heated than over adjacent areas it expands, and in consequence becomes lighter, volume for volume, than the air over neighbouring areas, and is forced upward and overflows aloft. The overflow or dispersion of the warmer and lighter air above gives origin to a reduction in barometric pressure, the column of mercury in a barometer being counterbalanced by the pressure of the air above it. Briefly stated, the air near the earth's surface flows towards regions of low, and away from regions of high barometric pressure, and winds are established. The directions taken by the winds are influenced or controlled in various ways.
The Planetary Winds.—The great movements in the atmosphere originate from differences in temperature between the warm equatorial and cold polar regions. This alone would cause the cold air from either pole to flow towards the equator as surface winds, and the warm air in the equatorial belt to ascend and overflow aloft towards either pole. The earth's rotation, however, influences the direction of these winds and causes them to be deflected from the lines of longitude which they would otherwise follow. In the northern hemisphere the air-currents are deflected to the right and in the southern hemisphere to the left of their initial directions. The best known examples of these planetary winds, as they are termed, are the trade-winds, which blow from the northeast in the northern and from the southeast in the southern hemisphere. Between these two belts of converging winds lies the equatorial belt of calm, some 300 miles wide, which also encircles the earth and is termed the doldrums.
In the quadrant of the earth's surface occupied by North America the climatic conditions are controlled in a large measure by the planetary winds. In the equatorial belt of calms the barometric pressure is lower than on either side, the temperature is uniformly high, the air is heavily charged with moisture, and torrential rains are frequent. In the belt of the northeast trades the weight of the air for a given area is greater than in the doldrums, the wind blows with remarkable uniformity both of direction and force, the sky is normally clear, and rain infrequent except when the warm moist air is forced upward either by local storms or on coming in contact with high land. The trade-winds blow across the West Indies, Mexico, and much of Central America. To the north of the trade-wind belt is a belt of prevailingly high barometrical pressure, light variable winds, narrower and less well defined than the doldrums, which encircles the earth in the region of the Tropic of Cancer. This belt of calms, although familiar to sailors, to whom it is known as the "horse latitudes," is ill-defined on the land, where its presence is masked by changes due to local conditions. To the north of the tropical calm belt the prevailing surface winds are from the westward, and owe their direction to the constant flow of the upper air-currents in their poleward journey, under the influence of the earth's rotation. This great belt of winds from the westward crosses the portion of North America including the United States and southern Canada, but it is subject to many disturbances. The northern portion of the continent extends into the little known polar region of prevailingly low barometrical pressure, where midsummer and midwinter calms normally prevail.
The great world-encircling currents of the atmosphere, namely, the trade-winds, blowing towards the southwest or west across the Caribbean and Mexican region, and the prevailing westerlies, or winds blowing in an easterly direction, over the broad temperate portion of North America, exert the main control on the climate of the continent.
The Seasons.—Of primary importance to the inhabitants of North America is the fact that the climatic belts determined by the inclination of the earth's axis to the plane of the ecliptic are subject to annual migration towards the north and south. In the torrid zone the equatorial belt of calms, with its humid and oppressively hot atmosphere, prevailing cloudiness, and heavy rains, and the belt of the northeast trades, with its prevailingly clear skies and refreshing breezes, do not occupy the same positions throughout the year, but migrate with the sun. The migration of these two strongly contrasted climatic belts brings to the otherwise remarkably uniform conditions of the atmosphere over the West Indies, Central America, and Mexico, two, in general well-defined, periods each year, namely, a wet and a dry season, the former occurring in the summer and the latter in the winter. It is to be borne in mind that between the tropics there are, with certain local exceptions, but two seasons each year, the leading contrasts of which are determined by differences in rainfall.
To the north of the Tropic of Cancer the seasonal changes are more varied than in the torrid zone, and contrasts in temperature become the most marked climatic feature; while precipitation, although in general somewhat evenly distributed throughout the year, is more abundant in winter than in summer. On account, however, of the greater diversity in the climatic changes experienced each year within the temperate zone, four seasons are recognised, the most distinctive features of which depend on changes in both temperature and humidity.
In the northern portion of the temperate zone, and extending over the arctic zone, the seasons are again reduced to two, summer and winter, the contrasted conditions pertaining mainly to temperature and light.
A marked variation, which has an important bearing not only on climate, but on the distribution of life encountered in passing from equatorial to polar regions, is found in the distribution of light. Between the tropics the number of hours of light and darkness each day is approximately equal; in the temperate zone there is considerable diversity from season to season, which increases with increase in latitude; and uniformity, of a different character than at the far south, again becomes prominent in the frigid zone, where the number of hours of light each day is greatly prolonged during the summer and correspondingly decreased during the winter. The extreme contrast occurs in the neighbourhood of the pole, where during the summer season the sun is continuously above, and in winter continuously below the horizon, or in familiar language, there is a six-months day (light) and a six-months night (darkness).
In going from the equatorial to north polar regions there is a general decrease in mean annual temperature, and in general a decrease also in precipitation, but great variations in these gradual changes, with increase in latitude, occur which are both continental and local in character. In winter the interior portions of the continent, and especially the plateaus and mountains, are colder than the lands in corresponding latitudes near the oceans; while in summer the reverse is true, the margin of the continent being cooler than the broad interior.
In this general view of the climatic zones and the normal changes they undergo we may note that the torrid zone is characterized by its simplicity and monotony of climatic conditions, although disturbed at times, especially in the West Indies, by occasional great cyclonic storms, termed hurricanes, which occur, however, at quite definite seasons. The temperate belt is equally well marked by its complex and frequently changing atmospheric conditions, the winds being subject to numerous and great variations, and storms of diverse character being frequent. The frigid zone, again, is without conspicuous variations except during the change from its monotonous summer to its still more uniform winter weather, and the reverse change six months later. The disturbances in the balance of atmospheric conditions at the far north, or the storms, are of a much less varied character than in the fickle temperate zone—thunder-storms and tornadoes, for example, being unknown.
It is the summer migration of a heated belt from the south northward across the temperate zone, and the equally conspicuous winter advances of cold from the north southward across the same broad region, which gives to the United States and the southern portion of Canada a conspicuously changeable climate. The temperate zone, so far at least as North America is concerned, deserves its name only when the mean of the yearly changes in temperature is considered, as much of it is hotter in summer than equal areas between the tropics, and in winter over all of its northern half the cold is, at times, nearly or quite as intense as during the same season in the far north. As a whole, the portion of the continent embraced in the temperate zone is characterized by its pronounced seasonal changes, including wide extremes of heat and cold over large areas, and by its frequently sudden and strongly marked weather changes during short periods of time. It is a highly suggestive fact that of all the great climatic zones the one having the most changeable climate, the greatest extremes of heat and cold, and the most frequent storms should be the one in which man has reached the highest development both of body and mind. Evidently it is the struggle for existence, when not too severe, which insures advancement. The part of North America most densely inhabited by descendants of Europeans, and the portion of the continent where intellectual development has made the greatest advance, is the east-central portion, where not only the variation of climate from season to season, but the weather changes from week to week and day to day are the most conspicuous.
Secondary Conditions influencing Climate.—While the primary conditions controlling the climate of North America in common with all other portions of the earth's surface depend on the relation of the earth to the sun, there are many secondary conditions to be considered. First in importance among these, so far as the broader features of the climate of the continent are concerned, is the unequal heating of land and water areas. During summer, more especially in the temperate zone, the land becomes more highly heated than the adjacent oceans, and an inflow of the cooler and moister air from the sea over the land occurs. In winter the land cools more quickly and to a greater degree than the adjacent waters, and the tendency of the heavier air over the land is to flow outward as surface winds. Continental winds are thus generated, similar in their origin to the familiar land and sea breezes of the ocean shore in summer, but on a large scale, which have an important bearing on the seasonal changes. The influence of the continental winds is sufficiently well marked to give North America two general classes of climate. One pertains to inland regions, is characterized by great contrasts in temperature and humidity between summer and winter, and is termed continental. The other pertains to the border of the land where, on account of the equalizing influence of large water borders, the contrast between the climate of summer and winter is less pronounced, and has received the general title of oceanic climate. The climate of the Dakotas, for example, is of the continental type, while that of New Jersey is of the oceanic type.
The unequal heating and cooling of adjacent portions of land areas also produces important atmospheric movements, as, for instance, when broad, treeless plains become more highly heated in summer than adjacent forested areas; or on account of rapid radiation become excessively cold in winter and lower the temperature of the air above them. In the first instance an inflow of cooler and heavier air from adjacent regions would be established; and in the second example the chilled air would tend to flow outward, thus, in each instance, establishing winds which usually acquire a more or less well-pronounced circular motion. The Prairie plains and the Great plateaus to the east of the Rocky Mountains become highly heated in summer, and together with several other similar regions in North America, meet the first of the conditions just considered; while the higher portions of the Great plateau, especially at the north, and the still more elevated mountains of Montana, Colorado, etc., become excessively cold in winter and illustrate the other extreme.
Mountains serve to deflect the winds blowing against them either to one side or upward, the former frequently producing important changes in direction of the surface air-currents, and the latter, by causing the air to rise, permits of its expansion and consequent cooling, thus favouring precipitation. For this and other reasons precipitation increases with elevation, at least until an altitude of many thousands of feet is reached, and the mountains are cooler and more humid than the adjacent valleys. The air-currents on passing over a mountain range and descending are warmed by compression, and having lost a part and in many observed instances a large percentage of the moisture they previously contained, become warm, drying winds. The chinook winds, as they are termed in America, are marked examples of the influence exerted by mountains on climatic conditions.
What are termed above the secondary conditions, tending to modify climate, produce such great changes in the distribution of rainfall, temperature, etc., and in the influence of the planetary winds, that the subdivision of the northern hemisphere into torrid, temperate, and frigid zones, while based on astronomical data, does not serve to represent actual conditions, except in a general way, in reference even to the single element of temperature expressed in these names. A comparison of the isotherms and of the distribution of precipitation as indicated on the preceding maps, with the parallels of latitude, shows at once that these two most important elements of climate are conspicuously independent of distance from the equator. A logical basis for subdividing the continent into climatic provinces must therefore be sought in other directions.