In order to obtain a comprehensive idea of the climate of a continent, it is obviously desirable to subdivide it into areas having more or less similar and distinct atmospheric conditions. The leading difficulty in making such subdivisions is the well-known fact that the climate of any region which may be chosen passes by insensible gradations into that of adjacent regions, and any boundaries that may be drawn are to a considerable extent artificial and arbitrary. While the true basis on which to establish climatic areas or provinces is the resultant of all the weather elements which go to make up the atmospheric conditions recognised under the broader term climate, so many factors have to be considered that it is extremely difficult to arrive at a general acceptable conclusion. The principal elements of the weather to be considered are, approximately, in the order of their importance, beginning with the one which exerts almost universal control—temperature, precipitation, the winds, absolute and relative humidity of the air, and evaporation.

In North America there are voluminous records in reference to each of these elements of the weather, embracing considerable lengths of time, and relating mainly to the United States and southeastern Canada, but not sufficient to enable one to subdivide the entire continent into climatic provinces. The lack of weather records embracing the entire continent may be supplied in part by what may be termed the natural records of atmospheric conditions as expressed by the flora and fauna, as well as by soil conditions and topography. In recognition of this principle, the climatic provinces of North America here adopted have been made to conform to its life-zones.

The major climatic divisions of North America here provisionally adopted are, in their general order from south to north, the Tropical, Lower Austral, Upper Austral, Transition, Boreal, and Arctic (Plate III). These several divisions are termed climatic provinces, and are assumed to coincide with those of the life-regions as mapped by C. Hart Merriam. The basis for classification is mainly temperature. In the main, the northern boundaries of the provinces and their higher limits in mountainous regions are determined by the temperature of the season of growth and reproduction among plants and animals; while their southern boundaries or lower limits on the mountains are determined by the temperature of a brief period during the hottest portion of the year. A more definite account of the reasons for choosing these limitations will be given later in discussing life areas. While the principal basis for establishing climatic provinces is temperature, many other conditions are also recognised, chief among which is precipitation. Several of the climatic provinces have two divisions, namely, a humid and an arid, the dividing line being approximately the one hundredth meridian. These are well defined and important in the tropical, austral, and transitional, but less definite and less well known in the boreal and arctic provinces. The regions embraced in the several climatic provinces named above, as well as their humid and arid divisions, so far as now understood, are shown on the accompanying map.

The Tropical Province (Plate III).—This is the most southern of the climatic provinces which it is convenient to recognise in North America, and includes the West Indies, Central America, and southern Mexico, together with a narrow strip on each coast of northern Mexico and the extreme southern ends of the peninsulas of Florida and of Lower California. While the land areas in this widely extended province present conspicuous differences, their climate in general is characterized by a high mean annual temperature with but moderate seasonal or daily variations, and by the occurrence in general of a wet and a dry season each year. The prevailing winds are the northeast trades. While the average yearly temperature is high, being in general about 80° F., the heat in summer is less intense than in many portions of the austral provinces. In winter the temperature does not fall sufficiently to produce frost, except on the higher mountains, which, in fact, belong to one or more of the other provinces named above. On ascending the mountains a rapid change to cooler and even to frigid zones is experienced. Snow occurs on the higher portions of the mountains of Central America and Mexico, and in a few instances, as on the great volcanic cones in sight from the City of Mexico, is perennial. On the mountains just referred to all of the several climatic provinces are represented by well-characterized zones, arranged one above another, and presenting in epitome the general changes one would experience in travelling from the Gulf of Mexico to the Arctic Ocean.

In marked contrast to the prevailing uniformity of temperature at corresponding elevations throughout the tropical provinces is the inequality in rainfall in reference to both seasonal periods and differences in geographical position. Of the annual changes, the most pronounced and characteristic is the alternation of wet and dry periods. During the summer season, or in general from May to October, the air is usually clouded for at least a portion of each day, and heavy downpours of rain occur. Throughout the remainder of the year the clear skies and refreshing breezes, such as on the ocean are normally characteristic of the trade-wind belt, prevail. The remarkable regularity with which these changes occur each year suggests at once that they are due to the migrations northward and southward with the sun of the great climatic belts encircling the equatorial region of the earth. In summer the equatorial belt of calms and heavy rainfall not only migrates northward and occupies a portion of Central America, bringing to Costa Rica and Nicaragua cloudy skies and abundant precipitation, but seems to be carried bodily still farther northward, so that the influence of the southeast trades of the southern hemisphere makes itself felt, and four somewhat well-marked annual changes occur, namely, two wet and two dry seasons. What is known of the climate of this extreme southern portion of the province under review indicates that the seasonal changes, especially on the Caribbean slope, are less well marked than in its central portion, and rain is frequently copious in nearly every month of the year. In the region just mentioned the mean annual precipitation, as at Greytown, for example, at the eastern extremity of the proposed Nicaragua Canal, is about 250 inches, and in exceptionally wet years reaches nearly or quite 300 inches. This is, so far as known, the heaviest annual precipitation in any portion of North America.

In the West Indies the rainy season begins, in general, in May and lasts until October. On the lowlands of eastern and south-central Mexico the wet season commences in June, and the rains increase until the end of July and end in November. This region lies to the north of the northern limit reached by the equatorial belt of calm, but the rains in the summer season are due to the same general influence, namely, the lowering of temperature in the northward-moving upper air-currents, and their effect on the trade-winds. In Cuba and along the northern coast of the Gulf of Mexico when the trade-wind belt migrates southward in winter the influences of the southward-moving storm from the Mississippi Valley and Atlantic States produces what are termed northers, which bring a chill and at times frost, more especially in Florida and on the higher portions of Cuba.

While the wide-reaching seasonal changes within the tropical province depend on the migrations of the planetary winds northward and southward with the sun, and are due primarily to astronomical conditions, there are equally well-marked variations in rainfall dependent on or regulated by geographical conditions. These are of two principal classes: First, elevation of the land; and second, the relation of broad land areas to the direction of the prevailing winds, each of which is abundantly illustrated in the tropical province.

The mountainous islands in the West Indies receive a heavy rainfall, especially on their windward or easterly slopes, while the low islands, and less markedly the southwestern border of the higher islands, are much less humid. The eastern side of Porto Rico, for example, has an abundant rainfall, and was originally clothed with a luxuriant native flora, including large forest-trees, while the lowlands on its southern and southwestern border are subject to drought, and irrigation is desirable to insure the growth of crops. Again, while the mountainous islands of the Lesser Antilles, with their luxuriantly forested slopes, present ideal pictures of tropical loveliness, low-lying Sombrero, Barbuda, St. Martin, and other similar islands are so arid that guano has accumulated on them to such an extent as to be of commercial importance. The reason for these striking contrasts within narrow geographical limits is readily seen in the influence of the highlands on the trade-winds. These air-currents blowing from the Atlantic are warm and contain a high percentage of moisture. As they advance, however, they invade regions that are progressively warmer and warmer, and the capacity of the air for moisture is correspondingly increased. For this reason the trade-winds in crossing low land become drying winds. When the warm humid air-currents are forced upward, however, they are cooled in part by contact with the land, but to a greater extent on account of expansion due to decreased pressure; the dew-point is thus lowered, and when saturation is reached precipitation follows. This explanation applies also to the low peninsula of Yucatan, which is within the trade-wind belt, and is exceptional among the lands of Central America, on account of its dearth of forests, but in this instance, and also in reference to the similar barren condition of the Bahama Islands, in addition to the lowness of the land, the influence of the underlying porous, limestone rocks on the vegetable growths should be considered. In the instances just mentioned the rain that falls is quickly absorbed by the cavernous rocks, and surface streams are rare.

In Central America the influence of mountains on the climate is much the same as in the West India islands; in fact, the narrow rugged belt of land connecting the two Americas may, from our present point of view, be considered as a great island situated mainly within the trade-wind belt, and similar to Jamaica, for example, in its influences on the winds blowing across it. The eastern slopes of the Central American mountains, together with the adjacent lowlands, with the exception, principally, of Yucatan, are well watered and clothed with dense vegetation, while on the western slopes, and especially over the narrow fringe of lowland adjacent to the Pacific, the rainfall is less than on the Caribbean coast, and the forests are open with many grass-covered areas which are favourable for agriculture. In the mountainous portions of the West Indies and of Central America, on account of the more healthy conditions on the leeward or drier sides, as compared with the windward or humid slopes of the mountains, the towns and the principal portion of the white inhabitants are located on their western borders. Owing to the great humidity and the long-continued high temperature during the hotter portion of the year throughout the tropical province, much of the lowland to the eastward of the high mountains is swampy and unhealthy. This low region in Mexico and Central America is known as the tierra caliente, or hot country; on the mountains and plateaus, or in general where the elevation is between 5,000 and 7,000 feet, is the cooler and remarkably salubrious tierra templada; and at still higher elevations occur the tierra frie. Owing to the decrease in temperature with elevation, and the fact that the moist warm air is forced to rise, and in consequence expands on passing over the highland, the rainfall probably increases with elevation through the three zones just referred to.

While the tropical province is characterized by the uniformity with which its atmospheric changes occur, it is nevertheless in part subject to occasional and exceedingly violent cyclonic storms termed hurricanes, which begin in the torrid zone, travel northward (Fig. 26, page 210), and make their influence felt in more than one of the climatic provinces into which North America is here divided. Thunder-storms, frequently of great violence, also occur, especially in the Central American region in summer, when the doldrums migrate northward.

The trade-wind belt broadens in crossing the southern portion of the North American continent, and on the west coast and during its greatest northward migration reaches to southern California. As we have seen, the lowlands not adjacent to mountains in the Caribbean region receive little or no rain from the trade-winds, for the reason that the prevailing air-currents are moving from cooler to warmer regions, and therefore have their capacity for moisture increased as they advance more rapidly than their thirst can be satiated. The trade-winds are thus normally drying winds. The same principle holds true for continents as well as islands. The trade-winds on reaching the eastern border of the Mexican plateau are forced upward and part with much of their moisture in the form of rain and snow, and on descending to the lower lands bordering the Pacific are desiccating winds. The conditions are thus much the same as on the lowlands situated to the leeward of the mountains of the West Indies. The narrow fringe of low-lying country on the west border of the main body of Mexico, together with the peninsula of Lower California and a portion of the southern extremity of California, are arid, although in immediate proximity to the ocean. The leading characteristics of the climate of this, the arid portion of the tropical province, are its small rainfall, clear atmosphere, high mean annual temperature, moderate seasonal changes, and active evaporation.

The most typical portion of the arid region bordering the Pacific from California southward lies in northwestern Mexico, where the mean annual precipitation on the western slope of the central table-land is from 15 to 20 inches, but gradually diminishes as one descends to the lower lands to the westward to 10 inches, and even to 5 inches or less. A great portion of the lowlands is practically a desert, although, as is common on the desert-like tracts of this continent, it is sparingly covered with detached clumps of cacti, yuccas, and other similar plants which live with the minimum of water.

The hot, arid lands of the southwest just referred to are similar in position, in reference to the relation of land areas to the trade-wind belts, as other trade-wind deserts, as they are termed, such as the Sahara, which occur on the leeward side of continents where they are crossed by these drying winds.

The Lower Austral Province (Plate III).—This province embraces the Atlantic and Gulf coasts of the United States from Connecticut southward, with the exception of the tropical portion of southern Florida; the Mississippi basin south of the mouth of the Ohio; and includes also many of the valleys amid the Pacific mountains from the northern boundary of California to central Mexico. The larger geographical conditions on which the boundaries of the province depend are the warm currents in the Atlantic, which ameliorate the temperature of the adjacent land as well as supply it with abundant moisture; the low elevation of the central continental basin, which permits of great extension northward of the climatic conditions originating about the Gulf of Mexico; and the mountainous character of the western portion of the province, where the valleys have a markedly different temperature and rainfall from the highlands.

The most southern portions of the lower austral province within the border of the United States has a semitropical climate and is characterized by its high mean annual temperature, abundant rainfall, and uniformity of weather conditions throughout the year. The summers are hot, but tempered by winds from the Gulf of Mexico and the Atlantic, and the winters, although normally mild and without snow, are varied at intervals by periods of cold which bring occasional frosts. This semitropical division of the lower austral province embraces a narrow strip of coast-land extending from South Carolina to southeastern Texas, with the exception of the extreme southern end of Florida, and forms a transition between the hotter and more humid tropical province to the southward and the cooler and less humid region to the northward.

As the saying is, a tree is judged by its fruit; in a similar way we may judge of the climate of a region by its products. The Gulf strip of the lower austral province is the home of a number of plants and animals not found farther north, as, for example, the cabbage palmetto and Cuban pine, and several species of birds and small mammals. Among the agricultural products of this narrow coastal belt which are suggestive of its climate are rice, sugar-cane, and sea-island cotton; of fruits it produces, especially in Florida, oranges, mandarins, lemons, limes, shaddocks, and pineapples.

The humid portion of the lower austral province extends northward from the Gulf strip, and presents a transition in climatic conditions between the semitropical Gulf coast and the more markedly temperate climate of the upper austral. This eastern division of the lower austral, characterized by its mild winters, general absence of snow, long hot summers with abundant rain, extends from eastern Virginia southward about the southern end of the Appalachian Mountains, and in the Mississippi Valley broadens so as to reach the mouth of the Ohio River. Its western limit is in central Texas, where the mean annual rainfall decreases to less than 20 inches, and forests give place to treeless, grass-covered plains and plateaus.

Through this eastern portion of the lower austral the mean annual precipitation is in general between 50 and 60 inches, and is so distributed throughout the year that each month receives approximately an equal share. In general, however, the rainfall is greater in winter than in summer. The mean annual temperature of the humid portion of the lower austral is from 60° to 68° F., the mean winter temperature 40° to 52°, and the mean summer temperature from 75° to 80° F. From these records it may be inferred that the conditions are favourable for the growth of trees. In its natural condition this entire region was clothed with a varied and beautiful forest, consisting largely of broad-leaved trees, but is also the home of the southern pine and the cypress. The characteristic crops are cotton and corn (maize).

The western or arid portion of the lower austral province embraces western Texas, a large area in northeastern Mexico, and circling about the southern extremity of the Pacific mountains in central Mexico, extends northward adjacent to the tropical border of the Pacific and the Gulf of Lower California, into Arizona and southern California. A detached area of this same province occupies the great Valley of California.

The leading feature in the climate of the extensive and irregular region just outlined is its aridity. The rainfall is too small to insure forest growths; the land is treeless, except along the streams, and irrigation is necessary for successful agriculture. With a sufficient amount of water for irrigation, a great variety of fruits, etc., may be raised, including many products usually considered as indicating tropical conditions, such as mangoes, dates, figs, citrus fruits, olives, pineapples, etc. Not only are the agricultural products numerous and varied, but the yield per acre under the most favourable conditions is far in excess of the best results reached in most regions where rain is relied on to furnish the requisite moisture. Under the prevailingly cloudless skies of the hot arid lands of the southwestern portion of the continent insolation is intense and the growth of vegetation phenomenal when the necessary amount of water is supplied. The land in its present condition presents great contrasts, ranging from desolate, sun-burned tracts which are almost absolute deserts, to the vivid green of irrigated fields and the deep shade of heavily fruit-laden orchards.

The Upper Austral Province (Plate III).—The portion of North America embraced in this climatic province lies principally in the central part of the United States, but includes also a narrow strip in southern Ontario, adjacent to the north shore of Lake Erie, and a large irregular area in the central plateau of Mexico. A marked feature of its geography is its extreme irregularity in the portion occupied by the Pacific mountains in the United States and Mexico. The reasons for this lie mainly in the influence of the relief of the land on climate, the direction of the prevailing winds, and varying distances from the ocean. It is a familiar fact that boreal and even arctic climatic conditions are met with on high mountains. The attention that is given to changes in climate with increase in altitude is no doubt largely due to the fact that the mountains present conditions which are exceptional and more or less novel as seen from our accustomed point of view. A person living in an elevated region, on descending into a deep valley, would be impressed with the reverse order in which the climatic zones occur. In making such a descent he would pass in succession from a boreal or perhaps arctic climate, through a transitional or cold temperate, to the warm temperate or upper austral province, and might even reach the semitropical division of the lower austral. In the Pacific mountains within the border of the United States the valleys are sufficiently deep to have the climatic conditions here ascribed to the upper austral, and in the southwestern portion of the United States descents may be made—as in the Great Valley of California and in the arid basins of southern Nevada, Arizona, etc.—sufficiently great to reach the lower austral. The valleys amid the Pacific mountains, which fall in the upper austral province, are in general low at the north in reference to sea-level, and become higher and higher at the south. For example, the upper austral region in central Washington is but 400 or 500 feet above the sea, while in Mexico it lies in general at an altitude of between 4,000 and 6,000 feet.

The upper austral province may be termed warm temperate, with a marked contrast between the heat of summer and the cold of winter. The summers are long, with an average temperature of 70° or 75°, while the winters are variable, with frequent cold periods when ice forms and snow-storms are not rare. The snow seldom remains on the ground for more than a few days at a time, however, except in the northeast, where the warm temperate climatic conditions of the province under review merge with those of the colder region to the northward embraced in the transition province.

The upper, like the lower austral, presents two well-marked divisions in reference to humidity—an eastern or humid and a western or semihumid portion; the dividing line is in the neighbourhood of the one hundredth meridian. In the eastern division the mean annual precipitation in the Piedmont region to the east of the Appalachians and on the coastal plain adjacent to the Atlantic in Maryland and New Jersey is from 40 to 80 inches, but decreases westward, and on the border of the Great plateaus in Kansas and Nebraska is about 20 inches. In the western division the annual precipitation is less than 20 inches, and agriculture without irrigation is uncertain and usually impossible. To the east of the one hundredth meridian the rain is somewhat evenly distributed throughout the year, although an increasing dryness of the summer is easily detected as one travels from east to west, but in the various upper austral valleys of the Pacific mountain region the precipitation is mostly during the winter, and the summers are practically rainless. The marked difference in precipitation between the humid and semihumid division of the upper austral province is recorded on the surface of the land by the vegetation. In the eastern division the entire region, with the exception of the prairies in the central part of the Mississippi basin, was originally clothed with a varied and beautiful forest, consisting mainly of broad-leaved trees, such as the hickory, maple, oak, etc., while the semihumid western division is treeless, except in immediate proximity to streams.

In the southern portion of the humid division of the upper austral province cotton is one of the staple products, but the northern limit of the region in which it can be successfully cultivated is soon reached as one travels northward. Tobacco is grown extensively in the southeastern and eastern portions of the province. The principal crop of the great central area in the Mississippi Valley is corn (maize). Successful wheat culture begins in the northern portion of the province, but the conditions favouring its cultivation increase to the northward and it becomes the characteristic and most valuable crop of the transition province.

In the western or semihumid division of the upper austral the variety of agricultural products that can be successfully cultivated with the aid of irrigation is greater than in its eastern portion, where irrigation is not generally practised. The northern limit at which tobacco, fruits, the vine, etc., may be advantageously cultivated in the west is greater than in the east. For example, in the east the northern limit at which tobacco is raised on a commercial scale is in Connecticut, while in the west it reaches a large size and excellent quality in central Washington. Various fruits, such as the peach, pear, plum, grape, etc., have their northern limit of successful cultivation in the east in western New York, southern Ontario, and southern Michigan, a region favourably influenced in this connection by the proximity of the Great Lakes; in the west these same fruits reach a high degree of perfection, and are produced in great abundance, with the aid of irrigation, in north-central Washington, fully 5 degrees of latitude farther northward.

On the whole, the upper austral province may be said to have a warm temperate climate, in which the summer season is longer and more pronounced in its characteristics than the winter season. It presents sufficient seasonal variations, however, to favour in a high degree both the physical and intellectual development of man.

The Transition Province (Plate III).—This, the transition region between the austral and boreal provinces, includes the cool temperate portions of North America. Like the austral provinces, its outlines are irregular, and in places it occupies detached or island-like areas, on account of the influence of mountains on climatic conditions. Its largest continuous area is situated along the northern border of the United States, but includes the southern portions of Assiniboia, Ontario, and Quebec, and extends from the Atlantic to the Pacific Ocean. From this main belt there is a marked extension southward along the Appalachian Mountains, which carries a cool temperate climate into northern Georgia, and another and much greater southward extension along the Pacific mountains, which reaches central Mexico. In the northern portion of the United States and adjacent parts of Canada, the region under consideration has, in general, an elevation of 1,000 feet or even less above the sea, but it rises when followed southward along the mountains, and in the southwestern portion of the United States and on the table-land of central Mexico attains an elevation of some 8,000 or 9,000 feet. On the higher portions of the mountains of Central America and the West Indies the climatic conditions are similar to those of the regions farther north here included in the transition climatic province, but these isolated areas are not sufficiently well known to be indicated on the accompanying map.

The mean annual temperature of the transition province is lower than that of the upper austral, and, although a precise average is not at present attainable, may be taken at about 45° F. The winters are long and cold, especially in the northern part of the United States and adjacent portions of Canada, the mean temperature being approximately 20° or 25°, but sudden and great variations are not uncommon. At times, and frequently for continuous periods of several days, the temperature falls to 20° or more below zero of the Fahrenheit scale, then again rises above freezing, and the frozen soil thaws and possibly becomes entirely freed from ice.

In common with the austral provinces, the one under consideration is divided into two portions, an eastern and a western, in reference to precipitation, the dividing line being a little to the eastward of the one hundredth meridian. While the western portion of the province is characterized by its small rainfall, precipitation is greater, mainly on account of greater elevation, than in the adjacent portions of the austral provinces, and in Oregon and Washington there is a markedly humid area.

Snow falls throughout the entire transition province, at least during exceptional winters, and in general during every winter. There is great variation in the depth of the snow from winter to winter, and also with geographical position. It is greatest in the northeast—that is, from the Atlantic coast westward to Wisconsin and Minnesota—and least on the Pacific coast and the southern portion of the Pacific mountains. Over the northeastern portion of the province the snow frequently whitens the ground for weeks, and even for two or three months continuously. A deep accumulation which remains for a long time unmelted is welcome, as it protects the roots of plants from sudden changes of temperature and prevents alternate freezing and thawing of their sap, which is injurious to their tissues in numerous instances. Great variation in the amount of snow that falls annually in a given locality is of common occurrence. A large proportion of the yearly accumulation frequently occurs during one or two great storms. For example, in January and February, 1898, there were two severe storms, accompanied by an unusual depth of snow, the first being most pronounced over New England, and the second in the region of the Great Lakes. Other storms increased the amount of snow so that at the beginning of March the average depth in Maine was 40 to 70 inches, in New Hampshire and Vermont 10 to 40 inches, and in Massachusetts 10 to 20 inches, while in Michigan and Wisconsin the general depth on level ground was 1 to 2 feet, with many drifts 10 to 15 feet deep.

The summers throughout the transition province are hot, with little rain in the western portion, but refreshing showers and occasional destructive storms in the humid eastern portion. Owing to the latitude of the main transcontinental belt of the province, the number of hours of sunlight each day in summer is increased beyond what it is in the main portions of the austral provinces, thus favouring the growth of vegetation. There is also a lengthening of the morning and evening duration of twilight, and magnificent sunrises and sunsets are frequent. The mean summer temperature is in the neighbourhood of 70° F., but hot spells, lasting for days, and even weeks, are of common occurrence. During these trying and frequently unhealthy intervals the temperature in the shade reaches or even exceeds 100° F., and sunstrokes or prostrations by reason of the heat, particularly in the cities, are numerous. The four seasons of the year are better marked and have more pronounced characteristics in this division of the continent than in any other, and it is the region of greatest seasonal climatic changes as well as of marked weekly and even daily variations in weather conditions. The most delightful months to most people are May, when the returning migratory birds are nesting, the trees unfolding their many tinted leaves, and the air laden with the perfume of multitudes of blossoms, and October, when the rich colours of ripened leaves give to the forests a marvellous variety and brilliancy of colour and the tranquil, hazy atmosphere is undisturbed by storms for days and even weeks together. This annual period of tranquil weather, extending frequently far into November, is known as Indian summer.

In the northern portion of the transition province the broad-leaved, deciduous trees of the central and eastern portions of the United States reach their northern limit, and become mingled with a southward extension of the conifers which form the major portion of the forest of Canada. A similar but less marked change occurs among the Pacific mountains, where the scattered growths of oaks, piñon pines, sycamores, etc., of the lower mountain slopes and stream sides mingle with the spruces and yellow and white pines of the more elevated region, where the climate is similar to that of central Canada. As remarked by Merriam, the province as a whole is characterized by comparatively few distinctive animals or plants, but rather by the occurrence together of southern species which there find their northern limit and northern species which there reach their southern limit. It embraces the northern portion of the truly agricultural lands of the continent. The plants of economic importance which there reach their highest stage of perfection are wheat, oats, and other cereals, the sugar-beet, numerous vegetables, the white potato, apples in great variety and abundance, cherries, plums, grapes, etc. It is the northern limit of corn, and includes nearly the entire area in which maple-sugar is produced. In the eastern portion of the province several varieties of native nuts, such as the beechnut, butternut, chestnut, hazelnut, hickory-nut, walnut, etc., grow wild and in great abundance; but nut-bearing hardwood trees are also a characteristic feature of the forests of the humid portion of the austral provinces.

In the western division of the province a humid area—embracing western Washington and Oregon, part of northern California, including the Coast Range of the same States—presents a marked contrast to the more widely extended and excessively irregular arid portion which surrounds the higher mountains and is for the most part remote from the ocean. Both the humid and arid divisions of the western part of the province are alike favourable for agriculture, as is shown by the vast and highly productive wheat-fields of the semihumid eastern portion of the States just named and the productive hop lands, orchards, and vineyards of their humid western portions.

The climate of a great land area not only finds expression in its fauna and flora, but in the industries and the intellectual development of its people. While it is difficult to translate man's physical and intellectual development into terms of climate, it is evident that the transition province favours both bodily and mental activity more than any of the other climatic provinces into which North America is here divided. Although the boundary between the upper austral and the transition provinces is indefinite, it is easily to be seen, from the geographical distribution of cities, agricultural population, manufactories, colleges, and other institutions of learning, etc., that the climate of the province under review is on the whole the one in which the greatest intellectual advance has been made and the one which holds out the greatest promise for the future.

The Boreal Province (Plate III).—This climatic division of North America extends in a broad belt diagonally across the continent from the eastern portion of Labrador nearly to the shore of Bering Sea, and is represented by detached areas in both the Atlantic and Pacific mountains far beyond its general southern limit. Its northern border, in the Continental basin, is marked by the cessation of forests, and on the mountains to the southward its upper limit coincides with the timber-line. Its leading climatic features are its low mean annual temperature—in general from 32° to 40° F.—its long, cold winters, and short, hot summers. The differences in mean annual precipitation in various parts of the province are less marked than in the several provinces previously noticed, but in the far north a cold arid division should be recognised. Although but few direct measures of precipitation are available for comparison, our general knowledge of the great boreal province and the character of its vegetation indicate that there is a decrease in precipitation from both the eastern and western borders of the continent towards the interior, and also from its central portion both northward and southward. The heaviest precipitation is on the Pacific coast, from California northward to southern Alaska, and the lightest precipitation is probably in the central Continental basin, near the northern limit of the province. Precipitation on the Pacific coast at low elevations is almost entirely in the form of rain, but on the mountains there is in winter deep snow which remains for a number of months unmelted. Throughout the portion of the province included in Canada and Alaska the snowfall is abundant, but heaviest towards the Atlantic coast. Along the northern margin of the province, as indicated by observations at a small number of stations, not only is the mean annual precipitation light, probably under 20 inches, but the winter snow is not deep, although it remains on the ground continuously for five or six months. In the main or northern portion of the boreal province, owing to the comparatively high latitude, the variation in the number of hours of light and darkness each day during a year becomes conspicuous. In summer the sun is above the horizon from eighteen to twenty-four hours each day, and in winter the hours of darkness are correspondingly increased. The year is divided into but two seasons, summer and winter, the distinctive features of spring and fall, so well marked in the upper austral and transition provinces, disappearing. On account of the low mean annual temperature, and especially because of the shortness of the growing season, agriculture is of small importance. Along its southern border, more especially in southeastern Canada and Newfoundland, such small fruits as currants, huckleberries, raspberries, blackberries, cranberries, etc., grow wild and yield abundant returns when cultivated. In favoured localities white potatoes, turnips, beets, and certain varieties of the apple, as well as the more hardy cereals, are cultivated with moderate success.

The Arctic Province (Plate III) comprises the cold, treeless plains sloping to the Arctic Ocean and the summits of the higher mountains at the south which rise above the transition province. The one controlling climatic feature is the low temperature, the mean for each year being 32° F. or lower. The winters are longer and more severe than in the boreal province, and the summers short and hot. Insolation, on account of the length of the days in summer of the main area of the province and the free exposure on the mountain summits to the southward, is intense, but its beneficial effect on vegetation is largely counterbalanced by the influence of the lingering snow and ice. In the mountainous regions of North America the arctic province is the birthplace of numerous glaciers. Although destitute of trees, the arctic, or arctic-alpine province, as it may be termed, is rendered glorious in numberless localities by the profusion and brilliancy of its flowering annuals.

In the broad, general movements of the atmosphere over North America embraced in what are termed the planetary and continental winds there are many disturbances due to more or less local changes in conditions, the most conspicuous of which are whirlwinds, chinook winds, thunder-storms, tornadoes, cyclones, and hurricanes. While some of these disturbances are local, as the whirlwind and tornadoes, and may not extend beyond the boundaries of the particular climatic provinces where they originate, others, as the cyclones and hurricanes, may affect the climate of several provinces.

Whirlwind.—A conspicuous, although minor feature in the atmospheric phenomena of the hot, dry plains and valleys, especially of the Mexican plateau and the Great Basin, and less markedly of the Great plateau to the east of the Rocky Mountains, is the occurrence of small whirlwinds which carry dust and light objects into the air in spiral columns that are not infrequently 2,000 or 3,000 feet high, and have a diameter of perhaps 50 to 100 feet. These small whirls of the air, in which some of the characteristic features of the intensely active tornadoes and widely destructive tropical hurricanes can be studied on a small scale, occur most commonly during hot summer afternoons, when from a commanding station half a dozen or more swaying columns may be seen moving in various directions over the parched valleys and sun-scorched plains. These columns not only move in various directions, showing that they are not due to the same immediate cause, but have different internal motions, some whirling from right to left, and others in the opposite direction.

The generally accepted explanation of these small whirlwinds is that the air over the surface of the deserts, which are frequently almost bare of vegetation and perhaps white with saline incrustations, becomes locally highly heated, especially when there is little or no wind, and is forced upward by the inflow of the surrounding cooler and heavier air. The inflowing currents have different velocities, and on meeting the strongest one gives a rotary or spiral motion to the ascending column, which acts like a chimney in allowing the escape upward of the hot air from below. A central vertical line frequently seen in the dust columns shows that a core of comparatively still air is present, about which the dust-charged air rises in a spiral course. If the conditions just outlined should be greatly increased in magnitude some of the leading features of tornadoes and even of hurricanes would be produced. In short, all of the winds cited above, except the chinook, are concentric, swirling movements in ascending air, due primarily to a local increase in temperature at the lower portion of the atmosphere.

Chinook Winds.—On the Great plateaus adjacent to the Rocky Mountains, and in similar situations to the eastward of the Sierra Nevada and Cascade Mountains, warm, drying winds frequently occur, especially in winter, when they bring a balminess as of spring. The remarkable feature of these interesting winds is that they come from the snow-clad mountains, but are warm and dry in contrast with the preceding condition of the air on the plains. The capacity of the air brought by these winds for moisture is so great that evaporation is active, and the snow in the valleys and over the broad plains disappears without visible melting. The change in the previously winter aspect of a region within the influence of these chinook winds, as they are termed, is truly surprising, and to their influence is due to a marked extent the value of the Great plateaus as stock-ranges, for the reason that the snow is removed from them so as to allow cattle to feed on the naturally dried grasses.

The chinook winds are the counterpart of the foehn winds of Switzerland, and are explained on the principle that descending air is made more dense by the increased pressure to which it is subjected, and its temperature correspondingly raised, its capacity for moisture being at the same time increased on account of its rise in temperature. The apparent anomaly of a warm, dry wind blowing from a snow-clad mountain range is no longer a mystery, if we consider that the air is drawn over the mountains towards a centre of low barometrical pressure owing to the wide-reaching influence of a cyclonic storm or other large atmospheric movement. The air as it rises in order to cross a mountain is cooled, largely on account of relief of pressure, and parts with a portion, possibly a large portion, of its moisture, which condenses on the mountain commonly as snow; on passing the mountain the air descends and is warmed by compression, and having less moisture than before, becomes a drying wind, which produces the sudden and surprising changes on the plains and valleys to the leeward.

The chinook winds of the western portion of Canada and the United States occur principally to the eastward of high mountains, for the reason that the prevailing air-currents of that region are from the west.

Thunder-Storms.—In the eastern portion of the United States and adjacent parts of Canada during the summer season the heating of the lower portion of the atmosphere, especially on still, sultry afternoons, causes ascending currents of warm, moist air, which become cooled as they rise, and give origin to vast masses of cumulus clouds. These magnificent "thunder-heads," as they are sometimes termed, illuminated by the full sunlight are most magnificent, and usually herald the coming of heavy showers, accompanied by frequently destructive lightning and heavy thunder. The bases of the clouds when seen from a distance are usually horizontal and may have curtain-like festoons beneath, due to falling rain; while aloft the white vapour boils upward in fleece-like masses, revealing a strong convectional ascent of moist air. The immediate cause of a thunder-storm is the rapid ascent of a column of warm moist air, which becomes cooled as it rises and the moisture contained in it condensed. The cause of the ascent of the air column, at least over plains and plateaus, is the heating of the air in contact with the earth. A layer of warm, and consequently light, air beneath a layer of cooler and heavier air furnishes unstable conditions which favour an overturning and an escape upward of the lighter air, which is forced to ascend much as the hot air in a chimney is made to flow upward by the pressure of cooler and heavier air around. The conditions preceding a thunder-storm are a stagnant atmosphere over a broad region where the lower layer of hot air is also charged with moisture. These conditions are frequently fulfilled on the plains of the Atlantic slope and southeastern portion of the continental basin in summer when warm moist air is drawn in from the Gulf region towards the centre of an area of low atmospheric pressure, and thunder-storms are there a characteristic feature. The storms usually advance northeastward, the direction being determined by the flow of upper air-currents, and move over the country with a breadth of from 10 to perhaps 100 miles, and send down copious supplies of refreshing rain.

Over the Great plateaus the air near the earth's surface is highly heated during the summer season, but it is deficient in moisture, and thunder-storms are rare, except for a brief period in late summer or fall when the normal conditions are disturbed.

Thunder-storms are almost unknown in the great Canadian-Alaska province and along the cool and humid northwest coast. They are also of rare occurrence in the hot and dry atmosphere of the Great Basin and Mexican plateau, but when they do come are of marked intensity, and pass under the name of "cloudbursts." At the far south, in the region brought under the influence of the equatorial belt of calm, thunder-storms are frequent and of great intensity.

An upward ascent of warm moist air, in much the same manner as described above, occurs about isolated mountains, particularly in the southern portion of the Rocky Mountain chain, and summer thunder-storms are there of frequent occurrence, especially in the afternoon, about the higher mountain-peaks, while the adjacent valleys are flooded with sunlight. Reference to this most striking phenomenon has already been made in describing the Park Mountain.

Tornadoes.—The fierce circular whirls in the air producing pendent, spirally twisting clouds, which when they touch the earth are of such intensity as to sweep away houses, trees, and nearly everything in their paths, are known to meteorologists as tornadoes, although popularly, but erroneously, termed cyclones. Storms of this character are of frequent occurrence in the United States to the east of the Great plateau, and are most numerous in the Mississippi Valley. Their path of destruction is seldom over half a mile wide, and as a rule they progress towards the northeast, in obedience to the movement of the upper air-currents, at a rate of from 20 to 40 miles an hour, and may cut a swath from a few miles to 20 or more miles long through forests, farms, villages, and towns. They occur usually in the afternoon, and sometimes in the earlier hours of the night, of warm, sultry days, especially in spring and early summer, but are not strictly confined to that portion of the year. The conditions which precede the coming of a tornado are, in general, the same as those in advance of a thunder-storm—that is, an indraft of hot, moist air beneath a cooler layer, thus establishing unstable conditions. An upward draft is started, the intensity of which becomes so great that the inflowing winds are given a rapid spiral motion about a calm centre. The tornado may be considered as a fully developed or exceptionally energetic thunder-storm, in which a spiral movement is established as in desert whirlwinds. The conditions for the origin of this class of dreaded and locally most destructive storms are best fulfilled in the central portion of the Mississippi basin, where they are somewhat frequent. They occur less commonly over the country to the eastward, and are unknown in the more northern and western climatic provinces, and, so far as the writer is aware, they have not been reported from the region to the north of the United States.

Cyclones.—This name is applied to the great atmospheric disturbances marked by an inflowing of air towards a centre of low barometric pressure from adjacent regions, commonly several hundred miles across, and an escape and overflow aloft. As in whirlwinds and tornadoes, there is a spiral movement established in the inflowing currents, but owing to the large size of the area of low pressure, this seldom reaches destructive violence. Cyclonic storms are of common occurrence, especially in the temperate zone, and bring to that region its characteristic diversity of weather. Most of the rain and snow-storms of the continent are due to the vast swirls of the atmosphere about areas of low atmospheric pressure, which cause air-currents from different directions and with different components of heat and moisture to move over the land.

The cyclonic storm of the Mississippi Valley, the Atlantic coast States, and southeastern Canada frequently originate in the Great plateau province, and are carried towards the Atlantic owing to the influence of the eastward-blowing winds of the upper atmosphere. At times these storms are of such magnitude and intensity that they cross the Atlantic and are observed in England and Scandinavia. The courses they follow may be traced from day to day on the weather-maps issued by the United States Weather Bureau, and from the directions they are likely to follow and the atmospheric conditions pertaining to their various parts predictions of surprising accuracy as to the changes which the weather in a given locality will experience can be made one or two days before the changes occur.

Hurricanes.—Cyclonic storms of the general nature of the tornadoes, but of vastly greater extent and intensity, originate occasionally during the latter portion of the summer season over the tropical portion of the north Atlantic, move slowly westward to the vicinity of the Lesser Antilles, where normally their courses bend northward, and then skirt the Atlantic coast of the United States and drift eastward under the influence of the eastward-flowing upper air-currents, and not infrequently make their influence felt in the western portion of Europe. Occasionally, on account of the presence of an area of high barometric pressure to the north of Cuba, the course of one of these tropical hurricanes, as they are termed, is rendered irregular, and it passes over the Atlantic States or is deflected still more and crosses the Gulf of Mexico before reaching the border of the continent, as was the case in September, 1900, when a large part of Galveston was destroyed. The normal paths of the tropical hurricanes as they approach the coast of the United States and the exceptional course of the one which passed over Galveston, are indicated on the map forming Fig. 26.