THE CLIMATIC ZONES AND THEIR SUBDIVISIONS

Classification by Latitude Circles: The Five Classic Zones; Klima as Used by the Greeks; Ptolemy’s Climates; Parmenides; Polybius; Posidonius; Aristotle; Eudoxus; Strabo; Hippocrates—Temperature Zones: Supan; Köppen; Gebelin—Wind Zones: Davis; Woeikof—Summary and Conclusions—Necessary Subdivisions of the Zones.

_Classification by Latitude Circles._ So great is the variety of climates to be found in different parts of the world that it has long been customary to classify these climates roughly into certain broad belts. These are the climatic zones. A simple grouping of this kind can, however, obviously take account only of the most general characteristics of the climates which are included within each zone. The five zones with which we are most familiar are the so-called torrid, the two temperate, and the two frigid zones. The torrid, or, better, the tropical zone, naming it by its boundaries, is limited on the north and south by the two tropics of Cancer and Capricorn, the equator dividing the zone into two equal parts. The temperate zones are limited towards the equator by the tropics, and towards the poles by the Arctic and Antarctic circles. The two frigid, or, better, the two polar zones, are caps covering both polar regions, and bounded on the side towards the equator by the Arctic and Antarctic circles.

These five zones are classified on purely astronomical or mathematical grounds. They are really zones of sunshine, or of solar climate. Within the tropical zone, the sun reaches the zenith at two different times in the year; its greatest possible zenith distance is 47°; the day is never less than ten and a half hours long. On the tropics themselves, the sun reaches the zenith but once a year. In the polar zones, the sun is below the horizon for twenty-four hours at least once in winter, and is above the horizon for the same length of time at least once in summer. On the polar circles, the noon altitude of the sun decreases to 0° on the shortest day. The temperate zone has conditions between these two extremes. At no point can the sun be in the zenith; nor, except on the polar circles, is there ever anywhere a twenty-four-hour day or night.

The tropical zone has the least annual variation of insolation. It has the maximum annual amount of insolation. Its annual range of temperature is very slight. It is the summer zone. Beyond the tropics the contrasts between the seasons rapidly become more marked. The polar zones have the greatest variation in insolation between summer and winter. They also have the minimum amount of insolation for the whole year. They may well be called the winter zones, for their summer is so short and cool that the heat is insufficient for most forms of vegetation, especially for trees. The temperate zones are intermediate between the tropical and the polar in the matter of annual amount and of annual variation of insolation. Temperate conditions do not characterise these zones as a whole. They are rather the seasonal belts of the world. These five zones further differ more or less from one another in the character of their animals and plants, and in the conditions of human life within their boundaries.

Taking the area of a hemisphere as unity, the relative areas of these zones are as follows:

Tropical 0.40 Temperate 0.52 Polar 0.08

This subdivision of the earth’s surface on the basis of the geometrical distribution of sunshine dates from the time of the early Greek philosophers and geographers, but it is impossible to determine with certainty just when and by whom the various suggestions in this connection were made. The famous geographer Ptolemy, who lived in the second century A.D., used different schemes at different times. In the lower latitudes the breadth of a _klima_, or zone, was fixed by the difference of a quarter of an hour in the length of the longest day, but in higher latitudes differences of half an hour, an hour, and finally a month were the determining factors.

Parmenides, who flourished about the middle of the fifth century B.C., proposed a five-zone division of the earth’s surface not very unlike our present system. These zones were a torrid zone, uninhabitable because of heat; two frigid zones, uninhabitable because of cold; and two intermediate zones, of moderate temperature, suitable for man. The exact limits assigned to these zones are not known with certainty; but it is reasonable to suppose that the Arctic circle was even then recognised as a natural boundary for the north polar zone, and it is pretty clear that the temperate zone was much smaller, and the torrid zone much larger, than in our present classification. (See Fig. 4.)

The exact boundaries of the different zones varied more or less for some time, as astronomical knowledge became more and more exact, and as the habitable area of the earth’s surface was gradually extended, but the scheme was generally adopted by later writers. Polybius (born about B.C. 204), however, divided his torrid zone into two parts by the equator, and Posidonius (born about B.C. 135) divided his torrid zone into three parts, making six and seven zones respectively. Aristotle (born B.C. 384) limited the torrid zone by the tropics, and the north temperate zone by the Arctic circle; but there is doubt whether he really meant the fixed Arctic circle which we know. He believed both temperate zones habitable, thus limiting the uninhabitable area to the astronomical tropical zone. Eudoxus, of Cnidus, who lived about B.C. 366, used a division of a quadrant of the earth’s circumference into fifteen parts, of which four belonged to the torrid, five to the temperate, and six to the frigid zone. The tropics were thus fixed at latitude 24°. Strabo (born about B.C. 54), opposed the prevailing view that the whole of the belt between the two tropics was uninhabitable, and also first clearly set forth the opinion that the temperature decreases with increasing altitude above sea-level, as well as with increasing latitude. Strabo also had some fairly distinct ideas regarding local differences of climate resulting from the influence of land and water and of mountain barriers, and noted several effects of climate upon man and upon vegetation. He appreciated the fact that the zones were zones of temperature as well as zones of sunshine. As early as about 400 B.C., Hippocrates had endeavoured to show a causal relation between sunshine and the topography of a district on the one hand and the characteristics of its inhabitants on the other. He also gave an outline of geographical pathology.[3]

[3] The older views regarding the climates and the habitability of the five zones were thus stated by Virgil (_Georgics_, i, 233-239, translation by Davidson): “Five zones embrace the heavens; whereof one is ever glowing with the bright sun, and scorched forever by his fire; round which the two farthest ones to the right and left are extended, stiff with cerulean ice and horrid showers. Between these and the middle zones, two by the bounty of the gods are given to weak mortals; and a path is cut through both, where the series of the signs might revolve obliquely.”

_Temperature Zones._ The classification of the climatic zones on the basis of the geometrical distribution of sunshine serves very well for purposes of simple description, but a glance at any isothermal chart shows at once that the isotherms do not coincide with the latitude lines. In fact, in the higher latitudes, the former often follow the meridians more closely than they do the parallels of latitude. The astronomical zones—_i. e._, the zones of light—therefore differ a good deal from the zones of heat. Hence it has naturally been suggested that the zones be limited by isotherms rather than by parallels of latitude, and that a closer approach be thus made to the actual conditions of climate.

Supan (see Fig. 5) has suggested limiting the hot belt, which corresponds to the old torrid zone, but is slightly greater, by the two mean annual isotherms of 68°—a temperature which approximately coincides with the polar limit of the trade winds and with the polar limit of palms. The latter is considered by Grisebach to be the truest expression of a tropical climate. The hot belt widens somewhat over the continents, chiefly because of the mobility of the ocean waters, whereby there is a tendency towards an equalisation of the temperature between equator and poles in the oceans, while the stable lands acquire a temperature suitable to their own latitude. Furthermore, the unsymmetrical distribution of land in the low latitudes of the northern and southern hemispheres results in an unsymmetrical position of the hot belt with reference to the equator, the belt extending farther north than south of the equator. The polar limits of the temperate zones are fixed by the isotherm of 50° for the warmest month. This is a much more satisfactory limit than the mean annual isotherm of 32°, which has also been suggested; for climates differing very widely from one another are found to have the same mean annual temperature of 32°. The latter value has chiefly a theoretical interest, but is of some practical importance in its relation to the regions of frozen ground. Summer heat is more important for vegetation than winter cold; and where the warmest month has a temperature below 50°, cereals and forest trees do not grow, and man has to adjust himself to the conditions in a very special way. The two polar caps are not symmetrical as regards the latitudes which they occupy. The presence of extended land masses in the high northern latitudes carries the temperature of 50° in the warmest month farther poleward there than is the case in the corresponding latitudes occupied by the oceans of the southern hemisphere, which warm less easily and are constantly in motion. Hence the southern cold cap, which has its equatorial limits at about lat. 50° S., is of much greater extent than the northern polar cap. So far as this south polar zone is concerned, the presence or absence of an Antarctic continent is immaterial; for such a land mass must be ice-covered, and hence cannot operate to raise the temperature as in the case of a land surface to which the sun’s rays have immediate access. The northern temperate belt, in which the great land areas lie, is much broader than the southern, especially over the continents. These temperature zones have real significance. They emphasise the natural conditions of climate more than can be the case in any subdivision by latitude circles, and they bear a fairly close resemblance to the old zonal classification of the Greeks.

In high latitudes, neither the mean annual temperature nor the temperature of the coldest month is nearly as important a climatic control over vegetation as is the temperature of summer, from the point of view of climate as a whole, and especially in relation to organic life. The summer temperatures determine habitability, the limits of plant growth, and the general conditions of human life. Hence, in the higher latitudes, zones bounded by mean annual isotherms are no great improvement over zones limited by latitude circles.

Another classification of temperature zones has been suggested by Köppen (see Fig. 6). In this, the length of time during which the temperature remains within certain fixed limits, these limits having well-marked relations to organic life, is taken into account. Two critical daily mean temperatures, 68° and 50°, and the duration of these temperatures for periods of one, four, and twelve months, are the factors in this classification. These temperatures are not reduced to sea-level. A normal duration of a temperature of 50° for less than a month fixes very well the polar limit of trees and the limits of agriculture. Near this line are found the last groups of trees in the tundras. A temperature of 50° for four months marks the limit of the oak, and also closely coincides with the limits of wheat cultivation. North of the tree limit, agriculture ceases, and man’s food is to be sought very largely in the sea. With the approach to this line, the period of plant growth is shortened more and more, agricultural operations become restricted, and occupations of other kinds are followed. These critical temperatures and their varying periods of duration form the basis of the following classification:

1. Tropical belt: all months _hot_ (over 68°). This is almost altogether within the tropics; it reaches, in round numbers, from latitude 20° N. to 16° S.

2. Sub-tropical belts: 4 to 11 months _hot_ (over 68°); 1 to 8 months _temperate_ (50°-68°.)

3. Temperate belts: 4 to 12 months temperate.

4. Cold belts: 1 to 4 months temperate; the rest _cold_ (below 50°).

5. Polar belts: all months cold.

The temperate belts of both hemispheres are further subdivided into three districts[4]—the steadily temperate belt[5] is found only on the oceans; the belt of hot summers[6] only on the continents; and the third, with moderate summers and cold winters,[7] extends around the world, with the exception of a notable interruption over Siberia.

[4] All characterised by having at least four months temperate (50°-68°), and not more than four months hot (over 68°).

[5] No month over 68° or below 50°.

[6] Has temperatures below 50° for one or more months.

[7] Separated from “permanently temperate” by the 50° isotherm for the coldest, and from “hot summers” by the 71.6° (22° C.) isotherm for the warmest month.

In the second of these subdivisions, except in eastern North America and Asia, the rainfall is generally deficient; irrigation is more or less necessary, and deserts and steppes characterise the continental portions. Only in the monsoon districts of southern and eastern Asia, of Brazil, and of south-eastern North America, do we find high temperatures combined with high relative humidity. The third subdivision above noted is now the chief seat of human development. Over a large part of the cold belt of the northern hemisphere, the ground is permanently frozen, thawing only a little on the surface in summer. Nevertheless, in portions of it trees and hardy cereals grow. The polar belts are, as a whole, outside the limits of tree growth.

Another suggestion has been made by Gebelin, who has proposed to select, as limits of the temperate zone, certain visible geographical boundaries, in contrast with the ideal climatic limits based upon the distribution of sunshine. On the oceans, the tropical circles serve as acceptable boundaries on the sides towards the equator, but on the continents the desert belts on both sides of the tropics are reasonable limits, although these deserts do not reach the eastern coasts of the continents. For the polar limits of the temperate zone, the tundras are chosen on the continents, and the summer ice-masses on the oceans.

_Wind Zones._ While a simple classification of the zones on the basis of temperature is an improvement upon any rigid scheme of division by latitude circles, the heat zones emphasise the element of temperature to the exclusion of such important elements as winds and rainfall. So distinctive are the larger climatic features of the great wind belts of the world, that a classification of climates according to wind systems has been suggested by Davis. As the rain belts of the world are closely associated with these wind systems, a classification of the zones by winds also emphasises the conditions of rainfall. In such a scheme, the torrid, or tropical zone, with its regularity of weather through the year, and the comparative simplicity of its climatic features, is bounded on the north and south by the margins of the trade wind belts, and is therefore larger than the classic torrid zone. This trade wind zone is somewhat wider on the eastern side of the oceans, and properly includes within its limits the equable marine climates of the eastern margins of the ocean basins, even as far north as latitude 30° or 35°.

Most of the eastern coasts of China and of the United States are thus left in the more rigorous and more variable conditions of the north temperate zone. Through the middle of the trade wind zone extends the sub-equatorial belt, with its migrating calms, rains, and monsoons. On the polar margins of the trade wind zone lie the sub-tropical belts, of alternating trades and westerlies. The temperate zones, with the great irregularity of their weather phenomena and their marked seasonal changes, embrace the latitudes of the stormy westerly winds, having on the equatorward margins the sub-tropical belts, and being somewhat narrower than the classic temperate zones. Towards the poles, there is no obvious limit to the temperate zones, for the prevailing westerlies extend beyond the polar circles. These circles may, however, serve fairly well as boundaries, because of their importance from the point of view of insolation. The polar zones in the wind classification, therefore, remain just as in the older five-zone scheme.

A compromise between the rigid division by latitude circles and the isothermal and wind classifications has been suggested by Woeikof, who objects to limiting the torrid zone by the tropics on the ground that the high temperatures of that zone, as well as its characteristic winds, extend beyond these parallels. Latitude 30° would be a more natural boundary; but as the westerlies, which are characteristic of the temperate zones, prevail there in winter, latitude 25° is chosen as a compromise between 231⁄2° and 30°. The polar zones are bounded by latitude 65°. When bounded by these several limits, the areas of the different zones are as follows:

Tropical Zone 417 Temperate Zones 490 Polar Zones 93 1000

_Summary and Conclusions._ Reviewing what has been said regarding the climatic zones, it would seem that, all things considered, a simple division by isotherms, such as that suggested by Supan (1896), is the best for general use. The early division by latitude circles, while it has the merits of great simplicity, and emphasises the all-important element of sunshine, is too arbitrary, and hence does not accord sufficiently well with the facts of actual climate. Nevertheless, we should not discard the classic zones without recognising that they have a real meaning in relation to solar climate. The grouping of the climatic zones according to wind systems has much to recommend it from a meteorological standpoint, but is not quite simple enough for general use. Its adoption involves an understanding of the great wind and calm belts of the world, and of the migration of these belts. The shifting of the boundaries of the torrid zone also brings in an element of uncertainty which is somewhat confusing, although, as a place in the sub-tropical belt really changes its climate with the seasonal change from westerlies to trades, and _vice versa_, it may reasonably be expected to change its zone. In other words, actual climatic conditions are recognised; and in any case, this is a more reasonable plan than to limit the torrid zone by means of the tropics, which arbitrarily cut across the trade wind belts and separate areas which are climatically the same. The temperature zones proposed by Köppen, while useful in special studies of plant distribution, are too detailed for general adoption.

Whatever climatic zones we adopt, we should certainly abandon the word _temperate_ altogether as the designation of the middle zone in each hemisphere, and substitute some such adjective as _intermediate_ for it. The words _torrid_ and _frigid_ should likewise disappear, and be replaced by _tropical_ or _equatorial_, and _polar_.

_Necessary Subdivisions of the Zones._ However we may classify them, the climatic zones are far from being uniform in character throughout their whole extent. Hence, no brief, simple description of the climate of a zone can be given. For this reason, suggestions have been made regarding subdivisions of the different zones. Thus, in the case of the classic north temperate zone, it has been proposed to subdivide it into sub-tropical, temperate, and sub-arctic, but the question how to limit these subdivisions is difficult to settle. A more rational scheme is that which, in view of the great differences in the climatic relations of land and water, recognises a first large subdivision of each zone into land and water areas. Then, as continental interiors differ from coasts, and as windward coasts have climates unlike those of leeward coasts, a further natural subdivision would separate these different areas. Finally, the control of altitude over climate is so marked that plateaus and mountains may well be set apart by themselves as separate climatic districts. If each of the zones, whether bounded by latitude circles, or by isotherms, or by wind systems, be considered under these general subdivisions, as close an approach to actual conditions of climate will be made as is possible in general description. Obviously, however, when the larger zones are subdivided to such an extent as is here suggested, we are dealing with a classification of climates rather than with climatic zones.