Climatic classification in forestry

by C. W. THORNTHWAITE and F. KENNETH HARE¹

¹C. W. Thornthwaite is Director of the Laboratory of Climatology, Centerton, N. J., U.S.A., and President, WHO Commission for Climatology: F. Kenneth Hare is Chairman of the Department of Geography, McGill University Montreal, Quebec, Canada.

CLIMATES, like nearly all other variable quantities, can be classified. But it is important to realize that the taxonomy of climate is not the same as that which arranges living organisms into species, genera and so to the phylum. However much the biologists may disagree as to what constitutes a species, there is no doubt that the living world is divisible into groups that are, on the whole, mutually distinct; between the genera Abies and Picea there are differences of morphology, of physiology and of genetic characteristics that are too strong to be lightly disregarded. But no such striking differences need separate adjacent climates. On the contrary, one climate tends to grade into another by continuous transition. Only rarely do wee find climatic boundaries that approach in sharpness the difference, for example, between Pinus and Taxus.

Clearly, the real analogy is between climatic classification and the classification of soils or natural vegetation; it is commonplace that the grasslands of the world normally grade into forest through a broad ecotone of park or savannah. It is difficult to classify rationally such a natural complex as soil or vegetation; yet such classification is necessary, and it is groping toward reality. There is no doubt that "coniferous forest" is a real entity, however difficult it may be to define its limits. It is likewise with climate. In the face of complex geographical variation of climate, it is necessary to recognize climatic types and their geographical equivalent, climatic regions. The purpose of a climatic classification is to provide a concise description of the various climatic types in terms of the truly active factors of climate, primarily those of moisture and heat. The value of any classification. depends, first, on the accuracy with which the climatic regions are identified and their boundaries located, and second, on the skill with which numerical parameters are selected to define these boundaries, as well as to specify conditions lying within them.

The forester and the silviculturist have a legitimate interest in this subject. For many years it has been customary to regard natural vegetation and soil as complex functions of climate, whether on the continental scale (as in Clements' climax formations) or on the purely local scale where the concept of site is applicable. That forest vegetation is distributed in response to certain external climatic controls and that forests are themselves major conditioners of microclimates and even macroclimates are by now well-known facts. But to the Food and Agriculture Organization, the question of climatic classification assumes an altogether more practical significance, because of its interest in tree-planting, especially of exotic species. The Asia-Pacific Forestry Commission, at its second session in Singapore, December 1-13, 1952, asked the World Meteorological Organization (WMO), through the Director of the Forestry Division of FAO, to recommend a standard scheme of climatic classification...

"... based on the relationships between climate and vegetation... Such a classification would be extremely useful to forestry agencies of our Member Governments as a guide for the choice of exotic species for trial plantings."²

²Quoted in Annex II to Recommendation 8, Abridged Final Report of the First Session, Commission for Climatology, World Meteorological Organization, Washington March 12-25, 1953, pp. 40-41.

In 1954, the Eighth General Conference of UNESCO in Montevideo discussed a plan of procedure for research in the humid tropics. The Proposed Programme and Budget for 1955 and 1956 included the following recommendation:

"Maps delimiting the humid tropical areas and showing those areas which experience similar climatic conditions are considered to be a prerequisite for the work of the programme."³

³Quoted from Proposed Programme and Budget for 1955 and 1956, UNESCO, 8th General Conference, Montevideo, 1954, paragraph 86.

This interest by different agencies within the United Nations Organization is but the reflection of a growing awareness in all countries of the need for accurate and detailed climatic information.

The present authors doubt whether any classification yet exists that deserves to be adopted as "standard." It may be a. long time before anything approaching a standard can be achieved. But a review of existing methods may be useful: that is what is attempted in this article.

Brief historical review

The earliest attempts at climatic classification in the modern sense were made in the mid-nineteenth century, chiefly by natural historians and biologists. From the very beginning the initiative in such classification has rested with the biologists, and it has been from these students of the living cover of the earth that most of the actual systems of classification have come. Both of the present authors have published reviews of this early phase with abundant literature citations, and only the briefest treatment is needed here.⁴ 1866, A. Grisebach published the earliest significant world map of vegetation regions. Coming as it did shortly after the great plant physiologist, Alphonse de Candolle, had published his massive investigations into the factors influencing the distribution of plant species, Grisebach's attempt at the classification of world vegetation was a challenge to students of climate to look for correspondences between temperature or precipitation data and the world distribution of vegetation. At the same time, among biologists there was extensive investigation of the effect of climate on the phenology, growth and development of plant species. The monographs of Carl Linsser are the outstanding monument to this phase; his work, concerned with the effects of temperature on phenology and rainfall on vegetation, led him to divide the world into climatic zones, thereby making himself the first to attempt a true classification of climates on the basis of vegetation zones. Later, de Candolle published a physiological classification of plants based on adaptation to climate in which the familiar terms "megathermal", "xerophilous", "mesothermal" and "microthermal" appeared for the first time.

⁴THORNTHWAITE, C. W.., Problems in the classification of climates, The Geographical Review, vol. 33, no. 2, 1943, pp. 233-255.

HARE, F K, Climatic classification, The London Essays in Geography; ed. L. D. Stamp and S. W. Wooldridge Harvard University Press, Cambridge, 1951, pp. 111-134.

TABLE 1. - CLIMATIC ZONES OF THE WORLD

Climates	Simbol	Köppen's name	Equivalent de Candolle zone	Climatic limits zone
Tree1	A	Topical rainy	Megathermal	Coldest month temperature above 18°C. (64°F.)
	C	Warm temperate	Mesothermal	Coldest month between -3°C 27°F) and 18°C (64°F.)
	D	Boreal	Microthermal	Coldest month below -3°C (27°F) warm est above 10°C(50°F.)
Snow2	E	Snow	Hekistothermal	Warmest month below 10°C. (50°F)
Dry3	B	Dry	Xerophilous	Annual rain-fall less than R.4

¹With enough rainfall for forest vegetation
²Too cool for tree growth.
³Too dry for tree growth,
⁴For derivation, see following text,

Thus by 1875 the idea that climates might be classified according to the type of vegetation or physiological response they produced was well established. But it was left to a St. Petersburg trained biologist, Wladimir Köppen to take this idea and elevate it into a primacy of place that it has never lost. Köppen's interest carried him into climatology, which he dominated for some 60 years, and his climatic classification continues in use even today among geographers. His first work on heat and plant growth was in the tradition of Linsser and de Candolle, but he soon turned to the task of defining world climatic regions objectively. His early training, and his familiarity with the works of Oscar Drude and Grisebach, led him to the idea that plants might serve as instruments integrating the various climatic elements, and that their distribution might therefore be assumed to identify climatic regions. This idea subsequently dominated his life's work and the thinking of a whole generation of climatologists.

Köppen first presented his classification in 1900, and thereafter modified it frequently and extensively until his last paper appeared in 1936. He took de Candolle's plant regions or vegetation zones and endeavored to find climatic correspondences for their boundaries, a search that continued the rest of his days. His primary division of the earth's surface was into five great zones following de Candolle, separated by certain critical values of temperature and precipitation. To use Köppen's own terms, these five zones comprise a "dry climate", a "snow climate", and three "tree climates", arranged as in Table 1.

Thus Köppen's limits are essentially simple averages of the directly observed values of the climatic elements. The "dry" climates (B) are those whose annual rainfall is less than a value that depends on the mean annual temperature. Köppen used many different formulae to determine this critical rainfall (R). In 1928 he adopted (as a final form) the value:

R = 0.44 (T-k),

where T = mean annual temperature, and

k = a constant whose value is determined by the seasonal concentration of rainfall.

Stations having rainfall greater than R thus fall into the humid group, which includes both tree and snow climates. Simple values of mean temperature for the coldest or warmest month separate them into A, C, D, and E groups. The primary climates are further subdivided according to the seasonal distribution of rainfall and temperature, again using averages of the simple measures of these quantities.

In spite of the wide currency it achieved, the Köppen system was always unsatisfactory, and is quite futile as a method for classifying forest climates. In the first place, it is a crude, blunt instrument; the regions it defines are large and unwieldy, and do not correspond to major divisions of the vegetation of the earth. Nor is there any point in trying to adjust the regional boundaries by shifting their numerical value as has often been done. Köppen's use of simple temperature and precipitation values to define boundaries entirely misses the point, as do all the schemes produced by his imitators and apologists. The fact that a particular isopleth of mean air temperature happens to follow a soil or vegetation boundary is to a large extent fortuitous. Any effective system must endeavor to answer the questions, "What are the real, active processes of climatic control? And how can suitable parameters for these processes be devised?

Climatic processes

The climate of a locality, considered statistically, is often regarded simply as "average weather", which is essentially a meteorologist's view. Climate considered in the light of physical processes is better thought of as the complex interaction of vegetation and atmosphere at the earth's surface, especially as expressed in the exchanges of energy, moisture and momentum between the surface and the atmosphere. We believe that a soundly based classification, of real value in the study of vegetation, ought to seek its parameters in these complex exchanges, and not in the raw observational data of the meteorologist.

In the new classification of climate which Thornthwaite proposed in 1948, he gave great prominence to natural evaporation, or evapotranspiration, to give it the term it merits over a vegetation-covered continents The principal parameter of Thornthwaite's classification is a climatic potential derived from the thermal regime, the potential evapotranspiration. To justify the cardinal role applied to these quantities, it will be necessary at this stage to digress upon the nature of evaporation over a land surface, since great confusion exists in the literature of the ecologist and plant physiologist on this point.

Evaporation - the change in state of water from liquid to vapor - represents an important mass transfer from ground to atmosphere, the reverse of precipitation in the hydrologic cycle. But it is also an important agency of energy transfer, since vast amounts of heat are required to bring the evaporation about, and are then transferred to the air with the vapor as latent heat. Thus natural evaporation is much more than the reverse of rainfall; it is also a reverse flow to the downward stream of radiation from suit and atmosphere that warms the soil surface. A single parameter, measured evaporation or computed potential, hence at one time demonstrates two of the principal exchanges between earth and atmosphere.

Natural evaporation from the land whether evaporation from ponds, lakes and rivers, intercepted rainfall from the leaves and stems of plants, direct-evaporation from the soil surface, or transpiration from green plant tissues, can proceed only when the vapor pressure of the ambient air is less than the vapor pressure at the evaporating surface, and can continue only while there is an external source of energy. The measurement of evaporation presents many difficulties. Instruments like the porous bulb atmometer and the evaporation pan are of little value in estimating evapotranspiration. They measure, rather arbitrarily, some function of the evaporating power of the air at the level of exposure. But no close relationship exists between this function and the actual upward flow of vapor from the nearby land-surface. To measure natural evaporation, the instrument must simulate closely the natural conditions of surface, exposure, albedo⁵ and water-supply, or else it must be designed to measure the actual flow of moisture from the ground to the atmosphere, as an electric meter measures consumed electricity.⁶ Actually, there are only two practicable ways of instrumentally measuring evaporation under natural conditions: the "vapor transport" method using measured values of wind speed and humidity at two levels and the method utilizing soil filled tanks or evapotranspirometers. The vapor transport method is still not entirely practical due to observational limitations but evapotranspirometers have been installed in many parts of the world, and observations are now available for a number of years.⁷ The problem of devising an evapotranspirometer below a tree-covered surface remains unsolved. It may be noted in passing, however, that stream discharge from watersheds with known rainfalls gives an indirect measure of evapotranspiration, though there is a long lag between the accumulation of surplus rainfall and its discharge from the watershed.

⁵For an explanation of this term see following text.

⁶THORNTHWAITE, C. W. ., and HOLZMAN, BENJAMIN., Measurement of evaporation from land and water surfaces U.S. Dept. Agric. Tech. Bull No. 817, 1942, 143 pp. -
THORNTHWAITE, C.W, ., with WILM, H. G. and others. Report of the committee on evaporation and transpiration, Trans. Amer. Geophys. Union, vol. 27, no. 5, 1946, pp.721-723.

⁷MATHER, J. R. The measurement of potential evapotranspiration, Publications in Climatology, The Johns Hopkins University Laboratory of Climatology, Seabrook, New Jersey, vol. VII, no. 1, 1954, 224 pp.

From this increasing inventory of evaporation measurements, certain conclusions are permissible. Evapotranspiration clearly depends on the:

(a) external supply of energy to the evaporating surface, principally by solar radiation;

(b) capacity of the air to remove the vapor, i.e., on wind speed, turbulent structure and the decrease of vapor concentration with height;

(c) nature of the vegetation, especially as regards its capacity to reflect incident radiation, the extent to which it fully occupies the soil, and the depth of its root system;

(d) nature of the soil, especially the amount of available water in the root zone.

Of these four, the meteorological controls (a) and (b) rank larger than the biotic and edaphic controls (c) and (d), a claim that will surprise many foresters. Actually (a) and (b) are closely related, since to a large extent the turbulent structure of the lower atmosphere depends on the same radiative and mass exchange processes that contribute the energy needed for evaporation. Similarly (c) and (d) are closely related.

There is no doubt that (a) is the master item in this list. Natural evaporation requires the latent heat of vaporization, and in practice this comes mainly from incoming solar radiation. Some of the radiation is reflected back from the surface, the percentage lost being known as the albedo. Some is used to heat the soil. Some goes to create convection within the air. The rest is used for evapotranspiration.

Different types of vegetation differ in their potential evapotranspiration because they absorb different amounts of solar radiation. Ångströms has given the albedos of a few different surfaces as follows: grass 0.26; oak woodland, 0.175; and pine forest, 0.14.8 Thus the pine forest might absorb 16 percent more energy than the grassland but the proportion available for evapotranspiration varies through a narrower percentage range. Most common garden vegetables and field crops absorb about the same amount of solar energy as grassland. Some types of forest may have a higher albedo than grass although we have no records of them; if such exist they would transpire less than grassland.

⁸ÅNGSTRÖM, A. The albedo of various surfaces of ground, Geografiska Annaler, H. 4, 1925, pp. 323-342. E

A grassland may easily consume 80 percent of the incoming net radiation for evapotranspiration when the soil is moist, and mature high forest cannot transpire much more. Table 2 shows how the available energy was partitioned during an experimental period in August 1953, over the Nebraska prairies.⁹

⁹THORNTHWAITE, C. W., and others. Micrometeorology of the surface layer of the atmosphere, the flux of momentum heat, and water vapor. Publications in Climatology, The Johns Hopkins University Laboratory of Climatology, vol. VII, no. 2, 1954, PP. 359-161.

TABLE 2. - ENERGY FOR CONVECTION, EVAPOTRANSPIRATION, AND STORAGE IN THE SOIL, TOGETHER WITH WATER CONTENT OF SOIL, OVER GRASSLAND, O'NEILL, NEBRASKA, U.S.A.

Date	Heat Used for Convection (C)	Heat Stored in Soil (S)	Heat Used for Evapotranspiration	Total (C+S+E)		Available Soil Moisture in Top 45 cm. of Soil
	(cal/cm2)	(cal/cm2)	(cal/cm2)	(cal/cm2)	(%)	(cm.)
Aug. 13-14	56.3	29.7	377.2	463.2	81	4.2
Aug. 18-19	59.1	4.8	287.8	342.1	84	3.6
Aug. 22	98.4	19.0	216.2	333.6	65	3.0
Aug. 25	181.9	41.5	131.8	355.2	37	2.7
Aug. 31	242.3	28.3	44.5	315.1	14	1.9

When the soil of this grassland was moist, over 80 percent of the available energy was utilized for evapotranspiration, whereas only 14 percent was consumed two weeks later when the soil had partially dried out. In other words, over moist soils, a grassland is an extremely efficient evaporator. Forest vegetation can do little better; for, even if all the energy is used for evapotranspiration, the total flow can only be 25 percent greater. Those forms of forest having a lower albedo than grassland will hence absorb more energy, and will have more available for evapotranspiration. But the difference is small.

The idea that forest vegetation, under moist conditions, differs little from grassland in the power to evapotranspire will strike an unfamiliar note to many readers. The literature is full of accounts of exceedingly high transpiration rates from isolated trees, set in pasture or on river banks; and potted seedlings have also produced high rates. But it can be shown that a large stand of trees cannot possibly maintain these rates, for to do so would involve many times the available energy. The isolated tree can get additional energy by absorbing reflected radiation and by picking up heat advectively from the air. The forest stand cannot.

It is under conditions of drought, when surface soil moisture is largely exhausted, that the deep-rooted forests are able to outdo the grasslands; for they can tap deeper layers of the soil, and so supply the demands of their leaf-zone longer. But otherwise, the meteorological factor dominates and the influence of vegetation type is of secondary importance. The logic of these paragraphs is the logic of the principle of the conservation of energy; it will not tolerate much argument.

A word is necessary about interception, which is of considerable importance in forests. From the point of view of the climatologist it matters little whether the water evaporating from a plant comes from the soil via the root system, or is merely intercepted rain. Both processes require the same quantity of energy, and both constitute evapotranspiration. It is true, of course, that the intercepted water will not have figured in the physiology of the plant; but, supposing that soil moisture is not actually at the wilting point, the energy consumed in evaporating the intercepted water would otherwise have been used to evaporate transpired water. In other words, intercepted rainfall is not lost to the plant; it must afford relief to the drain on soil moisture.

Thus, to sum it up, we can fairly claim that evapotranspiration is the key process in the exchanges between earth and atmosphere. Any effective system of climatic classification in forest or grassland regions must necessarily seek to express this process and to use it as the central parameter. Even Köppen's crude aridity factor, like those of Lang, de Martonne, Szymkievicz, Emberger, and others, attempted to express the balance between rainfall and "evaporation", without being too clear about what evaporation is actually being considered. But the importance assigned to evaporation by these writers simply considered the mass exchange; water evaporated was water lost to the soil which would have to be replenished. Important also, however, is the role of evaporation in the energy balance of the surface, and it is this aspect that is emphasized here because of its novelty.

Precipitation is easily measured by means of rain gauges and has been recorded in most settled areas of the world. It is not easy to measure evapotranspiration however; in fact, no weather service in the world yet determines this important element and the little that we know about its distribution has been pieced together from various scattered determinations. So many difficulties exist, however, that it is still necessary to refer to other climatic data in order to determine the distribution of potential evapotranspiration.

The most reliable measurements of evaporation and transpiration which can be related to climatic elements in an effort to obtain a valid and practical empirical relationship are based on the monthly or seasonal data from irrigation and drainage projects and on daily observations from carefully operated evapotranspirometer tanks. Thorntwaite has found that when adjustments are made for variations in day length there is a close relation between mean temperature and potential evapotranspiration. Through study of the available data he produced a formula that permits the computation of potential evapotranspiration for any place whose latitude is known and for which temperature records are available. The formula is given and its use described elsewhere.¹⁰ Work is proceeding in several places toward the development of a new formula that is based on physical principles; in the meantime, the present empirical formula is being widely used in various water balance studies.

¹⁰THORNTHWAITE, C. W. An approach toward a rational classification of climate. The Geographical Review vol. 38, 1948, pp. 55-94.

FIGURE 1. - March of precipitation and evapotranspiration at four selected stations. (1)

FIGURE 1. - March of precipitation and evapotranspiration at four selected stations. (2)

FIGURE 1. - March of precipitation and evapotranspiration at four selected stations. (3)

FIGURE 1. - March of precipitation and evapotranspiration at four selected stations. (4)

The 1948 Thornthwaite classification

Thornthwaite's first world classification of climate was presented in 1931-1933.¹¹ It differed from earlier attempts by others in that it assigned the major role to the moisture factor. In 1948 he proposed an entirely new system based fundamentally on the ideas presented above^.12 He assigned the central place in this new classification to the potential evapotranspiration (PE), which he defined as the evapotranspiration that would occur from a vegetation-covered surface if soil moisture conditions were adequate for unrestricted transpiration. Work has been continued on this system since its first publication not only by the present authors but also by climatologists, biologists, and agriculturists throughout the world. Certain improvements have been worked out and adopted and others are in prospect.

¹¹THORNTHWAITE, C. W. The climates of North America according to a new classification. The Geographical Review, vol. 21, 1931, pp. 633-655.
THORNTHWAITE, C. W. The climates of the earth. The Geographical Review, vol. 23, 1933, pp. 433-440.

¹²THORNTHWAITE, C. W.. An approach toward a rational classification of climate. The Geographical Review, vol. 38, 1948, pp. 55-94.

When the potential evapotranspiration is compared with the precipitation and allowance is made for the storage of water in the ground and its subsequent use, periods of moisture deficiency and excess are clearly revealed and an understanding of the relative moistness or aridity of a climate is obtained. In some stations, precipitation is always more than the evapotranspiration so that the soil remains full of water and a water surplus s occurs. In other places, month after month, precipitation is less than potential evapotranspiration, there is not enough moisture for the vegetation to use and a moisture deficit d occurs. Stations with both wet and dry seasons, or with cold seasons of low water-need, normally show:

1. a period of full storage, when precipitation exceeds water-need and a moisture surplus, s, accumulates;

2. a drying season when stored soil moisture and precipitation are used in evapotranspiration, storage is steadily diminished, the actual evapotranspiration falls below the potential and a moisture deficiency, d, occurs;

3. a moistening season when precipitation again exceeds water-need and soil moisture is recharged.

The values of s and d can be computed during the budgeting.

It was originally assumed for convenience only that the root zone of the soil contained a maximum of 10 cm of water in storage when fully moistened and that this moisture would be used at the potential rate as long as any of it remained. Actually, it is known that the moisture holding capacity of the soil available for use by the roots may be much greater than 10 cm. and that, as soil moisture is utilized, the rate of evapotranspiration will diminish. Recent work¹³ suggests that at least 30 cm. depth of water will be available for use by deep-rooted mature plants in most normal soils and that the evapotranspiration rate which diminishes as the soil dries is proportional to the amount of water in the soil. When the soil moisture is reduced to 50 percent of capacity, the actual evapotranspiration rate will be only 50 percent of the potential rate. Although the two procedures give somewhat comparable values of actual evapotranspiration, water surplus, and deficit, as would be expected from a consideration of the assumptions themselves, the new procedure is preferable since it is more realistic than the older empirical one and depicts more exactly the processes going on in nature.

¹³THORNTHWAITE, C. W., and MATHER, J. R. The water budget and its use in irrigation. Manuscript prepared for the U. S. Dept. of Agric. Yearbook 1955.

Using the new procedure it is possible to work out a water balance sheet from climatological data alone showing at all times the soil moisture condition and providing values of moisture surplus and deficiency.. Figure 1 compares the precipitation with both potential and actual evapotranspiration at four selected stations while Table 3. gives the water balance computations for two of them. The various operations indicated in Table 3, are relatively straightforward. When the soil moisture is at field capacity, actual and potential evapotranspiration are the same, and all precipitation in excess of the potential evapotranspiration is realized as water surplus. When precipitation does not equal potential evapotranspiration, the difference is made up in part from soil moisture storage; but as the soil becomes drier the part not made up is larger. This is the water deficit, the amount by which actual and potential evapotranspiration differ. The soil moisture storage change cannot be determined directly but must be obtained from an appropriate table.

Finally a moisture index, Im, is derived from the relationship of s, d, and PE, all three quantities being given their annual values. This index is the basis for the division of the world into moisture provinces. The divisions suggested are as follows:

	Moisture Province	Moisture Index (Im)
A	Perhumid	100 and above
B4	Humid	80 to 99.9
B3	Humid	60 to 79 9
B2	Humid	40 to 59.9
B1	Humid	20 to 39.9
C2	Moist subhumid	0 to 19.9
C1	Dry subhumid	- 19.9 to 0
D	Semiarid	- 39.9 to - 20
E	Arid	- 60 to - 40

The moist and dry climates are thus separated by the moisture index of zero.

The second index used to define climatic provinces is the annual PE itself. Extensive research at the Laboratory of Climatology has shown that the growth of cultivated crops is highly correlated with the accumulation of PE. For this reason, the annual PE can be regarded as a sort of growth potential for the region.¹⁴ Whether an equally high correlation will ultimately be established with forest vegetation remains to be seen

¹⁴THORNTHWAITE, C. W. Climate in relation to planting and irrigation of vegetable crops, Proceedings of the 17th International Geographical Congress, Section on Climatology, August 8-15, 1952. Preprinted, pp. 46-51 Also, Publications in Climatology, The Johns Hopkins University Laboratory of Climatology, vol. 5, no. 5, 1952.

The thermal provinces defined by means of PE values are given below:

	Thermal Province	Annual PE
E	Frost	0 to 14.2 cm.
D	Tundra	14.3 to 28.5 cm.
C1	Microthermal	28.6 to 42.7 cm.
C2	Microthermal	42.8 to 57.0 cm.
B1	Mesothermal	57.1 to 71.2 cm.
B2	Mesothermal	71.3 to 85.5 cm.
B3	Mesothermal	85.6 to 99.7 cm
B4	Mesothermal	99.8 to 114.0 cm.
A	Megathermal	> 114.0 cm.

Thus a single parameter, annual potential evapotranspiration, because of its dependence on the energy balance, is made to serve in both the moisture and thermal indices. It is worthy of emphasis that the indices which define these "provinces" also specify at every point a thermal growth potential and the degree of moistness or aridity of the climate. In other words, the indices are continuously distributed about the earth, and do not exist merely along boundaries.

Correlation with forest distribution

The task of correlating the indices of the Thornthwaite classification with observed vegetation distribution and with soils over the earth has hardly begun. The graphical representation of the patterns of distribution of both vegetation and soil as related to climate presented in Figure 2 is adapted from Thornthwaite's 1931 classification. Eastern United States is made up almost entirely of humid climate and together with southeastern Canada, comprises one of the largest humid regions in the world. Within this region several distinct forest types have developed, in part, due to variations in the thermal index but largely because of variations in the moisture index. The humid climate exhibits a wide range in effective moisture between its subhumid and perhumid boundaries and it has accordingly been divided into four subregions. Within each of these subdivisions there is a northern and a southern zone. The forest types associated with these climatic regions are approximately as in Table 4.¹⁵

¹⁵THORNTHWAITE, C. W. Atlas of Climatic Types in the United States 1900-1939. U.S. Dept. Agri. Misc. Pub. 421, 1941, 7 pp. 96 plates.

TABLE 3. - WATER BUDGET FOR SELECTED STATIONS (in centimeters)

TABLE 4. - MOISTURE SUBDIVISIONS OF HUMID CLIMATE, MOISTURE INDICES, AND CHARACTERISTIC FOREST TYPES IN NORTHERN AND SOUTHERN ZONES, UNITED STATES

Climate	Index	Northern Zone	Southern Zone
B4	80- 99.9	Spruce - fir	(absent in eastern U.S.)
B3	60 - 79.9	Birch - beech - maple-hemlock	Oak - chestnut
B2	40- 58.9	Beech - maple	Oak - pine
B1	20 - 39 9	Oak - hickory	Oak - hickory

The moisture factor, and hence the moisture index, is of importance principally in temperate and tropical latitudes. In the economically valuable coniferous forests of the sub-arctic, thermal control appears to outweigh the significance of moisture. In a recently published series of articles Hare has shown that the broad physiognomic subdivisions of the Boreal Forest are highly correlated with the distribution of annual PE.¹⁶ He suggests these divisions:

¹⁶HAKE F. K. The Boreal conifer zone, Geographical Studies, vol. 1, no. 1, 1954, PP. 4-18.

FIGURE 2. Graphical representation of relation between climate, vegetation and soil.

Forest Sub-Zone	Annual PE	Vegetation
	(cm)
Tundra		Tundra
	31
Forest-tundra		Tundra on interfluves; woodland in valleys.
	35
Woodland		Predominatly Cladonia-rich open woodland, closed-crown forest in isolated groves
	42
Forest		Closed-crown forest occupying most of the mesic sites.
	52
Mixed temperate forests		Forest dominated by non-Boreal species, typically deciduous broadleaves.

The attempts at correlation of climate and vegetation that are presented in the foregoing paragraphs are highly tentative, and the authors will warmly welcome the collaboration of readers of this paper in establishing firmer and more useful relationships. Actually, since the development of climax vegetation formations and of mature soil types are most closely related to the march of soil moisture, we feel that the present

Thornthwaite classification represents a step in the right direction. It is also reasonably certain that no system of classification exists, that is more soundly based. In particular, in establishing climatic analogues for the guidance of programs of exotic plant introduction, the indices described above seem likely to be of considerable value.

As part of a long-time program which the Laboratory of Climatology has under way,¹⁷ climatic maps are being prepared which include precipitation, potential evapotranspiration, water surplus, water deficit, and the moisture regions for all parts of the world on a scale that is consistent with the density of the climatic network. Maps of several areas have been made on a scale of 1: 1,000,000. Maps of the entire continent of Africa have been completed on the new American Geographical Society base map in scale 1: 3,000,000. It is anticipated that the mapping of the entire earth will be completed within a short time.

¹⁷This program is supported in part by the Geography Branch, Office of Naval Research, United States Navy.

These maps may help meet the expressed needs of silviculturists in FAO which were quoted earlier in this paper, as well as such groups as the Advisory Committee on Arid Zone Research and the Humid Zone Research panel of UNESCO. It is probably quite important that these new maps be made available to all workers who have need for climatic information.

Since it is clear that whether grasses and other herbaceous plants occupy an area alone or share it with trees and shrubs depends on the soil moisture regime, it is necessary to go further than merely to obtain a useful and rational climatic classification. It is necessary to develop more fully the water balance approach, to determine the actual influence of soil moisture on climax vegetation formations and to make maps of these active controls of vegetation distribution. Thus, foresters should not hope for a complete solution to problems of plant distribution in a "standard" climatic classification but should seek information on the soil moisture regime, and understand the importance of periods of moisture deficiency and surplus in the formation of vegetation complexes. Maps of the distribution of these latter elements would be more important than conventional maps of climatic regions.^l8

¹⁸THORNTHWAITE, C. W. Grassland climates, Publications in Climatology, The Johns Hopkins University Laboratory of Climatology, vol. 5, no. 6, 1952.

Finally, the authors express the hope that the conceptual framework of forest ecology will approach that of the climatologist more closely in the future. It is clear that such a convergence already exists. Thornthwaite has pointed out that the detailed texture of local climates, based on variations of slope, soil characteristics, roughness, and albedo, requires what he calls a topoclimatological approach.¹⁹

¹⁹THORNTHWAITE, C. W. Topoclimatology, Proceedings of the Toronto Meteorological Conference, 9-15 September 1953, AMS and RMS, 1954, pp. 227-232.

This approach, in effect, an effort to bring climatology down to earth, has an obvious parallel in the "site" concept of the forest ecologist. Indeed, it might be possible to go so far as to say that the two ideas are opposite sides of the same coin. On the obverse, there are climatic influences of the moisture, energy and momentum exchanges, and on the reverse are the biotic and edaphic controls of relief, aspect, slope, soil texture and the composition of the vegetation. If the topoclimatological approach is pursued, there is little doubt that both groups, climatologists and forest ecologists alike, will get far closer together in their thinking than they are today.

SUN HARNESSED FOR COOKING

Sunshine may be the cheapest available "fuel" for millions of families in underdeveloped areas who cannot afford shiny new stoves or even wood.

A solar energy scientist at New York University has conceived a practical, economical stove that cooks by sunlight. The Ford Foundation is backing the project with a grant to the Research Division of New York University's College of Engineering for detailed research and development of a sun stove.

The project as reported to FAO ranges beyond engineering aspects. Concurrent with the scientific development of the cooker, therefore the social, psychological and economic effects of introducing and integrating such a new device into the customs of the peoples for whom it is intended will need to be determined.

Important also, and another area of the study, is an industrial and materials survey of these countries. What materials are available for incorporation into the sun stove? What. Skills are available for its manufacture? What is the country's industry potential for its mass production?

Cost is a crucial factor in introducing solar cooking to underdeveloped areas. The most successful solar cooker developed to date involves a costly parabolic reflector.

The new stove design eliminates the need for parabolic reflectors.. Furthermore, it has the advantage of retaining cooking heat for an hour or so after the sun has gone down, the time when the evening meal is cooked and its heat-storing capacity largely eliminates the necessity of changing the position of the stove frequently to catch direct sunshine.

The stove is a closely insulated box, roughly triangular in shape. Four ordinary flat mirrors fan out from the tilted face of the stove. At the rear of the stove is a removable drawer in which the food is placed.

The mirrors reflect sunlight down through the tilted face of the stove, concentrating it in the interior, which is filled with special heat-absorbing chemicals. The principle of the stove is "heat of fusion" , or "heat of transformation." All materials when melting require large amounts of heat to change from solid to liquid forms. During melting, the temperature of the material does not change but remains at melting point. The problem in the solar stove and other heat storage devices is to devise materials with relatively high heats of fusion. In the sun stove, therefore, the sun's heat produces a succession of changes in the heat-storage salts from solid to liquid state. The changes give off the heat used for cooking.

Preliminary models of this stove developed temperatures up to 300° F(148° C) on days when the outdoor temperature was under 70° F (21° C). From 250° to 300° F (120°-148° C) is considered ample for average cooking operations. Higher temperatures needed for frying and browning have been developed in the stove on clear days. The areas for which the stove is intended enjoy direct sunlight most of the year.

The New York University research group believes the* stove can be developed so that it can be manufactured to sell for $ 5. The least expensive solar cooker devised to date sells for about $ 16. Besides cost and conformity to local cooking customs the stove will be designed to meet these requirements: durability, ease of operation and cleaning; simplicity and portability; ability to operate in early evening, and little attention required during cooking.