Meaning and Scope of Climatology—Relation of Meteorology and Climatology—Literature of Climatology—The Climatic Elements and their Treatment—Solar Climate—Physical Climate.

_Meaning and Scope of Climatology._ The word _klima_ (from κλίνειν, to incline), as used by the Greeks, originally referred to the supposed slope of the earth toward the pole, or to the inclination of the earth’s axis or of the sun’s rays. It may, perhaps, have had reference to the different exposures of mountain slopes. Later, probably after Aristotle’s time, it came to be used as about equivalent to our _zone_, but at first it was simply a mathematical or an astronomical term, not associated with any idea of physical climate. A change of latitude in those days meant a change of _climate_. Such a change was gradually seen to mean a change of atmospheric conditions as well as a change in length of day. Thus _klima_ came to have its present meaning.

An excellent illustration of the ancient meaning of the word _klima_ is found in the system of climates proposed by the famous geographer, Ptolemy. This was a division of the earth’s surface between equator and north pole into a series of climates, or parallel zones, separated by latitude circles and differing from one another simply in the length of their longest day. Ptolemy’s subdivision of the earth’s surface was really nothing but an astronomical climatic table.

_Climate_, as we use the term, is the resultant of the average atmospheric conditions, or, more simply, it is the average condition of the atmosphere. _Weather_ is a single occurrence, or event, in the series of conditions which make up the _climate_. The _climate_ of a place is in a sense its average weather. The average values of these atmospheric conditions can be determined only by means of careful observations, continued for a period sufficiently long to give accurate results. Climatology is the study or science of climates.

_Relation of Meteorology and Climatology._ Meteorology and climatology are interdependent. It is impossible to distinguish very sharply between them. Each needs the results obtained by the other. In a strict sense, meteorology deals with the physics of the atmosphere. It considers the various atmospheric phenomena individually, and seeks to determine their physical causes and relations. Its view is largely theoretical. The aspect of meteorology which is of most immediate practical importance to man is that which concerns weather-forecasting.

When the term meteorology is used in its broadest meaning, climatology is a subdivision of meteorology. Climatology is largely descriptive. It aims to give as clear a picture as possible of the interaction of the various atmospheric phenomena at any place on the earth’s surface. It rests upon physics and geography, the latter being a very prominent factor. Climatology may almost be defined as geographical meteorology. Its main object is to be of practical service to man. Its method of treatment lays most emphasis on the elements which are of the most importance to life. Climate and crops, climate and industry, climate and health, are subjects of vital interest to man. No other science concerns man more closely in his daily life.

_Literature of Climatology._ Scientific climatology is based upon numerical results obtained by systematic, long-continued, and accurate meteorological observations. The essential part of its literature is therefore found in the collections of data published by the various meteorological services and observatories. In addition, large numbers of short sketches and notes on climate, partly the more or less haphazard accounts of travellers, partly the more careful studies of scientific observers, are scattered through a wide range of geographical and other publications. The only comprehensive text-book of climatology is the _Handbuch der Klimatologie_ of Professor Julius Hann, of the University of Vienna. This is the standard book on the subject, and upon it is based much of the present volume, and of other recent discussions of climate. The second edition of this work, in three volumes, was published in 1897 (Stuttgart, Engelhorn). The first volume deals with general climatology, and has been translated into English.[1] The second and third volumes are devoted to the climates of the different countries of the world. Woeikof’s _Die Klimate der Erde_ (Jena, Costenoble, 1887) is also a valuable reference book. The first part concerns general relations of climate, particularly to rivers and lakes, to vegetation, and to snow-cover, while the second part deals with the climates of special areas. The standard meteorological journal of the world, the _Meteorologische Zeitschrift_ (Braunschweig, Vieweg, monthly), is indispensable to anyone who wishes to keep in touch with the latest publications on climatology, for it contains the most complete record of such literature, as well as a large number of original notes and discussions. The newest and most complete collection of charts is that in the _Atlas of Meteorology_ (London, Constable, 1899), in which also there is an excellent bibliography. For the titles of more recent publications reference may be made to the _International Catalogue of Scientific Literature_ (annual volume on _Meteorology_); or to the more frequent bibliographical lists in the _Meteorologische Zeitschrift_; the _Monthly Weather Review_ (Washington, U. S. Weather Bureau); the _Quarterly Journal of the Royal Meteorological Society_ (London), and the _Halbmonatliches Litteraturverzeichniss der “Fortschritte der Physik”_ (Braunschweig, Vieweg, twice a month).

[1] By R. De C. Ward. London and New York, Macmillan, 1903.

_The Climatic Elements and their Treatment._ Climatology has to deal with the same groups of atmospheric conditions as those with which meteorology is concerned, viz.: temperature (including radiation); moisture (including humidity, precipitation, and cloudiness); wind (including storms); pressure; evaporation, and also, but of less importance, the composition and the chemical, optical, and electrical phenomena of the atmosphere. The characteristics of each of these so-called _climatic elements_ are set forth in a standard series of numerical values, based on careful, systematic, and long-continued meteorological records, corrected and compared by well-known methods. Various forms of graphic presentation, by curves, or by wind roses, etc., are employed to emphasise and simplify the numerical results. Instructions concerning the use, exposure, hours of observation, and corrections of the ordinary meteorological instruments; as well as for obtaining the usual numerical results, are published by the various governmental meteorological services. In Hann’s _Handbook of Climatology_, Vol. I, will be found a general discussion of the methods of presenting the different climatic elements, and of the reasons for adopting the accepted scheme of presentation. The most complete guide in the numerical, mathematical, and graphic treatment of meteorological data for climatological purposes is Hugo Meyer’s _Anleitung zur Bearbeitung meteorologischer Beobachtungen für die Klimatologie_ (Berlin, Springer, 1891).

Climate deals first of all with _average_ conditions, as is apparent from the definition given above. But _means_ may be made up of very different values of the elements which go into them, and therefore a satisfactory presentation of a climate must include more than mere averages. It must take account, also, of regular and irregular daily, monthly, and annual changes, and of the departures, mean and extreme, from the average conditions which may occur at the same place in the course of time. The mean minimum and the mean maximum temperature or rainfall of a month, or a season, are important data, not in any way replaced by a knowledge of the mean monthly or seasonal temperature and rainfall. Further, a determination of the frequency of occurrence of a given condition, or of certain values of that condition, is important, for periods of a day, month, or year, as, for example, the frequency of winds according to direction or velocity; or of different amounts of cloudiness; or of temperature changes of 5, or 10, or more degrees; the number of days with and without rain or snow in any month, or year, or with rain of a certain amount, etc. The probability of occurrence of any condition, as of rain in a certain month; or of a temperature of 32°, for example, is also a useful thing to know concerning a climate. In the past, climatology has been too much concerned with monthly, seasonal, and annual averages. An important addition to the usual climatic summaries would be the introduction, for all regions in which the cyclonic or storm control of weather conditions is characteristic, of the cyclonic unit, so that, for example, the average duration and value of cyclonic ranges of temperature in the several months, or the proportion of the annual rain and snowfall received from cyclonic storms and from local thunderstorms, might be determined.[2]

[2] See R. DeC. Ward: _Suggestions Concerning a More Rational Treatment of Climatology_. Report Eighth International Geographic Congress, Washington, D. C., 1904, pp. 277-293.

_Solar Climate._ The sun is clearly the principal control of climates on the earth’s surface. The general distribution of temperature, as well as the seasonal and diurnal changes, all depend upon changes in the intensity of sunshine. Hence a brief consideration of the distribution of insolation over the earth’s surface is essential to a proper understanding of climates. Climate, in so far as it is controlled solely by the amount of solar radiation which any place receives by reason of its latitude, is called _solar climate_. Clearly, all places on the same latitude circle would have the same solar climate, for the intensity and amount of insolation depend upon the angle of incidence of the sun’s rays, and upon the length of day, and both of these depend upon latitude. Solar climate alone would prevail if the earth had a homogeneous land surface, and if there were no atmosphere. For under these conditions, and without air or ocean currents, the distribution of temperature at any place would depend solely on the amount of energy received from the sun, and upon the loss of heat by radiation. And these two factors would have the same value at all points on the same latitude circle.

The relative amounts of insolation received at different latitudes and at different times have been carefully determined. The values all refer to conditions at the upper limit of the earth’s atmosphere, _i. e._, without the effect of absorption by the atmosphere. The accompanying diagram (see Fig. 1) shows very clearly the distribution of insolation in both hemispheres at different latitudes and at different times in the year. The latitudes are given at the left margin and the time of year at the right margin. The values of insolation are shown by the vertical distance above the plane of the two margins.

At the equator, where the day is always twelve hours long, there are two maxima of insolation at the equinoxes, when the sun is vertical at noon, and two minima at the solstices, when the sun is farthest off the equator. The annual curves show that the values do not vary much through the year, because the sun is never very far from the zenith, and day and night are always equal. There is a slight difference in the insolation at the two maxima, owing to a difference in the sun’s distance, the earth’s orbit being an ellipse and not a circle. The earth is nearer the sun in the winter of the northern hemisphere, and therefore the spring maximum is somewhat greater than the autumn maximum. The varying distance from the sun also explains the fact that the maxima of insolation do not come exactly on the dates of the equinoxes.

LATITUDES]

These conditions are clearly brought out in curve 1 of Fig. 2, which shows the annual march of insolation on the equator. The law of the distribution of insolation would be simple if the sun were always on the equator, for the angle of insolation and the length of day and night would then always remain the same. But under existing conditions, both the angle of insolation and the length of day are constantly changing, and the interaction between these two controls becomes very complex. As the latitude increases, the angle of insolation becomes more oblique, and the intensity of insolation decreases, but at the same time the length of day rapidly increases during the summer, and towards the pole of the hemisphere which is having its summer the gain in insolation from the latter cause more than compensates for the loss by the former. The double period of insolation, above noted for the equator, prevails as far as about lat. 12° N. and S.; at lat. 15° the two maxima have united, and the same is true of the minima. Take the case of an intermediate latitude, like 45° N. (see curve 2, Fig. 2). Here there is one minimum, in December, when the sun is south of the equator, and one maximum, in June, when the sun is north. The slight displacement of this maximum and minimum from the exact date of the two solstices is due to the difference in the sun’s distance. At the north pole (curve 3, Fig. 2), there is one maximum at the summer solstice, and no insolation at all while the sun is below the horizon. The distribution of insolation at different latitudes on the same day is also interesting. On June 21, for example (see Fig. 1), the equator has a day twelve hours long, but the sun’s maximum altitude is only 661⁄2°, _i. e._, it does not reach the zenith, and the amount of insolation is less than at the equinox. On the northern tropic, however, the sun is vertical at noon, and the day is between thirteen and fourteen hours long. Hence the amount of insolation received at this latitude on June 21 is greater than that received on the equinox at the equator. As one passes from the tropic to the pole the sun stands lower and lower at noon, and the value of insolation would steadily decrease with latitude if it were not for the increase in the length of day. Going polewards from the northern tropic on June 21, the value of insolation increases for a time, because, although the sun is lower, the number of hours during which it shines is greater. A maximum value is reached at about lat. 431⁄2° N. The decreasing altitude of the sun then more than compensates for the increasing length of day, and the value of insolation diminishes, a minimum being reached at about lat. 62°. Then the rapidly increasing length of day towards the pole (the day being twenty-four hours long beyond the Arctic circle) again brings about an increase in the value of insolation, until a maximum is reached at the pole which is greater than the value received at the equator at any time. (See Fig. 2, in which the curves are all drawn on the same scale). The length of day is the same on the Arctic circle as at the pole itself, but while the altitude of the sun varies during the day on the former, being at the horizon at midnight and highest at noon, the altitude at the pole remains 231⁄2° throughout the twenty-four hours. The result is to give the pole a maximum (See Fig. 3, curve marked 1.00.). On June 21, there are therefore two maxima of insolation, one at lat. 431⁄2° and one at the north pole. From lat. 431⁄2° N., insolation decreases to zero on the Antarctic circle, for sunshine falls more and more obliquely, and the day becomes shorter and shorter. Beyond lat. 661⁄2° S. the night lasts twenty-four hours. On December 21 (see Fig. 1), the conditions in southern latitudes are similar to those in the northern hemisphere on June 21, but the southern latitudes have higher values of insolation because the earth is then nearer the sun.

At the equinox, the days are equal everywhere, but the noon sun is lower and lower with increasing latitude in both hemispheres until the rays are tangent to the earth’s surface at the poles (except for the effect of refraction). Therefore, the values of insolation diminish from a maximum at the equator to a minimum at both poles. From the fact that the southern hemisphere has its summer in perihelion and its winter in aphelion, it follows that there is a greater difference between the seasonal values of insolation south of the equator than north of it. In other words, the solar climate of the southern hemisphere is more severe than that of the northern. Nevertheless, owing to the fact that the earth moves more rapidly around its orbit when nearest the sun, both hemispheres receive equal amounts of insolation at the same latitudes, and in the mean of the year, both have the same amount of insolation.

JUNE 21]

The values of insolation thus far considered have reference to the upper limit of the earth’s atmosphere, or to the earth’s surface assuming that no atmosphere exists. The effect of the atmosphere is to weaken the sun’s rays. The more nearly vertical the sun, the less the thickness of atmosphere traversed by the rays. The values of insolation at the earth’s surface, after passage through the atmosphere, have been calculated. They vary much with the condition of the air, as to dust, clouds, water vapour, etc. In Fig. 2, the broken lines, 4, 5, and 6, show the values of insolation at the equator, lat. 45° N., and the north pole, allowing for a loss of 25% during the passage through the atmosphere, _i. e._, with a coefficient of transmission 0.75. This is higher than that usually observed, even under very favourable conditions, with the sun in the zenith. As a rule, even when the sky is clear, about one-half of the solar radiation is lost during the day by atmospheric absorption. The great weakening of insolation at the pole, where the sun is very low, is especially noticeable. The effect of the atmosphere is also shown in Fig. 3. The upper curve represents the total quantity of insolation received at the earth’s surface with a coefficient of transmission of 1.00 (_i. e._, no loss). Under such conditions, as already noted, there are two maxima on June 21, at lat. 431⁄2° N. and at the north pole. The second curve corresponds to a coefficient of transmission of 0.75, which is also used in the broken curves of Fig. 2. Under these conditions, there is but one maximum, at about lat. 36° N., and the north pole has only 49% of the total radiation emitted by the sun. The third curve is based on a coefficient of transmission of 0.50, and shows one maximum at lat. 32° N., the pole receiving only 18% of the total amount which reaches the upper limit of the atmosphere at that point. The curves 0.75 and 0.50 show that, taking the atmosphere into account, even in midsummer the amount of insolation decreases from between lats. 30° and 40° to the pole. The following table (after Angot) shows the effect of the earth’s atmosphere (coefficient of transmission 0.7) upon the value of insolation received at sea-level.

VALUES OF DAILY INSOLATION AT THE UPPER LIMIT OF THE EARTH’S ATMOSPHERE AND AT SEA-LEVEL.

---------------+--------------------------+------------------------- | Upper limit of atmosphere| Earth’s surface Lat. +---------+------+---------+---------+------+-------- | Equator | 40° | N. Pole | Equator | 40° | N. Pole ---------------+---------+------+---------+---------+------+-------- Winter Solstice| 948 | 360 | 0 | 552 | 124 | 0 Equinoxes | 1000 | 773 | 0 | 612 | 411 | 0 Summer Solstice| 888 | 1115 | 1210 | 517 | 660 | 494 ---------------+---------+------+---------+---------+------+--------

The following table gives, according to Zenker, the relative thickness of the atmosphere at different altitudes of the sun, and also the amount of transmitted insolation.

RELATIVE DISTANCES TRAVERSED BY SOLAR RAYS THROUGH THE ATMOSPHERE, AND INTENSITIES OF RADIATION PER UNIT AREAS

Altitude of Sun. -----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- 0° | 5° | 10° | 20° | 30° | 40° | 50° | 60° | 70° | 80° | 90°

Relative Lengths of Path through the Atmosphere. -----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- 44.7 |10.8 | 5.7 | 2.92| 2.00| 1.56| 1.31| 1.15| 1.06| 1.02| 1.00

Intensity of Radiation on a Surface Normal to the Rays. -----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- 0.0 | 0.15| 0.31| 0.51| 0.62| 0.68| 0.72| 0.75| 0.76| 0.77| 0.78

Intensity of Radiation on a Horizontal Surface. -----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- 0.0 | 0.01| 0.05| 0.17| 0.31| 0.44| 0.55| 0.65| 0.72| 0.76| 0.78

_Physical Climate._ It is clear that the distribution of insolation, just considered, explains many of the large facts of the distribution of temperature—for example, the decrease of temperature from equator to poles; the double maximum of temperature on and near the equator; the increasing seasonal contrasts with increasing latitude, etc. But it is equally apparent that the distribution of temperature often does not follow the distribution of insolation closely, for, if it did so, the two poles would be warm at the times of their respective maxima of insolation. The high values of _insolation_ at the poles do not correspond to high temperatures, as will be seen in a later chapter (VI). The old view which thus explained an “open polar sea” was erroneous. The distribution of insolation suggests a subdivision of the earth’s surface into three distinct belts. In one, within about 12° of the equator, there are two maxima and two minima. In a second, there is one maximum; and for part of the year the absence of the sun reduces the amount to zero. In a third, the conditions are intermediate; there is one maximum and one minimum, but there is no time when the value of insolation decreases to zero. Of the second and third of these belts, there are two divisions, one in the northern and one in the southern hemisphere. It will be noted that the tropics, the polar, and the temperate zones roughly correspond to these insolation belts.

The regular distribution of solar climate between equator and poles which would exist on a homogeneous earth, whereby similar conditions prevail along each latitude circle, is very much modified by the unequal distribution of land and water; by differences of altitude; by air and ocean currents; by varying conditions of cloudiness, and so on. Hence the climates met with along the same latitude circle are no longer all alike. Solar climate is greatly modified by atmospheric conditions and by the surface features of the earth, and what is known as _physical climate_ is the result. The uniform latitudinal arrangement of solar climatic belts is interfered with. Physical climate results from the reaction of the earth’s surface features upon the atmosphere. According to the dominant control, in each case, we have solar, continental, marine, and mountain climates. In the first named, latitude is the essential; in the second and third, the effect of land or water; in the fourth, the effect of altitude.