CHAPTER IV
CLIMATE AND WEATHER
To the superficial observer those daily variations in the atmospheric conditions in any one locality which we sum up under the term weather, may appear to occur without order or regularity, but detailed quantitive study soon shows that even British weather displays constancy in its irregularity. The existence of such basal constancy, indeed, lies at the root of all intelligent utilisation of the soil. The irresponsible amateur gardener may lightheartedly assume that a particular spring will be “early,” but the professional is not easily induced to abandon his rule that such and such operations must not be undertaken before certain fixed dates. The farmer, if he is to avoid bankruptcy, must know within what limits the first autumn frost is likely to make its appearance, and when the last spring one may be expected.
Collective experience, then, whether expressed in the meteorologist’s figures or in a less accurate form, leads us to the conclusion that for every locality on the earth’s surface there is a certain fixed average succession of weather, which we sum up in the term climate.
In the case of both climate and weather our knowledge may be summed up in such general terms as “wet” or “dry,” “warm” or “cold,” and so forth, or we may borrow the meteorologist’s notations, and express the facts in degrees of temperature, inches of rainfall and of pressure, percentages of humidity, and so on. But it should be understood that such figures can be used by the geographer with justification only when he is himself aware, and can assume that his audience is aware, of the significance of the figures in connection with the processes of erosion and the phenomena of life. To say that the mean January temperature of a particular place is 30° F., is only a convenient shorthand way of saying that in this place in winter plant life is arrested, water is ice-bound, and most animals sleep or migrate. In other words, the use of the figures assumes a certain knowledge of biology and of physics on the part of the audience.
We do not propose here to treat either climate or weather with any fullness, for there is a volume in the series specially devoted to these and kindred subjects. All that will be attempted, therefore, is to discuss one or two important climates with the object of considering later their respective effects on the distribution of other phenomena on the surface of the globe. This is the more worth doing in that the subject is one which has had a great deal of attention devoted to it in recent years.
Certain points in regard to climate, e. g. the fact that the regions of the earth near the equator get more solar heat than those nearer the poles, and that parts of the globe are subjected to variable winds, as contrasted with those regions where the extraordinarily regular winds called “trades” blow, have of course been known for long enough. But not till the latter half of the nineteenth century did the civilised nations begin regular meteorological observations, and these observations are still scanty for the uncivilised and partially civilised regions. The meteorological raw material necessary for the exact study of climates has thus only been available for a comparatively short period, and is still incomplete.
We may begin with that type of climate which has so profoundly influenced the civilisation of western Europe, and therefore also the new civilisations of America, Australia, South Africa, and so on. This is the type called Mediterranean, because it reaches its best development and has been most studied round the Mediterranean area. But it also occurs in California, in parts of Chile, in South Africa round Cape Colony, and in south and south-western Australia. Generally, it is characteristic of lands lying on the western side of continents, in the latitudes between tropical and temperate, and is therefore sometimes called the maritime sub-tropical climate. The term maritime is applied because, as we shall see, for some part of the year oceanic influences prevail, sub-tropical indicates the position in latitude.
A very curious illustration of the similarity of climate in the different regions named is to be found in the fact that in parts of the Mediterranean area two introduced American plants, the agave and the prickly pear, are more obvious and abundant than most native plants; while in California, Cape Colony and southern Australia the cultivated plants are chiefly of Mediterranean origin.