For temperatures a few figures may suffice. In London the mean January temperature is 39° F., while it is only 36° F. at Paris. In Nice the mean January temperature is 45°, which is about the same as that of Athens, and rather less than that of Naples. In January, then, the temperature of Nice is only 6° higher than that of London. In July the mean temperature at London is 62°, as against 73° at Nice and over 80° at Athens. In other words, owing to our mild winters and cool summers, there is far more difference between British and Mediterranean temperatures in summer than in winter. In the Mediterranean region itself the difference between the temperatures of summer and winter increases as we pass eastwards, so that it is especially to the west that characteristically Mediterranean conditions occur, i. e. mild, frost-free winters, and summers which for the latitude are not excessively hot. This feature also has been of importance in the development of the Mediterranean civilisations.
We have treated the climate of the Mediterranean area in some detail, as an example of the methods and results of modern climatology. We may note much more briefly the characteristics of one or two other climatic provinces.
Mediterranean influences, expressed in winter rains, are continued eastward into Mesopotamia and even into Persia, the rain always becoming scantier, and desert conditions tending to supervene. Still further east, however, we come to a region where the rainfall is abundant, and where the population is once more dense. These are the monsoon countries, including India and China, where the usually plentiful rainfall again permits the land to nourish man abundantly.
Excluding Africa south of the Sahara from consideration, we may indeed say that the Old World has two regions of abundant rainfall and dense population, the one to the west and the other to the south-east, separated from each other by warm and cold deserts. Each of these two regions has given rise to its own civilisation, each has produced its own types of cultivated plants and domestic animals, and the root differences between the two must be regarded as largely the result of climatic conditions.
The monsoon countries are so named because of the regular seasonal reversal of the winds, which blow from land to sea in winter and from sea to land in summer, affording an example of a land and sea breeze on the gigantic scale. The result is that, subject to local modifications, the summer winds are moisture-bringing, and the winter winds are dry. Whereas, then, in the Mediterranean the heat of summer is largely wasted, from the agriculturist’s point of view, on account of the scarcity of the water necessary for growth, in monsoon regions, unless the rain fail, as it sometimes does, the hot season is the moist season, and, therefore, other things being equal, growth must be faster here than in the Mediterranean area. The monsoon countries extend over a great stretch of latitude, and therefore temperature conditions vary greatly, while the great variety of surface-relief produces here abnormally heavy rainfall, and there desert conditions. The essential contrast with the Mediterranean type is, however, the summer rainfall.
Taking the globe as a whole we find that summer rainfall is more common than winter, and in addition to occurring in monsoon regions, it tends to occur in tropical regions generally. As we approach the equator from the tropics we find that the total fall increases, and tends to show two maxima, which occur when the sun is overhead, i. e. at the equinoxes. For our particular purpose, however, the climatic conditions in tropical and equatorial regions generally, though of great importance to the climatologist, are not of great interest, for except in monsoon countries the hot parts of the earth do not show the most highly developed human societies.
Let us turn next to that part of Europe which is outside the reach of Mediterranean influences. Here we find that the rain is distributed throughout the year, and is usually abundant, though it decreases in passing eastwards from the seaboard. Temperatures are naturally lower than in the Mediterranean basin, and winter frost plays an important part in determining the choice of cultivated plants. As the figures which we have already quoted for London and Paris suggest, the winter cold increases on passing eastward. Paris is colder in winter than London, though it lies south of it. Vienna is again colder than Paris. But the increase in winter cold is compensated for by an increase in the summer heat. In other words, as the distance from the sea increases in Europe the climate becomes drier and more extreme.
This observation naturally leads up to a consideration of the effect of the proximity of the sea upon climate. Water heats more slowly than land, but also cools more slowly, and therefore the proximity of large masses of water has, speaking generally, a moderating influence upon climate, producing the so-called maritime climate. In the case of the British Isles this effect is very marked, because the ocean to the west of us is unusually warm, and the circulation of the atmosphere is such that the prevailing winds of winter blow towards us from the warmer parts of this ocean, while the fact that the summer winds often have a northerly component helps to keep the summer temperatures down.
The peculiar conditions of the British Islands illustrate the fact that climate does not depend upon latitude alone, but may be greatly modified by local conditions, especially by the distribution of land and water, and the direction of the wind.
Let us now sum up what has been said in regard to the main types of climate found in Europe. Round the Mediterranean basin we have an area with mild winters and warm summers, where the rain tends to fall during the winter season, making summer a period of drought. This climate extends beyond the limits of Europe into Northern Africa and Western Asia, and is separated from the regions of tropical climate, which have no winter and have rains at the hottest season, by a belt of desert.