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.

The main features of the Mediterranean climate may be briefly summarised. The most important character, next to the mild temperature, is the fact that no rain (or very little) falls in summer, the growing season further north, which is here largely a period of cessation of plant growth. The rain, which tends to be scanty or even absent in the interior of land masses, e. g. in Spain and Asia Minor, and also to the south, e. g. in the Desert of Sahara, in the Mediterranean region proper falls in the winter months. It is this winter rainfall and the summer drought which define the Mediterranean type of climate.

The reason for this seasonal distribution of rainfall is as interesting as the fact itself, and to understand it we must turn to the circulation of air on the surface of the globe.

In the following description we shall restrict ourselves, for the sake of clearness, to the Mediterranean region itself, the region where the Mediterranean type of climate is developed over the largest area, and where, for many reasons, it is most important. But it must be noted that the conditions which give rise to the Mediterranean type of climate are the same wherever it occurs, though in the Mediterranean area they are greatly modified by the great inland sea of that name, which carries oceanic conditions far into the land.

We must note, first, that at all seasons those regions of the earth which are directly beneath the vertical rays of the sun are heated most intensely. Therefore the air over these regions, being rendered light by heating, rises, and a belt of low pressure is thus formed. Only at the equinoxes does this belt of high temperature, low pressure, and light winds or calms, coincide with the equator. In the northern summer it moves north with the sun; in the northern winter it travels south with the sun, being always over what is called the heat equator. Into this belt of low pressure air from north and south, where the pressure is greater, tends to rush in, and we have thus formed the constant or “trade” winds, which, owing to the deflection produced by the earth’s rotation, appear as the north-east trades in the northern hemisphere and the south-east in the south. These winds are dry winds, because they blow from colder to warmer latitudes, and they accompany the equatorial low-pressure belt in its north and south movements.

In the northern summer the trade winds may extend northward to lat. 35° or even 40°, while in winter their northern limit is 10° to 15° further south. A glance at the map, then, will show that in summer the Mediterranean area is within or near the sphere of action of the dry trade winds, which are continental, sweeping into the region after having blown over land surfaces.

We must next consider the atmospheric movements in the region to the north of the trade wind belt. An area of more or less permanent low pressure, best marked in winter, exists in the North Atlantic, in about 60° N. lat., and draws the air into it in the direction known as counterclockwise, that is, in the direction opposite to that of the hands of the clock. The result is the production of the winds which appear off the coast of western Europe as the warm south-westerly winds of winter, while they appear off the coast of North America as cold northerly winds. In the southern hemisphere, where, as we have seen, there is less land to interfere with the development of the atmospheric circulation, these winds form the prevailing westerlies.

In the Atlantic these south-westerly winds obviously blow in a direction opposite to the north-east trades, whence the name of anti-trades often given to them. As they blow across the broad Atlantic they arrive off Europe saturated with moisture. As they come from lower latitudes they are warmth bringing. In winter these winds reach the Mediterranean area owing to the southern shift of the trades, and bring moisture with them; while in summer they lie more to the north, and though their moisture affects the coast of Portugal it does not reach the greater part of the Mediterranean area.

Within that area the northern limit of the rainless summer may be said, in a rough sense, to correspond with about the 40th parallel of latitude, though it varies according to local conditions in the different peninsulas. To the north of this line, therefore, the climate is more or less affected even in summer by the anti-trades.

It must not be supposed that the region of the trade winds and of the anti-trades lie side by side. Between the two there is a zone of variable winds, but in general terms we can explain the peculiarities of the Mediterranean rainfall by saying that the region lies within or just at the edge of the dry trades in summer, and within the zone of the moist anti-trades in winter.

Let us next consider how the area is demarcated from the surrounding regions. There is of course no hard and fast line, but we can indicate in broad outline the meteorological limits. To take the absolutely rainless summer as the limit would cut out, as we have suggested above, the greater part of the northern shore of the Mediterranean, except the southern halves of all the great peninsulas. Quite generally, however, we may say that the northern limit of the Mediterranean region, in its western half, is defined by the occurrence of considerable summer rain. That is, it is bounded to the north by a region which is within reach of the rain-bringing anti-trades in summer as well as in winter, and which has a lower temperature than the Mediterranean region proper. To the east the region is limited by deserts, for the westerlies of winter can only carry their moisture a certain distance inwards, and though they are greatly assisted by the long, eastward-stretching, inland sea, yet there comes a time when all their load of moisture is lost, and desert conditions supervene.

To the south the desert again forms the boundary, though here for a different cause. North Africa behind the Atlas is permanently within the trade-wind belt, that is, it is permanently subjected to the action of drying winds, and its rainfall is therefore small or nil. Similarly in California the southern limit of the Mediterranean zone of climate is the desert region of Arizona, Mexico, and the north of Lower California. A similar band of desert separates the Mediterranean zone from the tropical region of summer rain in the other places where the Mediterranean type occurs.

This may be summed up as follows:—Defining the Mediterranean climate only by its rainfall, we may say that it prevails over lands both to the north and south of that sea, and these have all or most of their rainfall in winter, when the winds, though typically westerly, are often stormy and rendered variable by local conditions. In the summer there may be no rain at all, or, to the north, small amounts. To the north the region passes gradually into that colder zone where rain occurs abundantly both in summer and winter, while to the east and south the rainfall diminishes greatly, and there is a gradual transition to desert conditions. To the west the boundary of the region is theoretically the ocean, but the western coastline owes to its peculiar position a more abundant precipitation, which makes the vegetation of, e. g., Portugal present quite a different appearance from that of southern Italy or Algiers. These peculiarities of rainfall the region owes to its position between two great wind systems, of which one gains the mastery in winter and the other in summer.

So far in this discussion we have spoken only of the distribution of the rainfall throughout the year, but there are other features of the Mediterranean climate which are almost as important in considering the effects of the climate on the life of the region. These are the amount of the rainfall, and the temperature.

Beginning with general points, it is very important to notice that the rainfall throughout the area as a whole is relatively scanty, except where special conditions, e. g. great elevation, or local rain-bringing winds, increase it. Translated into terms of plant life this means that continuous forests of the type so characteristic of the greater part of Europe till man interfered, are relatively rare within the limits of the Mediterranean region. Looking at the same fact from the human standpoint we may say that the rainfall is often so scanty that irrigation is necessary before man can prosper. These two facts, that Mediterranean man had not to clear forests before he planted and sowed, as the Teutons were obliged to do, and that he had often to bring water artificially before his crops would grow, have been of supreme importance in the evolution of Mediterranean civilisation. Even at this stage it is interesting to note that France in this, as in many other respects, has shared in two civilisations, for her territory to the south shows Mediterranean characters, and elsewhere resembles the cool temperate zone of Europe.

The next general point of importance is that of temperature. As was to be expected from its latitude the basin of the Mediterranean is a relatively warm region. Local conditions, and especially the presence of a great mass of water, make the winter exceptionally mild, while the summers, though not excessively hot they are considerably cooler than those of similar latitudes in Asia, are yet warm and sunny. The result is that, given water artificially supplied, or given crops which can take water from the deeper layers of the soil, the region is productive, the destructive frost of the north not being a menace. This relative easiness of life in the more favoured parts of the region has been of great importance in its history.

We may give next some actual figures to illustrate what has been said about temperature and rainfall. Let us begin with rainfall, and in order to have a basis of comparison we may first note that Edinburgh has a mean annual rainfall of about 28 inches, and London one of about 25 inches. In other words, when the total amount of rain which falls in any one year is estimated for many years in either of these places, these totals added together and divided by the number of years of observation, the quotient is the figure given. The figures show that the rainfall in London is less than that in Edinburgh, while in Paris it is less than in either.

Passing now to consider the Mediterranean area we find that, speaking generally, the rainfall diminishes, for the reasons already explained, in passing from west to east, and in passing from north to south. Thus Gibraltar, at one end of the basin has a fall of 32" per annum, as compared with one of 15" at Athens near the other extremity. Genoa in the north has the heavy fall of 51", while Biskra in Algiers has only 8".

There are many local variations, due to local causes, and in comparing the falls with those of Edinburgh and London we must remember that the higher temperatures mean much greater evaporation. Sunny Naples has about 4" more rain in the year than Edinburgh, and has 7" more than foggy London, but yet has not a wet climate.

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.

The western seaboard of Europe has a maritime climate, the sea tempering the winter, but diminishing the summer heat. The prevailing winds are westerly, and the rainfall is typically abundant and distributed throughout the year. On passing inwards this type of climate changes into the continental type, with cold winters and hot summers, and diminishing rainfall. Though precipitation occurs at all seasons of the year, it tends to be greatest in summer, giving, e. g. in parts of the Balkan States, a type eminently suited to the cereal maize, which needs more summer rain than wheat.

If we bear in mind that North America is a large continent, and Europe a very small one, and that while Europe has no eastern seaboard, it is the eastern seaboard of America which faces Europe, we may realise that the climates of North America show a remarkable analogy to the European. On the western side we have in British Columbia and California respectively the same two types of maritime climate which occur in Europe, that is, British Columbia has a mild equable climate with abundant and equally distributed rainfall, and California has a Mediterranean climate.

At the eastern side the conditions are a little different, and show us that the mere presence of the sea is not sufficient to produce a “maritime” climate. The prevailing winds in eastern North America are off the shore; they cannot therefore carry oceanic influences landwards. To the north the winds tend to have a northerly component, and cold currents of water also stream out of the Arctic and chill eastern North America. The result is that we find that Labrador, though lying in the latitude of Great Britain, has a very severe climate. Further south the conditions are of the “continental” character even on the seaboard, the winters being very cold and the summers hot. Rainfall is equally distributed throughout the year, but on passing inland it diminishes in amount and tends to be limited to the warm season. The diminution would be much more obvious than it actually is were it not that the existence of the large Gulf of Mexico, and also the size of the North American continent, give rise in the south to a monsoon effect, which greatly increases the rainfall of the south-eastern corner of the States. Further to the west, in the lee of the great barrier of the Rocky Mountains, the rainfall is slight.

Incidentally, we may notice that the eastern seaboard of the great Eurasian continent also has a more extreme climate than the western, offering in this respect an analogy to the conditions which prevail on the eastern and western halves of temperate North America. The cause in both cases is the same—the direction of the prevailing winds.

We cannot close this chapter without some reference to weather, a subject of more geographical importance than is generally realised. In speaking of climate we have used figures which were invariably means, i. e. have been obtained by averaging a great number of observations. But where a great number of mean figures are used in a discussion, it is always found that the different averages are based upon varying numbers of observations, and are therefore not strictly comparable with one another. There is always a risk that such figures may mask facts of real geographical importance. No doubt some of the difficulties will disappear with the progress of meteorological science, which will enable the geographer only to select figures which are strictly comparable. Meantime, however, observations for long periods are rare, and the meteorologist must be content to take the figures which are available. For this reason as well as for others, it is advisable to add to the somewhat abstract study of means, that is, of climate, some note upon the actual conditions, that is, upon weather.

We may take British weather, which has become a proverb on account of its variableness, as a text for a brief discussion of the subject.

The daily variations in our weather, as all who have read weather reports know, are chiefly determined by the movements of areas of low pressure or cyclones, which mostly come to us from the Atlantic, and continue eastwards past us, often towards the Baltic. We have already noted the occurrence of what we have called a permanent area of low pressure in the North Atlantic, but this “permanent area” in point of fact is due chiefly to the constant passage here of cyclones, or moving areas of low pressure.

The causes of the eastward displacement of these depressions are interesting. One cause is the general eastward movement of the atmosphere in this region, produced in the fashion already described. This movement necessarily raises the pressure to the west of the depression, owing to the influx of fresh air, while the onward movement of the air in front of the depression lowers the pressure there, and so produces displacement. Again, the air is sucked into a depression in the direction opposite to the hands of a clock, and a moment’s reflection will show that this means that the winds to the east of the depression are southerly and those to the west of it northerly. The warm southerly winds in front tend to lower the pressure by warming the air, while the cold northerly winds behind it raise the pressure by cooling the air. This again produces a displacement of the depression towards the east (see fig. 10).

The fact just described has an interesting practical result. If after a day or night of storm and rain, the temperature falls, we know that the depression causing the storm has passed us, and that we are feeling the effects of the colder winds in its rear. If the thermometer suddenly rises again, then a new depression is approaching, and we are feeling its warm breath before its winds reach us. The clearness and chilliness of the air after a stormy or windy period gives us one of our commonest meteorological sensations, and produces a marked psychical effect, reflected in much of our literature.

One other reason for the eastward motion of the cyclones with us is that they seem to prefer damp air, and so tend to follow the North Sea and pass towards the Baltic, where they often die away.

In the British area, though the depressions move faster in winter than in summer, they have only a mean speed of about 16 miles an hour, while in the United States their mean speed is 25 miles per hour, and their effects are often disastrous except when discounted by the warnings of the Weather Bureau.

In the case of the British Isles cyclones are most frequent and best marked in winter, and they are of great importance in producing our mild and windy winters. In summer they travel further northwards, and as a rule affect our climate less. When, however, from causes still inadequately known, they are better marked in summer than usual, we have a “bad” summer, that is, one which is wet and relatively windy.

The fact that the English Channel is one of the favourite tracks of cyclones has been an important element in protecting the British Islands from foreign invasion, while we all know that it is also a factor in diminishing free intercourse with the Continent.

The second point of importance about our weather is the periodic occurrence at some part of our area of anticyclones, or areas of high pressure, out of which the winds stream gently in the same direction as the hands of[108]
[109] the clock. These areas of high pressure do not display the same tendency to move as do the cyclones, and are most frequently merely displaced by advancing cyclones. For reasons into which space does not permit us to go fully here, anticyclones have a very different effect in summer and in winter. In winter they may bring to us the continental cold, and make our weather abnormally severe, though often bright and fine. On the other hand, in summer they bring to us continental warmth, so that “good” summers are those in which anticyclonic conditions are most frequent, while “severe” winters are due to the same cause. Anticyclones also sometimes induce a curious form of inversion, in that places to the north of a given spot may have temporarily a higher temperature than places to the south. It is such facts which are entirely masked by “mean” figures.

We do not as yet understand the causes which make cyclones sometimes more numerous or better marked than usual, which cause them sometimes to cross our area, and at other times to travel too far north or too far south to influence our weather. It is possible that further investigation in the future may unravel this problem; it is practically certain that a freer use of wireless telegraphy, and the establishing of meteorological stations in northern seas, would give weather forecasting a definiteness and accuracy which it does not yet possess.

We cannot follow this interesting subject further here, but we have said enough to illustrate its geographical significance. As a science or sub-science by itself it will form the subject of a special volume in this series. It may be enough to point out that the Daily Weather Report, published by the Meteorological Office at a cost of one penny, and reproduced in some daily newspapers, is a document well worth the careful study of those with any interest in geography.