THE CHARACTERISTICS OF THE ZONES: II.—THE TEMPERATE ZONES

General: “Temperate” Zones—Temperature—Pressure and Winds—Rainfall—Humidity and Cloudiness—Seasons: Their Effects on Man—Weather—Climatic Subdivisions—South Temperate Zone—Sub-tropical Belts: Mediterranean Climates—North Temperate Zone: Western Coasts—Interiors—Eastern Coasts—Mountain Climates.

_General: “Temperate” Zones._ The so-called “temperate” zones occupy about one-half of the earth’s surface. As a whole, they are _temperate_ only in that their mean temperatures and their physiological effects are intermediate between those of the tropics and those of the polar zones. The modifications of solar climate which result from the distribution and influence of land and water are greatest in the temperate zones. The north temperate zone includes the greatest known extremes of temperature. If the use of the word “temperate” were not so firmly established it would be well to change the name to _intermediate_, or to _middle_.[21]

[21] North-middle and south-middle would then distinguish the zones in the two hemispheres. (See W. M. Davis: _The Temperate Zones_, _Journ. Geogr._, vol. i, 1897, pp. 139-143.) “Temperate” does, however, apply fairly well to the south temperate zone.

A marked changeableness of the weather is a striking characteristic of these zones. Apparently irregular and haphazard, these continual weather changes nevertheless run through a fairly systematic series, although they are essentially non-periodic. Climate and weather are by no means synonymous over most of the extra-tropical latitudes.

_Temperature._ The mean annual temperatures at the margins of the north temperate zone differ by more than 70°. The ranges between the mean temperatures of hottest and coldest months reach 120° at their maximum in north-eastern Siberia, and 80° in North America. A January mean of -60° and a July mean of 95°, and maxima of over 120° and minima of -90°, occur in the same zone. In the districts of lowest winter minima, the mean summer temperatures exceed 85°, and in portions of the districts of highest mean summer maxima, the mean winter minima fall below 32°. Such great ranges characterise the extreme land climates. Under the mild influence of the oceans, the windward west coasts have much smaller ranges than the interiors; the seasonal differences increase inland. The annual ranges in the middle and higher latitudes exceed the diurnal, the conditions in much of the torrid zone being exactly reversed. Over much of the oceans of the temperate zones the annual range is less than 10°. In the south temperate zone there are no extreme ranges, the maxima, slightly over 30°, being near the margin of the zone in the interior of South America, south Africa, and Australia. In these same localities, the diurnal ranges, however, rival those of the north temperate zone.

The north-eastern Atlantic ocean and north-western Europe are about 35° too warm for their latitude in January, while north-eastern Siberia is 30° too cold. The lands north of Hudson’s Bay are 25° too cold, and the waters of the Alaskan Bay 20° too warm. In July, and in the southern hemisphere, the anomalies are small. The lands which are the centre of civilisation in Europe average too warm for their latitudes. These lands are the most truly “temperate” portion of the north temperate zone. The north-west coast of North America is much the same. The diurnal variability of temperature is greater in the north temperate zone than elsewhere in the world, and the same month may differ greatly in its character in different years. One winter in higher latitudes may have much snow, and temperatures below normal; the next may give much rain instead of snow, and the ground remain unfrozen. One summer may be very favourable for crops; the next may give a poor harvest.

From the point of view of temperature, these zones may be considered in three divisions: (1) the sub-tropical, (2) the “temperate” latitudes, and (3) the sub-polar. The annual temperature curve has one maximum and one minimum. In the continental type, the times of maximum and minimum are about one month behind the maximum and minimum insolation dates. In the marine type, the retardation may amount to nearly two months. Coasts and islands have a tendency to a cool spring and warm autumn; continents, to similar temperatures in both spring and fall.

_Pressure and Winds._ The prevailing winds are the “westerlies,” which occupy about as much of the earth’s surface as do the easterly trades. The westerlies are, however, much less regular than the trades. They vary greatly in velocity in different regions and in different seasons, from a light wind to a gale of fifty or more miles an hour. They are stronger in winter than in summer. They are much interfered with, especially in the higher northern latitudes, by seasonal changes of temperature and pressure over the continents, whereby the latter establish, more or less successfully, a system of obliquely outflowing winds in winter and of obliquely inflowing winds in summer. On the eastern coast of Asia there is a complete reversal in wind direction at the opposite seasons, but usually the seasonal shift is much less than 180°. In summer, when the lands have low pressure, the northern oceans are dominated by great oval areas of high pressure, with outflowing spiral eddies, while in winter, when the northern lands have high pressure, the northern portions of the oceans develop cyclonic systems of inflowing winds over their warm waters. All these great continental and oceanic systems of spiraling winds are important climatic controls.

The westerlies are also much confused and interrupted by storms. Hence their designation of _stormy westerlies_. A constant succession of cyclones, and the accompanying anticyclones, travelling along with the prevailing westerlies, causes the latter very frequently to change direction in order to become part of a cyclonic or an anticyclonic whirl. In these storms, velocities of eighty or more miles an hour may be reached at sea. So common are such interruptions that the prevailing westerly wind direction is often difficult to discern without careful observation. Cyclonic storms are most numerous and best developed in winter. The irregular pressure changes usually wholly mask the faint diurnal variation of the barometer which is so characteristic of the tropics, and which becomes less and less marked with increasing latitude. Although greatly interfered with near sea-level by continental changes of pressure, by cyclonic and anticyclonic whirls, and by local inequalities of the surface, the eastward movement of the atmosphere remains very constant aloft. The drift of the higher clouds, and wind observations on mountains, show clearly that the upper currents blow with great steadiness from westerly points, the departures being temporary, and under the control of passing cyclones or anticyclones. The south temperate zone is chiefly water. Hence the westerlies are but little distorted by continental effects. They are strong and steady, and almost as regular as the trades. “Roaring forties” is a well-known designation for the southern middle latitudes, and between latitudes 40° and 60° S. the “brave west winds” blow with a constancy and a velocity found in the northern hemisphere only on the oceans, and then in a modified form. Storms, frequent and severe, characterise these southern hemisphere westerlies, and easterly wind directions are temporarily noted during their passage. Voyages to the west around Cape Horn against head gales, and in cold, wet weather, are much dreaded. South of Africa and Australia, also, the westerlies are remarkably steady and strong. The winter in these latitudes is stormier than the summer, but the seasonal difference is less than that north of the equator.

Between trades and westerlies lies a debatable belt of high pressure, shifting seasonally. Within it, stormy westerlies and drying trades alternately hold sway. It is the sub-tropical belt, a favoured climatic region, where invalids seek health, and an escape from the rigors of a cold winter is found by many who have time and means to leave their northern homes.

_Rainfall._ Rainfall is fairly abundant over the oceans, where evaporation is large, and also over a considerable part of the lands (30-80 inches, and more). It comes chiefly in connection with the usual cyclonic storms, or in thunderstorms, but altitude often serves locally to increase this precipitation. So great are the differences, geographic and periodic, in rainfall, produced by differences in temperature, topography, cyclonic conditions, etc., that none but the most general rules can be laid down. The equatorward margin of the temperate zone rains is clearly defined on the west coasts, at the points where the coast deserts are replaced by belts of light or moderate rainfall. Bold west coasts, on the polar side of lat. 40°, are very rainy, having 100 inches and more a year in the most favourable situations. The hearts of the continents, far from the sea, and especially when well enclosed by mountains, or when blown over by cool ocean winds which warm in crossing the land, have light rainfall (less than 10-20 inches). East coasts, receiving rain from moist winds blowing in from the adjacent oceans as monsoons, or in front of cyclonic storms, are wetter than interiors, but drier than west coasts. Winter is the season of maximum rainfall over oceans, islands, and west coasts, for the westerlies are then most active, cyclonic storms are then most numerous and best developed, and the cold lands chill the inflowing damp air. At this season, however, the low temperatures, high pressures, and tendency to outflowing winds over the continents are unfavourable to rainfall, and the interior land areas, as a rule, then have their minimum. The warmer months bring the maximum rainfall over the continents. Then conditions are favourable for inflowing damp winds from the adjacent oceans; there is the best opportunity for convection; thunder-showers readily develop on the hot afternoons; the capacity of the air for water vapour is greatest. Continents, from equator to higher latitudes, thus have a tendency to maximum rainfall in the warm season; summer rains, as a whole, predominate over the lands. The marine type of rainfall, with a winter maximum, extends in over the western borders of the continents, and is also found in the winter rainfall of the sub-tropical belts. These winter rains are in some respects like the winter rains on windward coasts in the trades. Coastal lands reached by them are well watered, and droughts need not be feared. Rainfalls are heaviest along the tracks of most frequent cyclonic storms.

For continental stations, the typical daily march of rainfall is shown in the accompanying data for Berlin and New York.

DAILY MARCH OF RAINFALL (THOUSANDTHS OF THE DAILY MEAN).

I. _Continental Type._

Hours. Berlin. New York. 12 P.M.—2 A.M. 76 79 2-4 83 85 4-6 74 79 6-8 69 80 8-10 62 74 10-Noon 68 81 Noon—2 P.M. 85 83 2-4 105 95 4-6 104 91 6-8 113 90 8-10 83 85 10-12 P.M. 78 78

The chief maximum is in the afternoon, and the secondary maximum comes in the night or early morning. The chief minimum comes between 10 A.M. and 2 P.M. Coast stations generally have a night maximum, and a minimum between 10 A.M. and 4 P.M., as illustrated in the following data for Valentia.

DAILY MARCH OF RAINFALL AT VALENTIA (THOUSANDTHS OF DAILY MEAN)

II. _Marine Type._

12 P.M.—2 A.M. 88 2-4 93 4-6 93 6-8 90 8-10 84 10-Noon 76 Noon—2 P.M. 74 2-4 75 4-6 80 6-8 82 8-10 82 10-12 P.M. 83

_Humidity and Cloudiness._ Arrhenius gives the mean cloudiness for different latitudes as follows:

70° N. 60° 50° 40° 30° 20° 10° Eq. 10° 20° 30° 40° 50° 60° S. 59 61 48 49 42 40 50 58 57 48 46 56 66 75

The higher latitudes of the temperate zones thus have a mean cloudiness which equals and even exceeds that of the equatorial belt. The amounts over the oceans and coasts are greater than inland. The belts of minimum cloudiness are at about lat. 30° N. and S. Over the continental interiors, the cloudiest season is summer, but the amount is never very large. Otherwise, winter is generally the cloudiest season, with a fairly high mean annual amount.

The absolute humidity, as a whole, decreases as the temperature falls. The relative humidity averages ninety per cent., more or less, over the oceans, and is high under the clouds and rain of cyclonic storms, but depends, on land, upon the wind direction; winds from an ocean or from a lower latitude being damper, and those from a continent or from a colder latitude being drier.

_Seasons: Their Effects on Man._ Seasons in the temperate zones are classified according to temperature—not, as in the tropics, by rainfall. The four seasons are important characteristics of these zones, especially of the middle latitudes of the north temperate zone. Here spring and autumn intervene as transition seasons between the colder winter with snow, and warmer summer with more or less rain. Towards the equatorial margins of the zones, the difference in temperature between summer and winter becomes smaller, and the transition seasons weaken and even disappear. At the polar margins, the change from winter to summer, and _vice versa_, is so sudden that there also the transition seasons disappear. These seasonal changes are of the greatest importance in the life of man.

_Weather._ An extreme changeableness of the weather, depending on the succession of cyclones and anticyclones, is another characteristic. For most of the year and most of the zone, settled weather is unknown. The changes are most rapid in the northern portion of the north temperate zone, especially on the continents, where the cyclones travel fastest. The nature of these changes depends on the degree of development, the velocity of progression, the track, and other conditions of the disturbance which produces them. The changes may be sudden and marked, or faint and slow; the wind may back or veer; the precipitation may be heavy or light; the wind velocity may be light, or of hurricane force; anticyclones may be clear, or may have clouds, and not infrequently, precipitation. There is an almost endless variety of such examples. The detailed study of these varying phases of cyclonic and anticyclonic weather controls belongs to meteorology. It suffices here to say that the particular weather types resulting from this control give the climates their distinctive character, and that the study of climate through these types is the only method of appreciating the actual conditions. Annual and monthly averages of the different climatic elements alone are misleading, and give but a lifeless picture. The cyclonic unit, although its period is irregular and of varying length, is an essential basis of computation and comparison.

The weather types vary with the season and with the geographical position. They result from a combination, more or less irregular, of periodic, diurnal elements, under the regular control of the sun, and of non-periodic cyclonic and anticyclonic elements. In summer, on land, when the cyclonic element is weakest and the solar control is the strongest, the dominant types are associated with the regular changes from day to night. Daytime cumulus clouds; diurnal variation in wind velocity; afternoon thunderstorms, with considerable regularity, characterise the warmest months over the continents and present an analogy with tropical conditions. Cyclonic and anticyclonic spells of hotter or cooler, rainy or dry, weather, with varying winds differing in the temperatures and the moisture which they bring, serve to break the regularity of the diurnal types. On the oceans, the diurnal characteristics are much less marked.

In winter, the non-periodic, cyclonic control is strongest. Local conditions of heat and cold become subordinate to the general control by the cyclone, which imports weather from a distance. The irregular changes from clear to cloudy, from warmer to colder, from dry air to snow or rain, extend over large areas and show little diurnal control. Spring and autumn are transition seasons and have transition weather types. In spring, the growing diurnal quality is marked by the increasing importance of local controls; the appearance of convectional phenomena such as spring rains; the struggle between the cyclonic and the solar controls of temperature, now one and now the other being paramount, but the latter gaining and the former losing. Cold spells, with cyclonic winds and clouds, recall winter. Warm spells, with marked diurnal temperature range, presage summer. In autumn, the decreasing frequency and importance of diurnal phenomena, such as thunder-showers, high afternoon temperatures, and the like; the active radiation and cooling during the longer nights, with resulting fogs; and the increasing control by the cyclone, point to winter’s coming.

Weather types thus differ with the seasons. They differ also in continental and marine climates. They differ according to topography and cyclonic and anticyclonic tracks. The oceans in the south temperate zone have a constancy of non-periodic cyclonic weather changes through the year which resembles only faintly that over the oceans of the northern hemisphere. Winter types differ little from summer types. The diurnal control is never very strong. Stormy weather prevails throughout the year, although the weather changes are more frequent and stronger in the colder months.

_Climatic Subdivisions._ From whatever point of view the temperate zones be considered, it is clear that there are fundamental differences between the north and south temperate. The latter is sufficiently individual to be given a place by itself. The marginal sub-tropical belts must also be considered as a separate group by themselves. The north temperate zone as a whole includes large areas of land, stretching over many degrees of latitude, as well as of water. Hence it embraces so remarkable a diversity of climates that no single district can be taken as typical of the whole. Its climate has been called “a crazy quilt of patches.” It is a zone of marked seasonal variations and of great extremes, annual, diurnal, cyclonic. The simplest and most rational scheme for a classification of these climates is based on the fundamental differences which depend upon land and water, upon the prevailing winds, and upon altitude. Thus there are the ocean areas and the land areas. The latter are then subdivided into western (windward) and eastern (leeward) coasts, and interiors. Mountain climates remain as a separate group.

_South Temperate Zone._ If the climate of the north temperate zone is “a crazy quilt of patches,” that of the south temperate is a piece of fairly uniform texture and appearance throughout. This is the effect of the large ocean surface. The whole meteorological régime is more uniform than in the northern zone. Although the solar climate of the southern hemisphere is more severe than that of the northern, the physical climate is very much less extreme. It has been pointed out that this zone may properly be called “temperate”; that its temperature changes are small; its prevailing winds are stronger and steadier than in the northern hemisphere; its seasons more uniform; its weather prevailingly stormier, more changeable, and more under cyclonic control. The uniformity of the climatic conditions over the far southern oceans is monotonously unattractive. The continental areas are small, and develop to a limited degree only the more marked seasonal and diurnal changes which are characteristic of lands in general. The summers are less stormy than the winters, but even the summer temperatures are not high. Such an area as that of New Zealand, with its mild climate and fairly regular rains, is really at the margins of the zone, and has much more favourable conditions than do the islands farther south. These islands, in the heart of this zone, have dull, cheerless, and inhospitable climates, with snow sometimes in midsummer. The zone enjoys a good reputation for healthfulness, which fact has been ascribed chiefly to the strong and active air movement, the relatively drier air than in corresponding northern latitudes, and the cool summers. It must be remembered, also, that the lands are mostly in the sub-tropical belt, which possesses peculiar climate advantages, as will be seen. The northern oceans repeat, in a much modified form, many of the characteristics of the south temperate oceans. Except to leeward of the broad lands, the northern oceans have the conservative features typical of marine climates the world over.

_Sub-tropical Belts: Mediterranean Climates._ At the tropical margins of the temperate zones, in the latitudes of the tropical high pressure areas, are the so-called sub-tropical belts. Far enough from the equator to be free from continued high temperatures, and near enough to it to be spared the extreme cold of higher latitudes, these transition belts are among the most favoured of the world. Their rainfall régime is alternately that of the westerlies and of the trades. They are thus associated, now with the temperate, and now with the tropical zones. In winter, the equatorward migration of the great pressure and wind systems brings these latitudes under the control of the westerlies, whose frequent irregular storms give a moderate winter precipitation. These winter rains recall the winter trade rains of the tropics, although their origin is different. They are not steady and continuous, but are separated by spells of fine, sunny weather. The amounts vary greatly.[22]

[22] In round numbers, Lisbon has 28.60 inches; Madrid, 16.50; Algiers, 28.15; Nice, 33.00; Rome, 29.90; Ragusa, 63.90.

In summer, when the trades are extended polewards by the outflowing equatorward winds on the eastern side of the ocean highs, mild, dry, and nearly continuous fair weather prevails, with general northerly winds.

The sub-tropical belts of winter rains and dry summers are not very clearly defined. They do not extend continuously around the world. They are mainly limited to the western coasts of the continents, and to the islands off these coasts, in latitudes between about 28° and 40°. Their degree of development and their importance vary in different longitudes. The sub-tropical belt is exceptionally wide in the eastern hemisphere, and reaches far inland there, embracing the countries bordering on the Mediterranean in southern Europe and northern Africa, including the Azores and the famous Riviera, and then extending eastward across the Dalmatian coast and the southern part of the Balkan peninsula into Syria, Mesopotamia, Arabia north of the tropic, Persia, and the adjacent lands. In the great eastward extension of the winter rains in this area, the development of secondary lows over the Mediterranean Sea is an important factor. The fact that the Mediterranean countries are so generally included in this belt has led to the use of the name “Mediterranean climates.” Owing to the great irregularity of topography and outline, the Mediterranean province embraces many varieties of climate, but the dominant characteristics are the mild temperatures, except on the higher elevations, and the sub-tropical rains.

On the west coasts of the two Americas, the sub-tropical belt of winter rains is clearly seen in California and in northern Chile, on the west of the coast mountain ranges. Between the region which has rain throughout the year from the stormy westerlies, and the districts which are permanently arid under the trades, there is an indefinite belt over which rains fall in winter. In southern Africa, which is controlled by the high pressure areas of the South Atlantic and South Indian oceans, the south-western coastal belt has winter rains, decreasing to the north, while the east coast and adjoining interior have summer rains, from the south-east trade. There is sub-tropical vegetation on the south-east coast, and a cool, dry climate on the south-west coast. Southern Australia is climatically similar to south Africa. In summer, the trades give rainfall on the eastern coast, which decreases inland. In winter, the westerlies give moderate rains, chiefly on the south-western coast. Northern Chile, California, south-western Australia, and the Cape province of Africa are thus all in the sub-tropical belt.

WINTER RAINS

M: Malta. W. A: Western Australia]

The sub-tropical climates follow the tropical high pressure belts across the oceans, but they do not retain their distinctive character far inland from the west coasts of the continents (except in the Mediterranean case), nor on the east coasts. On the latter, summer monsoons and the occurrence of general summer rains interfere, as in eastern Asia and in Florida, and to some extent in South America east of the Andes.

Strictly winter rains, with a maximum in December or in June, according to the hemisphere, are typical of the coasts and islands of this belt. The more continental areas have a tendency to spring and autumn rains. The rainy and dry seasons are most marked at the equatorward margins of the belt, and thus recall the tropical characteristic of dry and wet, rather than cold and hot seasons. With increasing latitude, the rain is more evenly distributed through the year, the summer becoming more and more rainy until, in the continental interiors of the higher latitudes, the summer becomes the season of maximum rainfall. The monthly distribution of rainfall in two sub-tropical regions is shown in the accompanying data and curves (see Fig. 23).

ANNUAL MARCH OF RAINFALL: SUB-TROPICAL TYPE (in thousandths of the annual mean).

Southern Italy Western Sicily Australia Malta

Latitude 32.3° S. About 38° N. January 14 130 February 18 93 March 30 98 April 64 75 May 150 35 June 183 23 July 168 8 August 166 28 September 93 73 October 59 133 November 32 144 December 23 160

The following table (from Supan), giving the seasonal distribution of rainfall in southern Europe, in percentages of the annual mean, shows very clearly the change in the rainfall season in going from north to south. In the northern Tyrol, the normal type of central Europe prevails. In Sicily, the summer is almost rainless: the sub-tropical type is fully developed.

SEASONAL DISTRIBUTION OF RAINFALL IN CENTRAL AND SOUTHERN EUROPE (in percentages of the annual mean).

Winter Spring Summer Autumn Northern Tyrol 16 24 37 23 Southern Tyrol 14 26 28 32 Po Valley 20 26 24 30 Central Italy 25 24 17 34 Southern Italy 31 25 11 33 Sicily 39 22 3 36 Malta 48 14 2 36

In Alexandria the dry season lasts nearly eight months; in Palestine, from six to seven months; in Greece, about four months.

The sub-tropical rains are peculiarly well developed on the eastern coast of the Atlantic Ocean, and are clearly illustrated in the accompanying diagram, after Supan (see Fig. 24).

The different types of rainfall are as follows:

I. North of lat. 40° N. Rain throughout the year. II. Lats. 40°-27° N. Dry in summer (sub-tropical rains). III. Lats. 27°-19° N. Always deficient in rainfall. IV. Lats. 19°-7° N. Dry in winter (tropical rains). V. Lats. 7°-1° N. Always rainy (equatorial belt). VI. Lats. 1° N.-17° S. Dry in winter (tropical rains). VII. Lats. 17°-30° S. Always dry. VIII. Beyond lat. 30° S. Dry in summer (sub-tropical rains). (IX. Always rainy on the oceans. The African west coast does not extend into this zone.)

COAST]

The winter rains which migrate equatorward are separated by the Sahara from the equatorial rains which migrate poleward. An unusually extended migration of either of these rain belts may bring them close together, leaving but a small part, if any, of the intervening desert actually rainless. The Arabian desert occupies a somewhat similar position. Large variations in the annual rainfall, and droughts, may be expected towards the equatorial margins of the sub-tropical belts. Irrigation is practised in many places.

TABLE OF MEAN MONTHLY TEMPERATURES FOR SELECTED SUB-TROPICAL STATIONS.[23]

----------+-----------------------+----------------------- | Continental | Insular +-----------------------+----------------------- | Bagdad | Cordoba | Bermuda | Auckland ----------+-----------+-----------+-----------+----------- Lat. | 33°19′ N. | 31°25′ S. | 32°20′ N. | 36°50′ S. Long. | 44°26′ E. | 64°12′ W. | 64°43′ W. | 174°51′ E. Altitude | 39 ft. | 1440 ft. | 148 ft. | 276 ft. | | | | January | 50.9° | 73.4° | 62.4° | 67.1° February | 53.1 | 72.3 | 61.9 | 67.5 March | 62.1 | 68.4 | 61.7 | 65.5 April | 69.3 | 61.0 | 64.4 | 61.5 May | 82.0 | 54.5 | 69.6 | 56.7 June | 89.6 | 49.1 | 74.8 | 53.2 July | 92.8 | 50.0 | 78.8 | 51.8 August | 92.7 | 54.3 | 80.1 | 52.2 September | 85.6 | 58.6 | 78.1 | 54.5 October | 76.5 | 63.5 | 73.4 | 57.4 November | 62.1 | 68.5 | 67.6 | 60.3 December | 52.5 | 71.8 | 63.7 | 64.8 | | | | Mean | 72.4 | 62.1 | 69.7 | 59.4 Range | 41.9 | 24.3 | 18.4 | 15.7 ----------+-----------+-----------+-----------+-----------

[23] Given to nearest tenth of a degree Fahr.

The main features of the sub-tropical rains east of the Atlantic are repeated on the Pacific coasts of the two Americas. In North America, the rainfall decreases from Alaska, Washington, and northern Oregon southwards to Lower California, and the length of the summer dry season increases. The mean annual rainfall (1871-1901) at Neah Bay, Wash., is 112.40 inches; at San Francisco, Cal., 22.83 inches, and at San Diego, Cal., 9.40 inches. At San Diego, six months (May-October) have each less than five per cent. of the annual precipitation, and four of these have one per cent. The southern extremity of Chile, from about latitude 38° S. southward, has heavy rainfall throughout the year from the westerlies, with a winter maximum. Northern Chile is persistently dry. In the intermediate area there are winter rains and dry summers. Neither Africa nor Australia extends far enough south to show the different members of this system well. New Zealand is almost wholly in the prevailing westerly belt. Northern India is unique in having summer monsoon rains, and also winter rains from weak cyclonic storms, which correspond to the sub-tropical winter rains.

From the position of the sub-tropical belts to leeward of the oceans, and at the equatorial margins of the temperate zones, it follows that their temperatures are not extreme. Further, the protection afforded by mountain ranges, as by the Alps in Europe and the Sierra Nevada in the United States, is an important factor in keeping out extremes of winter cold. The annual march, and ranges, of temperature depend upon position with reference to continental or marine influences. This is seen in the accompanying data and curves for Bagdad, Cordoba, Bermuda, and Auckland (see Fig. 25).

SUB-TROPICAL STATIONS

Bd: Bagdad. Ba: Bermuda. A: Auckland. C: Cordoba]

Autumn is, as a rule, a good deal warmer than spring, as in all the eastern Mediterranean basin, the Canaries, and Madeira. This basin is particularly favoured in winter, not only in the protection against cold afforded by the mountains, but also in the high temperature of the sea itself. The southern Alpine valleys and the Riviera are well situated, having good protection and a southern exposure. The coldest month usually has a mean temperature well above 32°. Mean minimum temperatures of about, and somewhat below, freezing occur in the northern portion of the district,[24] and in the more continental localities minima a good deal lower have been observed. (At San Diego, Cal., the absolute minimum is 32°; at San Francisco, 29°.) Mean maximum temperatures of about 95° occur in northern Italy, and of still higher degrees in the southern portions. Somewhat similar conditions exist in the sub-tropical district of North America. Under the control of passing cyclonic storm areas, hot or cold winds, which often owe some of their special characteristics to the topography, bring into the sub-tropical belts, from higher or lower latitudes, unseasonably low or high temperatures. These winds have been given special names (mistral, sirocco, bora, chamsin, leste, leveche, pampero, southerly burster, etc.)

[24] Nice, 30.4°; Rome, 25.7°; Palermo, 32°; Athens, 28.8°.

These belts enjoy abundant sunshine, being among the least cloudy districts in the world. The accompanying data and curve, giving an average for ten stations, show the small annual amount of cloud, the winter maximum and the marked summer minimum, in a typical sub-tropical climate. (Fig. 26).

MONTHLY DISTRIBUTION OF CLOUDINESS IN A SUB-TROPICAL CLIMATE (EASTERN MEDITERRANEAN, LAT. 33.8° N.)

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year 4.9 4.6 3.8 3.7 2.8 1.3 1.1 1.3 1.8 2.5 4.0 4.7 3.0

CLIMATE

(Eastern Mediterranean)]

The winter rains do not bring continuously overcast skies; and it has been well said that the problem of securing a maximum rainfall with a maximum number of clear days has been solved on the southern Alpine slopes. A summer month with a mean cloudiness of 0.1 is not exceptional in the drier parts of the sub-tropics. The winter cloudiness in northern Italy is 5.0 to 6.0; in summer, 3.0 to 4.0. Cairo has an annual mean of 1.9, and in June it has 0.8. Biskra, on the northern margin of the Sahara, has 264 clear days. In the central valley of California, the number of clear days is similarly very large.

With prevailingly fair skies, even temperatures, and moderate rainfall, the sub-tropical belts possess many climatic advantages which fit them for health resorts. The long list of well-known resorts on the Mediterranean coast, and the shorter list for California, bear witness to this fact.

_North Temperate Zone: West Coasts._ Marine climatic types are carried by the prevailing westerlies on to the western coasts of the continents, giving them mild winters and cool summers, abundant rainfall and a high degree of cloudiness and relative humidity. North-western Europe is particularly favoured because of the remarkably high temperatures of the North Atlantic Ocean, and because of the influence of the winds controlled by the low pressure area off Iceland. In January, north-western Europe has temperatures from 20° to 40° in excess of the normal for the latitude. The north-western coast of North America has temperatures more than 10° too warm for the latitude. January means of 40° to 50° in the British Isles and on the northern French coast occur in the same latitudes as those of 0° and 10° in the far interior of Asia. In July, means of 60° to 70° in the former contrast with 70° and 80° in the latter districts. The conditions are somewhat similar in North America. Along the western coasts of North America and of Europe the mean annual ranges are under 25°,—actually no greater than some of those within the tropics. Irregular cyclonic temperature changes are, however, marked in the temperate zone, while absent in the tropics. The data and curves for the Scilly Isles and for Thorshavn, Faroe Islands, illustrate the insular type of temperature on the west coasts (see Fig. 27). In the Faroes the mean maximum is 65.1°, and the mean minimum 16°. It will be noted that the poleward decrease in the mean annual and the mean winter temperatures is very slow between latitudes 50° and 62° N. on the west coast of Europe.

STATIONS IN THE TEMPERATE ZONES

S. I: Scilly Isles. P: Prague. C: Charcow. S: Semipalatinsk. K: Kiakhta. B: Blagoweschtschensk. Sa: Sakhalin. T: Thorshavn. Y: Yakutsk]

TABLE OF MEAN MONTHLY TEMPERATURES FOR SELECTED STATIONS IN THE TEMPERATE ZONES.

LEGEND: (A) = Scilly Isles (B) = Prague (C) = Charcow (D) = Semipalatinsk (E) = Kiakhta (F) = Blagoweschtschensk (G) = Sakhalin (H) = Thorshavn, Faroe Isles (I) = Yakutsk, E. Siberia

----------+-------------------------------------------------+ | Along lat. 50° N. | +--------+----------------------------------------+ | West | Continental | | Coast | | +--------+---------+-------+------+-------+-------+ | (A) | (B) | (C) | (D) | (E) | (F) | ----------+--------+---------+-------+------+-------+-------+ Lat. | 49°55′ | 50°5′ | 50°2′ |50°24′| 50°21′| 50°15′| Long. |6°20′ W.|14°26′ E.|36° 11′|80°13′|106°31′|127°38′| Alt. (ft.)| 98.4 | 662.7 | 413.4 |593.8 | 2526 | 360.9 | Jan. | 45.7° | 29.8° | 16.5° | 0.5°| -15.9°| -13.9°| Feb. | 45.7 | 32.0 | 22.1 | 1.8 | -5.4 | -3.3 | March | 46.0 | 37.8 | 29.3 | 14.4 | 16.9 | 14.4 | April | 48.7 | 47.3 | 44.8 | 38.3 | 34.3 | 34.7 | May | 52.5 | 55.9 | 58.8 | 57.2 | 48.7 | 49.6 | June | 57.9 | 63.3 | 65.1 | 68.0 | 63.1 | 63.7 | July | 60.8 | 66.7 | 69.6 | 72.0 | 66.4 | 70.5 | Aug. | 61.2 | 65.3 | 66.4 | 67.3 | 61.7 | 65.8 | Sept. | 58.6 | 58.8 | 56.1 | 54.9 | 48.0 | 53.2 | Oct. | 54.0 | 48.7 | 45.5 | 38.1 | 32.0 | 34.2 | Nov. | 49.6 | 37.6 | 33.8 | 20.1 | 11.8 | 9.7 | Dec. | 47.3 | 31.3 | 23.2 | 6.1 | -2.7 | -9.2 | Year | 52.3 | 47.9 | 44.3 | 36.5 | 29.9 | 30.8 | Range | 15.5 | 36.9 | 53.1 | 71.5 | 82.3 | 84.4 | ----------+--------+---------+-------+------+-------+-------+ | At lat. 62°N. +---------+---------+----------- | East | Insular |Continental | Coast | | +---------+---------+----------- | (G) | (H) | (I) ----------+---------+---------+----------- Lat. |50°50′ | 62°2′ | 62°1′ Long. |142°7′ E.|6°44′ W. | 129°43′ E Alt. (ft.)| 180.4 | 29.5 | 328.1 Jan. | -0.4° | 37.8° | -45.2° Feb. | 5.0 | 38.1 | -35.0 March | 15.8 | 37.8 | -10.7 April | 31.1 | 41.9 | 15.1 May | 41.4 | 45.0 | 40.3 June | 50.7 | 49.5 | 58.5 July | 60.3 | 51.4 | 65.8 Aug. | 62.2 | 51.3 | 59.7 Sept. | 53.6 | 48.7 | 42.3 Oct. | 39.6 | 43.9 | 15.8 Nov. | 22.5 | 40.6 | -21.3 Dec. | 7.3 | 38.1 | -41.1 Year | 32.4 | 43.7 | 12.0 Range | 62.6 | 13.6 | 111.0 ----------+---------+---------+-----------

MONTHLY DISTRIBUTION OF RAINFALL (IN THOUSANDTHS OF THE ANNUAL MEAN). TEMPERATE ZONE.

---------+-------------------------+------------------------- |Continental Summer Rains | Coast Rains +------------+------------+------------+------------ | Moderate | Marked | Uniform | Fall and | | Summer |Distribution| Winter | | Maximum | | Rains +------------+------------+------------+------------ | Central | Northern | Atlantic | North-west | Europe | Asia | Coast, | Europe | North of | | North | | Alps | | America | ---------+------------+------------+------------+------------ Lat. |About 50° N.|About 55° N.|About 40° N.|About 60° N. January | 57 | 20 | 84 | 100 February | 56 | 17 | 77 | 80 March | 68 | 18 | 85 | 72 April | 71 | 35 | 70 | 56 May | 92 | 75 | 80 | 58 June | 115 | 235 | 81 | 64 July | 121 | 215 | 96 | 70 August | 117 | 122 | 87 | 80 September| 82 | 133 | 84 | 102 October | 75 | 58 | 91 | 110 November | 74 | 40 | 86 | 102 December | 72 | 32 | 79 | 106 ---------+------------+------------+------------+------------

The monthly distribution of rainfall, with the marked maximum in the fall and winter which is characteristic of the marine régime, is illustrated in the last column of the table above, for north-western Europe, and in the corresponding curve (see Fig. 28).

On the northern Pacific coast of North America, the distribution is similar. Thus at Olympia, Washington, there is a distinct cold season maximum, as appears in the following data:

MONTHLY DISTRIBUTION OF RAINFALL AT OLYMPIA, WASHINGTON (IN THOUSANDTHS OF THE ANNUAL MEAN).

January 159 February 135 March 95 April 65 May 44 June 31 July 13 August 13 September 53 October 86 November 124 December 182

In the southern hemisphere, the western coasts of southern South America, Tasmania, and New Zealand show the same type.

The cloudiness and relative humidity average high on western coasts, with the maximum in the colder season. The difference in general rainfall conditions between the west coast, typified by the exaggerated case of Valentia, in south-western Ireland, and the Mediterranean, is seen in the number of rainy days in each district. Valentia has nearly 250. In the Mediterranean, they may be set down as about 100, in round numbers.

The west coasts, therefore, including the important climatic province of western Europe, and the coast provinces of north-western North America, New Zealand, and southern Chile, have, as a whole, mild winters, equable temperatures, small ranges, and abundant rainfall, fairly well distributed through the year. The summers are relatively cool, especially on the Chilean coast.

C. E: Central Europe. A: Northern Asia. N. A: Atlantic Coast of North America. N. W. E: North-west Europe]

_Continental Interiors._ The equable climate of the western coasts changes, gradually or suddenly, into the more extreme climates of the interiors. In Europe, where no high mountain ranges intervene, the transition is gradual, and broad stretches of country have the benefits of the tempering influence of the Atlantic. In North America, the change is abrupt, and comes on crossing the lofty western mountain barrier. The data in the table on page 136, and the corresponding curves in Fig. 27, illustrate well the gradually increasing severity of the climate with increasing distance inland in Eurasia. Central Europe is seen to lie between the modified marine climate of the west coast and the continental conditions of Russia and Siberia. Its mean temperatures do not differ very much from those on the coast, but the seasons are more sharply contrasted.

The continental interiors of the north temperate zone have the greatest extremes in the world. Towards the Arctic circle, the winters are extremely severe, and January mean temperatures of -10° and -20° occur over considerable areas. At the cold pole of northern Siberia a January mean of -60° is found. Mean minimum temperatures of -40° occur in the area from eastern Russia, over Siberia and down to about latitude 50° N. At Verkhoyansk, an important town just beyond the Arctic circle, the absolute minimum is below -90°. Over no small part of Siberia minimum temperatures below -70° may be looked for every winter. Thorshavn and Yakutsk (see table on page 136) are excellent examples of the temperature differences along the same latitude line. The winter in this interior region is dominated by a marked high pressure. The weather is prevailingly clear and calm. The ground below a slight depth is frozen the year around, over wide areas. The moderate snowfall is sufficient, with the continued cold, to make sleighing possible for immense stretches all over the country. The frozen rivers can be crossed without bridges. This unifying influence, of easy winter communication, has been most important in Russian history, as Leroy-Beaulieu has pointed out. The extremely low temperatures are not disagreeable except when the steppes are swept by icy storm winds (buran, purga), carrying loose snow, and often resulting in loss of life.

In the North American interior, the winter cold is somewhat less severe. The lowest January mean temperatures are -30°, in the extreme northern portion of the continent. Mean annual minima of -40° occur down into the northern interior portion of the United States. The lowest is about -60°, near Great Bear Lake, with an absolute minimum of about -72°. North American winter weather in middle latitudes is often interrupted by cyclones, which, under the steep poleward temperature gradient then prevailing, cause frequent, marked, and sudden changes in wind direction and temperature over the central and eastern United States. Cold waves and warm waves are common, and blizzards resemble the buran or purga of Russia and Siberia. With cold northerly winds, temperatures below freezing are carried far south towards the tropic. The January mean temperatures in the southern portions of the continental interiors average about 50° or 60°.

In summer, the northern continental interiors are warm, with July means of 60° and thereabout. These temperatures are not much higher than those on the west coasts, but as the northern interior winters are much colder than those on the coasts, the interior ranges are very large. The mean annual extreme ranges exceed 150° in northern North America and 170° in Siberia. Mean maximum temperatures of 85° occur beyond the Arctic circle in north-eastern Siberia, and beyond latitude 60° in North America. In spite of the extreme winter cold, agriculture extends remarkably far north in these regions, because of the warm, though short, summers, with favourable rainfall distribution. The July isotherm of 50° is about the northern limit of tree growth. Beyond a zone of stunted tree growth, comes the tundra. The summer heat is sufficient to thaw the upper surface of the frozen ground, and vegetation prospers for its short season. At this time, great stretches of flat surface become swamps. The southern interiors have torrid heat in summer, temperatures of over 90° being recorded in the south-western United States and in southern Asia. In these districts the diurnal ranges of temperature are very large, often exceeding 40°, and the mean maxima exceed 110°.

In South America, the interior of Argentina has moderate mean annual ranges (20°-30°); the mean maxima reach 95°-100° and even higher, and the mean minima fall below 23°. The west coast has smaller ranges (less than 20°); lower mean maxima (77°-86°), and higher mean minima (32°-23°).

The winter maximum rainfall of the west coasts becomes a summer maximum in the interiors. The change is gradual in Europe, as is the change in temperature, but more sudden in North America. The curves for central Europe and for northern Asia (see Fig. 28) illustrate these continental summer rains. The summer maximum becomes more marked with the increasing continental character of the climate. Thus, while June to August in central Europe supply about thirty-five per cent. of the annual precipitation, in northern Asia, excluding the coast, they give nearly sixty per cent. The rains of Asia are actually comparable, in relative intensity, at their maximum, with the rains of the tropics. In Bengal, _e. g._, June to August give only fifty-seven per cent. of the annual rainfall. The winter dry season of Asia is, however, very different from a tropical dry season, because of the difference in conditions of vegetation and of snow-cover. In North America, Nebraska, a state which is typical of a considerable district of summer rains, receives about sixty per cent. of the annual rainfall in the months of April, May, June, and July.

The change in rainfall season with increasing distance from the Atlantic Ocean in Eurasia is well brought out by Supan in the following table:

TABLE SHOWING SEASONAL DISTRIBUTION OF RAINFALL IN EURASIA (IN PERCENTAGES OF THE ANNUAL MEAN).

Winter Spring Summer Autumn Ireland 28 21 24 27 Western England 28 19 24 29 Eastern England 23 19 28 30 North-western Germany 23 22 31 24 Central Germany 20 23 34 23 Eastern Germany 19 22 37 22 Western Russia 16 21 39 24 Central Russia 16 22 37 25 Western Siberia 13 13 42 32 Eastern Siberia 9 12 58 21

There is also a well-marked decrease in the amount of rainfall inland. In western Europe, the rainfall averages 20-30 inches, with much larger amounts (reaching 80-100 inches and even more) on the bold west coasts, as in the British Isles and Scandinavia, where the moist Atlantic winds are deflected upwards, and also locally on mountain ranges, as on the Alps. There are small rainfalls (below 20 inches) in eastern Scandinavia and on the Iberian peninsula. Eastern Europe has generally less than 20 inches; western Siberia about 15 inches, and eastern Siberia about 10 inches. In the southern part of the great overgrown continent of Asia, an extended region of steppes and deserts, too far from the sea to receive sufficient precipitation, shut in by mountains, and controlled in summer by drying northerly winds, receives less than 10 inches a year, and in places less than 5 inches. In this interior district of Asia, population is inevitably small, and suffers under a condition of hopeless aridity.

The North American interior has more favourable rainfall conditions than Asia, because the former continent is narrower. The heavy rainfalls on the western slopes of the Pacific coast mountains correspond, in a general way, to those on the west coast of Europe, although they are heavier (over 100 inches at a maximum). The close proximity of the mountains to the Pacific, however, involves a much more rapid decrease of rainfall inland than is the case in Europe, as may be seen by comparing the isohyetal lines in the two cases. The rain-shadow influence of the Pacific coast Cordilleras extends about half-way across the continent. A considerable interior region is left with deficient rainfall (less than 10 inches) in the south-west. The eastern portion of the continent is freely open to the Atlantic and the Gulf of Mexico, so that moist cyclonic winds have free access, and rainfalls of over 20 inches are found everywhere east of the 100th meridian. These conditions are much more favourable than those in eastern Asia. The greater part of the interior of North America has the usual warm-season rains. In the interior basin, between the Rocky and Sierra Nevada mountains, the higher plateaus and mountains receive much more rain than the desert lowlands. Forests grow on the higher elevations, while irrigation is necessary for agriculture on the lowlands. The rainfall here comes chiefly from thunderstorms.

In southern South America, the narrow Pacific slope has heavy rainfall (over 80 inches). East of the Andes the plains are dry (mostly less than 10 inches). The southern part of the continent is very narrow, and is open to the east. It is also more open to the west than is the country farther north, owing to the decreasing height of the mountains southward. Hence the rainfall increases somewhat to the south, coming in connection with passing cyclones. Tasmania and New Zealand have most rain on their western slopes.

In a typical continental climate, the winter, except for radiation fogs, is very clear, and the summer is the cloudiest season, as is well shown in the following data and curve for eastern Asia. In a more moderate continental climate, such as that of central Europe, and much of the United States, the winter is the cloudiest season (see Fig. 29).

MONTHLY DISTRIBUTION OF CLOUDINESS IN CONTINENTAL CLIMATES.

I. Eastern Asia. 10 stations. Lat. 56.5° N. Long. 115° E. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year. 3.1 3.4 3.9 4.7 5.7 5.6 6.2 6.0 5.5 5.4 4.8 4.2 4.9

II. Central Europe. Hungarian Plain. Lat. 47° N. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year. 6.5 5.9 5.7 5.6 5.4 5.3 4.4 4.2 4.6 5.8 6.6 6.9 5.6

In the first case, the mean cloudiness is small; in the second, there is a good deal of cloud all the year around.

The vast continental interiors, whose climatic features have here been outlined, can obviously be subdivided into smaller climatic provinces almost indefinitely, as pointed out in Chapter III.

MOUNTAIN CLIMATES: TEMPERATE ZONES.

E: Central Europe. A: Eastern Asia. M: Mountain]

_East Coasts._ The prevailing winds carry the marine climate of the oceans on to the western coasts of the temperate zone lands. They also carry the continental climates of the interiors off over the eastern coasts of these same lands, and even for some distance on to the adjacent oceans. The east coasts, therefore, have continental climates, with modifications resulting from the presence of the oceans to leeward, and are necessarily separated from the west coasts, with which they have little in common. On the west coasts of the north temperate lands the isotherms are far apart. On the east coasts, they are crowded together. The east coasts share with the interiors large annual and cyclonic ranges of temperature. At latitude 55° N., for example, the east coast of Asia has a mean annual range which is four times as large as that of the west coast. A glance at the isothermal maps of the world will show at once how favoured, because of its position to leeward of the warm North Atlantic waters, is western Europe as compared with eastern North America. A similar contrast, less marked, is seen in eastern Asia and western North America. In eastern Asia, there is some protection, by the coast mountains, against the extreme cold of the interior, but in North America there is no such barrier, and severe cold winds sweep across the Atlantic coast states, even far to the south. Owing to the prevailing offshore winds, the oceans to leeward have relatively little effect. In the north-east, the cold water is effective in giving cooler summers than farther south.

As already noted, the rainfall increases from the interiors towards the east coasts. In North America, the distribution through the year is very uniform, with some tendency to a summer maximum, as in the interior (see Fig. 28).

In eastern Asia the winters are relatively dry and clear, under the influence of the cold offshore monsoon, and the summers are warm and rainy, with the northward extension of the south-east monsoon, which reaches as far as lat. 60° N. The summer maximum of rainfall on this coast is clearly shown in the following data (Trabert):

MONTHLY DISTRIBUTION OF RAINFALL. EAST COAST OF ASIA. (IN PERCENTAGES OF ANNUAL MEAN). Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year. 2 2 3 6 8 10 12 21 16 11 6 3 19.3 ins.

Rainfalls of 40 inches are found on the east coasts of Korea, Kamchatka, and Japan, while in North America, which is more open, they reach farther inland. Japan, although occupying an insular position, has a modified continental, rather than a marine climate. The winter monsoon, after crossing the water, gives abundant rain on the western coast, while the winter is relatively dry on the lee of the mountains, on the east. Japan has smaller temperature ranges than the mainland.

_Mountain Climates._ The mountain climates of the temperate zone have the usual characteristics which are associated with altitude everywhere. If the altitude is sufficiently great, the decreased temperature gives mountains a polar climate, with the difference that the summers are relatively cool, while the winters are mild, owing to inversions of temperature in anticyclonic weather. Hence the annual ranges are smaller than over lowlands. At such times of inversion, the mountain tops often appear as local areas of higher temperatures in a general region of colder air over the valleys and lowlands. The increased intensity of insolation aloft is an important factor in giving certain mountain resorts their deserved popularity in winter (_e. g._, Davos and Meran). Of Meran it has been well said that from December to March the nights are winter, but the days are mild spring. Mountains provoke rainfall, even in arid continental interiors, and thus we have well-forested plateaus and mountain slopes rising above desert lowlands. The diurnal ascending air currents of summer usually give mountains their maximum cloudiness and highest relative humidity in the warmer months, while winter is the drier and clearer season. This is shown in the data below (see Fig. 29).

MONTHLY DISTRIBUTION OF CLOUDINESS. MOUNTAIN CLIMATE. (CENTRAL EUROPE. ALPINE SUMMITS. 8500 FT. LAT. 47° N. SEVEN STATIONS.)

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year. 5.0 5.3 6.0 6.5 7.0 6.7 6.1 5.8 6.0 6.0 5.5 5.1 5.9

The clouds of winter are low; those of summer are higher. Hence the annual march of cloudiness on mountains is usually the opposite of that on lowlands.