Part 5

At the equator, where the day is always twelve hours long, there are two maxima of insolation at the equinoxes, when the sun is vertical at noon, and two minima at the solstices when the sun is farthest off the equator. The values do not vary much through the year because the sun is never very far from the zenith, and day and night are always equal. As latitude increases, the angle of insolation becomes more oblique and the intensity decreases, but at the same time the length of day rapidly increases during the summer, and towards the pole of the hemisphere which is having its summer the gain in insolation from the latter cause more than compensates for the loss by the former. The double period of insolation above noted for the equator prevails as far as about lat. 12° N. and S.; at lat. 15° the two maxima have united in one, and the same is true of the minima. At the pole there is one maximum at the summer solstice, and no insolation at all while the sun is below the horizon. On the 21st of June the equator has a day twelve hours long, but the sun does not reach the zenith, and the amount of insolation is therefore less than at the equinox. On the northern tropic, however, the sun is vertical at noon, and the day is more than twelve hours long. Hence the amount of insolation received at this latitude is greater than that received on the equinox at the equator. From the tropic to the pole the sun stands lower and lower at noon, and the value of insolation would steadily decrease with latitude if it were not for the increase in the length of day. Going polewards from the northern tropic on the 21st of June, the value of insolation increases for a time, because, although the sun is lower, the number of hours during which it shines is greater. A maximum value is reached at about lat. 43½° N. The decreasing altitude of the sun then more than compensates for the increasing length of day, and the value of insolation diminishes, a minimum being reached at about lat. 62°. Then the rapidly increasing length of day towards the pole again brings about an increase in the value of insolation, until a maximum is reached at the pole which is greater than the value received at the equator at any time. The length of day is the same on the Arctic circle as at the pole itself, but while the altitude of the sun varies during the day on the former, the altitude at the pole remains 23½° throughout the 24 hours. The result is to give the pole a maximum. On the 21st of June there are therefore two maxima of insolation, one at lat. 43½° and one at the north pole. From lat. 43½° N., insolation decreases to zero on the Antarctic circle, for sunshine falls more and more obliquely, and the day becomes shorter and shorter. Beyond lat. 66½° S. the night lasts 24 hours. On the 21st of December the conditions in southern latitudes are similar to those in the northern hemisphere on the 21st of June, but the southern latitudes have higher values of insolation because the earth is then nearer the sun.

[Illustration: From Davis's _Elementary Meteorology_. FIG. 1.--Distribution of Insolation over the Earth's Surface.]

At the equinox the days are equal everywhere, but the noon sun is lower and lower with increasing latitude in both hemispheres until the rays are tangent to the earth's surface at the poles (except for the effect of refraction). Therefore, the values of insolation diminish from a maximum at the equator to a minimum at both poles.

The effect of the earth's atmosphere is to weaken the sun's rays. The more nearly vertical the sun, the less the thickness of atmosphere traversed by the rays. The values of insolation at the earth's surface, after passage through the atmosphere, have been calculated. They vary much with the condition of the air as to dust, clouds, water vapour, &c. As a rule, even when the sky is clear, about one-half of the solar radiation is lost during the day by atmospheric absorption. The great weakening of insolation at the pole, where the sun is very low, is especially noticeable. The following table (after Angot) shows the effect of the earth's atmosphere (coefficient of transmission 0.7) upon the value of insolation received at sea-level.

_Values of Daily Insolation at the Upper Limit of the Earth's Atmosphere and at Sea-Level_.

+-----------------+---------------------------+--------------------------+ | | Upper Limit of Atmosphere.| Earth's Surface. | | Lat. +----------+------+---------+---------+------+---------+ | | Equator. | 40°. | N. Pole.| Equator.| 40°.| N. Pole.| +-----------------+----------+------+---------+---------+------+---------+ | Winter solstice | 948 | 360 | 0 | 552 | 124 | 0 | | Equinoxes | 1000 | 773 | 0 | 612 | 411 | 0 | | Summer solstice | 888 | 1115 | 1210 | 517 | 660 | 494 | +-----------------+----------+------+---------+---------+------+---------+

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

_Relative Distances traversed by Solar Rays through the Atmosphere, and Intensities of Radiation per Unit Areas_.

+------------------+----+----+----+----+----+----+----+----+----+----+----+ |Altitude of sun | O° | 5° | 10°| 20°| 30°| 40°| 50°| 60°| 70°| 80°| 90°| | | | | | | | | | | | | | |Relative lengths | | | | | | | | | | | | | of path through | | | | | | | | | | | | | the atmosphere |44.7|10.8|5.7 |2.92|2.00|1.56|1.31|1.15|1.06|1.02|1.00| | | | | | | | | | | | | | |Intensity of | | | | | | | | | | | | | radiation on a | | | | | | | | | | | | | surface normal | | | | | | | | | | | | | to the rays | 0.0|0.15|0.31|0.51|0.62|0.68|0.72|0.75|0.76|0.77|0.78| | | | | | | | | | | | | | |Intensity of | | | | | | | | | | | | | radiation on | | | | | | | | | | | | | a horizontal | | | | | | | | | | | | | surface | 0.0|0.01|0.05|0.17|0.31|0.44|0.55|0.65|0.72|0.76|0.78| +------------------+----+----+----+----+----+----+----+----+----+----+----+

_Physical Climate._--The distribution of insolation explains many of the large facts of temperature distribution, for example, the decrease of temperature from equator to poles; the double maximum of temperature on and near the equator; the increasing seasonal contrasts with increasing latitude, &c. But the regular distribution of solar climate between equator and poles which would exist on a homogeneous earth, whereby similar conditions prevail along each latitude circle, is very much modified by the unequal distribution of land and water; by differences of altitude; by air and ocean currents, by varying conditions of cloudiness, and so on. Hence the climates met with along the same latitude circle are no longer alike. Solar climate is greatly modified by atmospheric conditions and by the surface features of the earth. The uniform arrangement of solar climatic belts, arranged latitudinally, is interfered with, and what is known as _physical climate_ results. According to the dominant control we have solar, continental and marine, and mountain climates. In the first-named, latitude is the essential; in the second and third, the influence of land or water; in the fourth, the effect of altitude.

_Classification of the Zones by Latitude Circles._--It is customary to classify climates roughly into certain broad belts. These are the climatic zones. The five zones with which we are most familiar are the so-called torrid, the two temperate, and the two frigid zones. The torrid, or better, the tropical zone, naming it by its boundaries, is limited on the north and south by the two tropics of Cancer and Capricorn, the equator dividing the zone into two equal parts. The temperate zones are limited towards the equator by the tropics, and towards the poles by the Arctic and Antarctic circles. The two polar zones are caps covering both polar regions, and bounded on the side towards the equator by the Arctic and Antarctic circles.

These five zones are classified on purely astronomical grounds. They are really zones of solar climate. The tropical zone has the least annual variation of insolation. It has the maximum annual amount of insolation. Its annual range of temperature is very slight. It is the summer zone. Beyond the tropics the contrasts between the seasons rapidly become more marked. The polar zones have the greatest variation in insolation between summer and winter. They also have the minimum amount of insolation for the whole year. They may well be called the winter zones, for their summer is so short and cool that the heat is insufficient for most forms of vegetation, especially for trees. The temperate zones are intermediate between the tropical and the polar in the matter of annual amount and of annual variation of insolation. Temperate conditions do not characterize these zones as a whole. They are rather the seasonal belts of the world.

[Illustration: From _Grundzüge der physischen Erdkunde_, by permission of Veit & Co. FIG. 2.--Supan's Temperature Zones.]

_Temperature Zones._--The classification of the zones on the basis of the distribution of sunshine serves very well for purposes of simple description, but a glance at any isothermal chart shows that the isotherms do not coincide with the latitude lines. In fact, in the higher latitudes, the former sometimes follow the meridians more closely than they do the parallels of latitude. Hence it has been suggested that the zones be limited by isotherms rather than by parallels of latitude, and that a closer approach be thus made to the actual conditions of climate. Supan[1] (see fig. 2) has suggested limiting the hot belt, which corresponds to, but is slightly greater than, the old torrid zone, by the two mean annual isotherms of 68°--a temperature which approximately coincides with the polar limit of the trade-winds and with the polar limit of palms. The hot belt widens somewhat over the continents, chiefly because of the mobility of the ocean waters, whereby there is a tendency towards an equalization of the temperature between equator and poles in the oceans, while the stable lands acquire a temperature suitable to their own latitude. Furthermore, the unsymmetrical distribution of land in the low latitudes of the northern and southern hemispheres results in an unsymmetrical position of the hot belt with reference to the equator, the belt extending farther north than south of the equator. The polar limits of the temperate zones are fixed by the isotherm of 50° for the warmest month. Summer heat is more important for vegetation than winter cold, and where the warmest month has a temperature below 50°, cereals and forest trees do not grow, and man has to adjust himself to the peculiar climatic conditions in a very special way. The two polar caps are not symmetrical as regards the latitudes which they occupy. The presence of extended land masses in the high northern latitudes carries the temperature of 50° in the warmest month farther poleward there than is the case in the corresponding latitudes occupied by the oceans of the southern hemisphere, which warm less easily and are constantly in motion. Hence the southern cold cap, which has its equatorial limits at about lat. 50° S., is of much greater extent than the northern polar cap. The northern temperate belt, in which the great land areas lie, is much broader than the southern, especially over the continents. These temperature zones emphasize the natural conditions of climate more than is the case in any subdivision by latitude circles, and they bear a fairly close resemblance to the old zonal classification of the Greeks.

_Classification of the Zones by Wind Belts._--The heat zones however, emphasize the temperature to the exclusion of such important elements as wind and rainfall. So distinctive are the larger climatic features of the great wind belts of the world, that a classification of climates according to wind systems has been suggested.[2] As the rain-belts of the world are closely associated with these wind systems, a classification of the zones by winds also emphasizes the conditions of rainfall. In such a scheme the tropical zone is bounded on the north and south by the margins of the trade-wind belts, and is therefore larger than the classic torrid zone. This trade-wind zone is somewhat wider on the eastern side of the oceans, and properly includes within its limits the equable marine climates of the eastern margins of the ocean basins, even as far north as latitude 30° or 35°. Most of the eastern coasts of China and of the United States are thus left in the more rigorous and more variable conditions of the north temperate zone. Through the middle of the trade-wind zone extends the sub-equatorial belt, with its migrating calms, rains and monsoons. On the polar margins of the trade-wind zone lie the sub-tropical belts, of alternating trades and westerlies. The temperate zones embrace the latitudes of the stormy westerly winds, having on their equator-ward margins the sub-tropical belts, and being somewhat narrower than the classic temperate zones. Towards the poles there is no obvious limit to the temperate zones, for the prevailing westerlies extend beyond the polar circles. These circles may, however, serve fairly well as boundaries, because of their importance from the point of view of insolation. The polar zones in the wind classification, therefore, remain just as in the older scheme.

_Need of a Classification of Climates._--A broad division of the earth's surface into zones is necessary as a first step in any systematic study of climate, but it is not satisfactory when a more detailed discussion is undertaken. The reaction of the physical features of the earth's surface upon the atmosphere complicates the climatic conditions found in each of the zones, and makes further subdivision desirable. The usual method is to separate the _continental_ (near sea-level) and the _marine_. An extreme variety of the continental is the _desert_; a modified form, the _littoral_; while altitude is so important a control that _mountain_ and _plateau_ climates are always grouped by themselves.

_Marine or Oceanic Climate._--Land and water differ greatly in their behaviour regarding absorption and radiation. The former warms and cools readily, and to a considerable degree; the latter, slowly and but little. The slow changes in temperature of the ocean waters involve a retardation in the times of occurrence of the maxima and minima, and a marine climate, therefore, has a cool spring and a warm autumn, the seasonal changes being but slight. Characteristic, also, of marine climates is a prevailingly higher relative humidity, a larger amount of cloudiness, and a heavier rainfall than is found over continental interiors. All of these features have their explanation in the abundant evaporation from the ocean surfaces. In the middle latitudes the oceans have distinctly rainy winters, while over the continental interiors the colder months have a minimum of precipitation. Ocean air is cleaner and purer than land air, and is generally in more active motion.

_Continental Climate._--Continental climate is severe. The annual temperature ranges increase, as a whole, with increasing distance from the oceans. The coldest and warmest months are usually January and July, the times of maximum and minimum temperatures being less retarded than in the case of marine climates. The greater seasonal contrasts in temperature over the continents than over the oceans are furthered by the less cloudiness over the former. Diurnal and annual changes of nearly all the elements of climate are greater over continents than over oceans; and this holds true of irregular as well as of regular variations. Fig. 3 illustrates the annual march of temperature in marine and continental climates. Bagdad, in Asia Minor (Bd.), and Funchal on the island of Madeira (M.) are representative continental and marine stations for a low latitude. Nerchinsk in eastern Siberia (N.) and Valentia in south-western Ireland (V.) are good examples of continental and marine climates of higher latitudes in the northern hemisphere. The data for these and the following curves were taken from Hann's _Lehrbuch der Meteorologie_ (1901).

Owing to the distance from the chief source of supply of water vapour--the oceans--the air over the larger land areas is naturally drier and dustier than that over the oceans. Yet even in the arid continental interiors in summer the absolute vapour content is surprisingly large, and in the hottest months the percentages of relative humidity may reach 20% or 30%. At the low temperatures which prevail in the winter of the higher latitudes the absolute humidity is very low, but, owing to the cold, the air is often damp. Cloudiness, as a rule, decreases inland, and with this lower relative humidity, more abundant sunshine and higher temperature, the evaporating power of a continental climate is much greater than that of the more humid, cloudier and cooler marine climate. Both amount and frequency of rainfall, as a rule, decrease inland, but the conditions are very largely controlled by local topography and by the prevailing winds. Winds average somewhat lower in velocity, and calms are more frequent, over continents than over oceans. The seasonal changes of pressure over the former give rise to systems of inflowing and outflowing, so-called continental, winds, sometimes so well developed as to become true monsoons. The extreme termperature changes which occur over the continents are the more easily borne because of the dryness of the air; because the minimum temperatures of winter occur when there is little or no wind, and because during the warmer hours of the summer there is the most air-movement.

[Illustration: FIG. 3.--Annual March of Air Temperature. Influence of Land and Water. (After Angot.)

M, Madeira. Bd, Bagdad. V, Valentia. N, Nerchinsk.]

_Desert Climate._--An extreme type of continental climate is found in deserts. Desert air is notably free from micro-organisms. The large diurnal temperature ranges of inland regions, which are most marked where there is little or no vegetation, give rise to active convectional currents during the warmer hours of the day. Hence high winds are common by day, while the nights are apt to be calm and relatively cool. Travelling by day is unpleasant under such conditions. Diurnal cumulus clouds, often absent because of the excessive dryness of the air, are replaced by clouds of blowing dust and sand. Many geological phenomena, and special physiographic types of varied kinds, are associated with the peculiar conditions of desert climate. The excessive diurnal ranges of temperature cause rocks to split and break up. Wind-driven sand erodes and polishes the rocks. When the separate fragments become small enough they, in their turn, are transported by the winds and further eroded by friction during their journey. Curious conditions of drainage result from the deficiency in rainfall. Rivers "wither" away, or end in sinks or brackish lakes.

Desert plants protect themselves against the attacks of animals by means of thorns, and against evaporation by means of hard surfaces and by a diminished leaf surface. The life of man in the desert is likewise strikingly controlled by the climatic peculiarities of strong sunshine, of heat, and of dust.

[Illustration: PLATE I. ANNUAL DISTRIBUTION OF TEMPERATURE AND PRESSURE.]

[Illustration: PLATE I. SEASONAL DISTRIBUTION OF TEMPERATURE AND PRESSURE.

TEMPERATURE JANUARY TEMPERATURE JULY PRESSURE & WINDS JANUARY PRESSURE & WINDS JULY]

_Coast or Littoral Climate._--Between the pure marine and the pure continental types the coasts furnish almost every grade of transition. Prevailing winds are here important controls. When these blow from the ocean, the climates are marine in character, but when they are off-shore, a somewhat modified type of continental climate prevails, even up to the immediate sea-coast. Hence the former have a smaller range of temperature; their summers are more moderate and their winters milder; extreme temperatures are rare; the air is damp, and there is much cloud. All these marine features diminish with increasing distance from the ocean, especially when there are mountain ranges near the coast. In the tropics, windward coasts are usually well supplied with rainfall, and the temperatures are modified by sea breezes. Leeward coasts in the trade-wind belts offer special conditions. Here the deserts often reach the sea, as on the western coasts of South America, Africa and Australia. Cold ocean currents, with prevailing winds along-shore rather than on-shore, are here hostile to rainfall, although the lower air is often damp, and fog and cloud are not uncommon.

_Monsoon Climate._--Exceptions to the general rule of rainier eastern coasts in trade-wind latitudes are found in the monsoon regions, as in India, for example, where the western coast of the peninsula is abundantly watered by the wet south-west monsoon. As monsoons often sweep over large districts, not only coast but interior, a separate group of monsoon climates is desirable. In India there are really three seasons--one cold, during the winter monsoon; one hot, in the transition season; and one wet, during the summer monsoon. Little precipitation occurs in winter, and that chiefly in the northern provinces. In low latitudes, monsoon and non-monsoon climates differ but little, for summer monsoons and regular trade-winds may both give rains, and wind direction has slight effect upon temperature.

The winter monsoon is off-shore and the summer monsoon on-shore under typical conditions, as in India. But exceptional cases are found where the opposite is true. In higher latitudes the seasonal changes of the winds, although not truly monsoonal, involve differences in temperature and in other climatic elements. The only well-developed monsoons on the coast of the continents of higher latitudes are those of eastern Asia. These are off-shore during the winter, giving dry, clear and cold weather; while the on-shore movement in summer gives cool, damp and cloudy weather.

_Mountain and Plateau Climate._--Both by reason of their actual height and because of their obstructive effects, mountains influence climate similarly in all the zones. Mountains as contrasted with lowlands are characterized by a decrease in pressure, temperature and absolute humidity; an increased intensity of insolation and radiation; usually a greater frequency of, and up to a certain altitude more, precipitation. At an altitude of 16,000 ft., more or less, pressure is reduced to about one-half of its sea-level value. The highest human habitations are found under these conditions. On high mountains and plateaus the pressure is lower in winter than in summer, owing to the fact that the atmosphere is compressed to lower levels in the winter and is expanded upwards in summer.

The intensity of insolation and of radiation both increase aloft in the cleaner, purer, drier and thinner air of mountain climates. The great intensity of the sun's rays attracts the attention of mountain-climbers at great altitudes. The vertical decrease of temperature, which is also much affected by local conditions, is especially rapid during the warmer months and hours; mountains are then cooler than lowlands. The inversions of temperature characteristic of the colder months, and of the night, give mountains the advantage of a higher temperature then--a fact of importance in connexion with the use of mountains as winter resorts. At such times the cold air flows down the mountain sides and collects in the valleys below, being replaced by warmer air aloft. Hence diurnal and annual ranges of temperature on the mountain tops of middle and higher latitudes are lessened, and the climate in this respect resembles a marine condition. The times of occurrence of the maximum and minimum temperature are also much influenced by local conditions. Elevated enclosed valleys, with strong sunshine, often resemble continental conditions of large temperature range, and plateaus, as compared with mountains at the same altitude, have relatively higher temperatures and larger temperature ranges. Altitude tempers the heat of the low latitudes. High mountain peaks, even on the equator, can remain snow-covered all the year round.