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Values of Daily Insolation at the Upper Limit of the Earth’s Atmosphere and at Sea-Level.

Lat. Upper Limit of Atmosphere. Earth’s Surface.
 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 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.

EB1911 - Climate Fig. 2.—Supan’s Temperature Zones.jpg
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<a name="FnAnchor_1b" id="FnAnchor_1b" href="#Footnote_1b">1</a> (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