CHAPTER VI

THE CHARACTERISTICS OF THE ZONES: III.—THE

POLAR ZONES

General: Relation to Man, Animals, and Plants-TemperaturePressure and Winds-Rain and Snow-Humidity, Cloudiness, and Fog-Cyclones and Weather-Twilight and Optical Phenomena-Physiological Effects.

General: Relation to Man, Animals, and Plants. The temperate zones merge into the polar zones at the Arctic and Antarctic circles or, if temperature is used as the basis of classification, at the isotherms of 50° for the warmest month, as suggested by Supan. The frequent use of maps on the Mercator projection tends to give us an exaggerated idea of the size of the polar zones. When limited by the polar circles, these zones occupy but 0.08 of the surface of each hemisphere, the whole area being 1.00. Astronomically they are distinguished by the fact that at all places within them the sun is above the horizon at least one full twenty-four hours each year, and below it the same length of time. This longer or shorter absence of the sun gives the climate a peculiar character, not found elsewhere. At the poles, the day

and the year are alike. These zones obviously have the most oblique insolation.

Our knowledge of polar climatology has advanced very rapidly in the last two decades. The fragmentary records of the earlier expeditions gave scattering information about the weather. The longer and more complete records of recent expeditions give much more accurate and satisfactory results. It is now becoming possible to see more clearly what the climatic conditions really are. But no satisfactory presentation of polar climatology is yet possible. We are still dealing with the meteorology of the polar zones, rather than with their climates. More is known of the Arctic than of the Antarctic. The admirable report, by the late Dr. Henrik Mohn, on the results of the Nansen North Polar expedition, embracing three years' observations, discussed with great care, and well illustrated by curves and charts, is a monumental piece of work. From the Antarctic, in recent years, English, Scotch, French, Swedish, Belgian, Norwegian, Australian, and German expeditions have brought back a great wealth of valuable meteorological material, the observations in several cases covering a whole year and also including records obtained by means of balloons. These data, which have been critically studied and compared, have made possible the construction of meteorological charts, and even of daily weather maps, for the far southern latitudes, hitherto largely a blank on our maps.

Beyond the isotherm of 50° for the warmest month,

forest trees and cereals do not grow. In the northern hemisphere this line is well north of the Arctic circle in the continental climate of Asia, and north of it in north-western North America. It is north of it also in northern Scandinavia, but falls well south in eastern British America, Labrador, and Greenland, and also in the North Pacific Ocean. In the southern hemisphere this isotherm crosses the southern extremity of South America, and runs nearly east and west around the globe.

In the Arctic climate, vegetation must make rapid growth in the short, cool summer. In the highest latitudes the summer temperatures are not high enough to melt snow on a level. Exposure is therefore of the greatest importance. Arctic plants grow and blossom with great rapidity and luxuriance where the exposure is favourable, and where the water from the melting snow can run off. The soil then dries quickly, and can be effectively warmed. On the other hand, when the water stands, it may freeze again and again, and the soil underneath has no opportunity to warm. Of Novaya Zemlya Baer has reported that the level surfaces are polar deserts, while the slopes at the foot of the mountains, unless covered with boulders, are like gardens in summer. Protection against cold winds is another important factor in the growth of this vegetation. Over great stretches of the northern plains the surface only is thawed out in the warmer months, and swamps, mosses, and lichens are found above eternally frozen ground.

Trees often grow in favourable conditions along streams when the intervening plains are typical tundras. Direct insolation is very effective in high latitudes. Where the exposure is favourable, snow melts in the sun even when the temperature of the air in the shade is far below freezing. It has been reported that at Assistance Bay (lat. 7412° N.), in March, when the air temperature was about 25°, snow near stones and other dark objects melted in the sun. Even the mean daily temperature of the snow surface may be higher than the air temperature. The injurious effect of polar climate upon vegetation, especially upon trees, has been attributed by Kihlmann to an insufficient water-supply furnished by the roots deep in the cold ground. From the upper parts of the tree, exposed to sunshine and wind, evaporation proceeds rapidly, and the tree dries up. Protective devices against excessive evaporation, not unlike those of desert plants, are found.

Arctic and Antarctic zones differ a good deal in the distribution and arrangement of land and water around and in them. The southern zone is surrounded by a wide belt of open sea; the northern, by land areas. The northern is therefore much affected by the conditions of adjacent continental masses. Nevertheless, the general characteristics are apparently much the same over both, so far as is now known, the Antarctic differing from the Arctic chiefly in having colder summers, and in the regularity of its pressure and winds. The cold Antarctic sum

mers are the chief cause of the poverty of the Antarctic flora. Both zones have the lowest mean annual temperatures in their respective hemispheres, and hence may properly be called the cold zones.

FIG. 30. JANUARY NORTH POLAR ISOTHERMS

Temperature. At the solstices, the two poles receive the largest amounts of insolation which any part of the earth's surface ever receives. It would seem, therefore, that the polar temperatures should then be the highest in the world, but as a matter of fact they