CHARACTERISTICS OF THE POLAR ZONES The temperate zones merge into the polar zones at the Arctic and Antarctic circles, or, if temperature be used as the basis of classification, at the isotherms of 5o°F for the warmest month, as suggested by Supan. The longer or shorter absence of the sun gives the climate a peculiar character. Beyond the isotherm of 5o°F 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 also in north-western North America and in northern Scandinavia, but falls well south in eastern British America, Labrador and Green land, and also in the north Pacific Ocean. In the southern hemis phere this isotherm crosses the southern extremity of South Ameri ca, and runs fairly east and west around the globe. There is a minimum of life, but more in the north polar than the south polar zone. Plants and land animals are few. Farming and cattle raising cease. Population is small and scattered. Man seeks his food chiefly in the sea and lives near the sea-coast. There are no per manent settlements at all within the Antarctic circle. Interior lands are deserted. Arctic vegetation must make rapid growth in the short, cool summer. In the highest latitudes summer temper atures are not high enough to melt snow on a level. Arctic plants grow and blossom rapidly and luxuriantly where the exposure is favourable, and the water from melting snow can run off ; the soil then dries quickly and can be effectively warmed. Protection against cold winds is another important factor. Over great stretches of the northern plains the surface only is thawed out in the warmer months, and swamps, mosses and lichens are found over perpetually frozen ground.
At the solstices the two poles receive the largest amounts of insolation which any part of the earth's sur face ever receives. Temperatures do not follow insolation in this case because much of the latter never reaches the earth's surface and because much energy which does peach the surface is expended in melting snow and ice of the polar areas. In the winter months there are three cold poles, in Siberia, in Greenland and at the pole itself. In January the mean temperatures at these three cold poles are —49°,
and —40° F respectively. The Siberian cold pole becomes a maximum of temperature during the summer, but the Greenland and polar minima remain throughout the year. In July the temperature distribution shows considerable uniform ity ; the gradients are relatively weak. A large area in the interior of Greenland, and one of about equal extent around the pole, are within the isotherm of 32°. For the year a large area around the pole is enclosed by the isotherm of —4°, with an isotherm of the same value in the interior of Greenland. The annual range of temperature is about '20' in Siberia, 80° in North America, 76° at the North Pole and 72° in Greenland. The North Pole obviously has a continental climate.
For the Antarctic our knowledge is still very fragmentary, and relates chiefly to summer months. Meinardus has determined mean temperatures of higher southern latitudes as follows :— Mean Temperatures of High Southern Latitudes S Lat 50 60 7O
Mean Annual . . . 41.9F 24.6F 8.1F —12.5F January . . . .
29.7 18.7 July
13-5 — I I.0
The whole southern hemisphere is colder than the northern. Antarctic summers are decidedly cold. Mean annual temperatures are about i o°, and minima of an ordinary Antarctic winter go to
and below, but so far no minima of the severest Siberian intensity have been noted. Maxima have varied between 35° and 50°. Temperatures at the South Pole itself suggest interesting speculations. It is likely that near the South Pole will prove to be the coldest point on the earth's surface for the year, as the distribution of insolation implies and as the conditions of land, ice and snow there suggest. The lowest winter and summer temperatures in the southern hemisphere will probably be found near the pole. Isotherms in the antarctic region bend polewards and equatorwards at different meridians, although much less so than in the Arctic.
The annual march of temperature in the north polar zone, for which we have the best comparable data, is peculiar in having a much retarded minimum in February or even in March—the re sult of the long cold winter. Temperature rises rapidly towards summer, reaching a maximum in July. Autumn is warmer than spring. The continents do not penetrate far enough into the Arctic zone to develop a pure continental climate in the highest latitudes. Verkhoyansk, latitude 67° 6'N., has an exaggerated continental type for the margin of the zone, with an annual range of 120°. One-third as large a range is found on Novaya Zemlya. Polar climate as a whole has large annual and small diurnal ranges, but sudden changes of wind may cause marked irregular temperature changes especially in winter. The smaller ranges are associated with greater cloudiness, and vice versa.
Pressure and winds follow a simpler scheme in the southern than in the northern hemisphere, because of the large extent of ocean. The southward decrease of pressure in temperate latitudes, previously described, ceases in about 6o°S., where a trough of low pressure encircles the globe, widen ing somewhat over the Ross and Weddell Seas, while still farther south pressure rises again. On the northern side of this trough westerly winds prevail, on the southern side easterly winds. Over the Antarctic continent itself periods of calm, and, in winter, intense cold alternate with periods of high winds, which often develop into blizzards. Blizzards are not equally developed on all parts of the Antarctic coast, their frequency depending on the local topography and distribution of pressure; they are especially numerous in Adelie Land. Winds of any force sweep away the surface layer of cold air and cause a marked rise of temperature. They whirl up the loose surface snow but are also accompanied by fresh snow, and visibility is limited to a few yards.
In the Arctic area wind systems are less clearly defined and pressure distribution is much less regular, on account of irregular distribution of land and water. The North Atlantic low-pressure area is more or less well developed in all months. Except in June, when it lies over Southern Greenland, this tongue-shaped trough of low pressure lies in Davis strait, to the south-west or west of Iceland, and over the Norwegian sea. In winter it greatly extends its limits farther east into the inner Arctic ocean, to the north of Russia and Siberia. The Pacific minimum of pressure is found south of Bering strait and in Alaska. Between these two regions a ridge of higher pressure extends from North America to eastern Siberia. This has been called by Supan the " Arktische Wind scheide." Pressure gradients are steepest in winter. The prevailing westerlies which in the high southern latitudes are so symmetri cally developed, are interfered with by the varying pressures over the northern continents and oceans in summer and winter. Iso baric and wind charts show that on the whole winds blow out from the inner polar basin, especially in winter and spring.
The amount of precipitation is compar atively slight in the polar zones, chiefly because of the small capacity of the air for water vapour at the low temperatures there prevailing; partly also because of the rarity of local convectional storms and thunder-showers. Even cyclonic storms cannot yield much precipitation. Extended snow and ice fields tend to give an exaggerated idea of the actual amount of precipitation. It must be remembered, however, that evaporation is slow at low temperatures, and melting is not excessive. Hence the polar store of fallen snow is well preserved ; interior snowfields, ice sheets and glaciers are produced. The commonest form of pre cipitation is naturally snow, the summer limit of which, in the northern hemisphere, is near the Arctic circle, with the exception of Norway. So far as exploration has yet gone into the highest northern latitudes, rain falls in summer, but probably over most of the Antarctic continent rain never falls. The snow of the polar regions is characteristically fine and dry. At low polar tempera tures flakes of snow are not found, but precipitation is in the form of ice spicules. The finest glittering ice needles often fill the air even on clear days, and in calm weather, and, gradually descend ing to the surface, slowly add to the depth of snow on the ground. Dry snow is also blown from the snowfields on windy days, inter fering with the transparency of the air.
Fog.—Absolute humidity is low in polar latitudes, especially in winter, on account of low tempera tures. Relative humidity varies greatly, and very low readings have often been recorded. Cloudiness seems to decrease somewhat towards the inner polar areas, after passing the belt of high cloudi ness in the higher latitudes of the temperate zones. In marine climates of high latitudes summer has the maximum cloudiness; winter is clearer. The summer maximum is associated with fogs, produced where warm, damp air is chilled by contact with ice. They are also formed over open waters, as among the Faroe islands, for example, and open water spaces, in the midst of an ice-covered sea, are commonly detected at a distance by means of "steam fog." Fogs are less common in winter, when they occur as radiation fogs, of no great thickness. The small winter cloudi ness, reported also from the Antarctic zone, corresponds with low absolute humidity and small precipitation. Coasts and islands bathed by warm water of the Gulf Stream drift are usually more cloudy in winter than in summer. The place of fog is in winter taken by fine snow crystals, which often darken the air like fog when strong winds raise the dry snow from the surfaces on which it is lying. Cumulus cloud forms are rare, even in summer. Stratus is probably the commonest cloud of high latitudes, often covering the sky for days without a break.
The north polar summer, in spite of its lack of warmth, is in some respects pleasant and healthful, but the polar night is monotonous, depressing and repelling. Sir W. E. Parry said that it would be difficult to conceive of two things which are more alike than two polar winters. An everlast ing uniform snow covering, rigidity, lifelessness and silence— except for the howl of the gale or the cracking of the ice. Small wonder that the polar night has sometimes unbalanced men's minds. The first effects are often a strong desire for sleep and indifference. Later effects have been sleeplessness and nervous ness, tending in extreme cases to insanity, anaemia and digestive troubles. Extraordinarily low winter temperatures are easily borne if the air be dry and still. Zero weather seems pleasantly refreshing if clear and calm. But high relative humidity and wind —even a light breeze—give the same degree of cold a penetrating feeling of chill which may be unbearable. Large temperature ranges are endured without danger in the polar winter when the air is dry. Under direct insolation skin burns and blisters; lips swell and crack. Thirst has been much complained of by polar explorers, and is due to the active evaporation from the warm body into the dry, relatively cold air. Polar air is singularly free from micro-organisms owing to lack of communication with other parts ; hence many diseases common in temperate zones, "colds" among them, are rare.
The climatic divisions of the world appear to be constant in their broad outlines, but are subject to minor fluctuations which have a duration from a few years up to many centuries. Some known fluctuations of climate have been irregular, but others recur with some regularity. The best known of these meteorological periodicities are the i 1-year sunspot cycle, best shown in the tropics, and the Bruckner cycle, of about
35 years, in temperate regions. In the geological past there have
been far more extensive changes (see e.g., CLIMATIC HISTORY).
BIBLIOGRAPHY.—Scientific climatology is based upon numerical Bibliography.—Scientific climatology is based upon numerical results, obtained by systematic, long continued, accurate meteorologi cal observations. The essential part of its literature is therefore found in the collection of data published by the various meteorological services. The only comprehensive text-book of climatology is the Handbuch der Klimatologie of the late J. von Hann (Stuttgart, 19o8 I I) . The first volume deals with general climatology, and has been translated into English (London and New York, 1903). Reference should be made to this book for further details than are here given. The second and third volumes are devoted to the climates of the different countries of the world. Koppen's Die Klimate der Erde and Kendrew's Climates of the Continents are valuable reference books. The standard meteorological journal of the world, the Meteor ologische Zeitschrift (Braunschweig, monthly), is indispensable. The Quarterly Journal of the Royal Meteorological Society (London), the Meteorological Magazine (London), and the Monthly Weather Review (Washington, D. C.) are also valuable. The best collection of charts is that in the Atlas of Meteorology (London, 1899) which gives an excellent working bibliography. See also the bibliography issued by the Royal Meteorological Society.
