THE CHARACTERISTICS OF THE ZONES: III.—THE POLAR ZONES

General: Relation to Man, Animals, and Plants—Temperature—Pressure 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. 741⁄2° 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 summers 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_.

_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 are nearly or quite the lowest. Temperatures do not follow insolation in this case because much of the latter never reaches the earth’s surface; because most of the energy which does reach the surface is expended in melting the snow and ice of the polar areas; and also because the water areas are large, and the duration of insolation is short. Hence the mean annual temperatures in the vicinity of the poles must be very low.

A set of monthly isothermal charts of the north polar area, based on all available observations, was prepared by Mohn and published in the volume on _Meteorology_ of the Nansen Expedition. These charts give the most authentic information now at hand regarding Arctic temperatures. 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°, -40°, and -40° respectively.

The Siberian cold pole becomes a maximum of temperature during the summer, but the Greenland and polar minima remain throughout the year. In April the lowest isotherm, -22°, is in Greenland, and the north pole is then within the area enclosed by -18.4°. In July the temperature distribution shows considerable uniformity; 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°. Hence the statement frequently made, that no places in the northern hemisphere have mean temperatures below freezing in July, is not correct. In October the interior of Greenland is enclosed by -13°, and at the pole we find -11.2°. 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, but a local area of -7.6° is noted in Greenland, and one of -11.2° is centred at lat. 85° N. and long. 170° E. It will be seen that the temperatures are relatively lower towards the eastern sides of the great continents. The ordinary mean annual isothermal chart shows, within the Arctic circle, temperatures of 40° off the Norwegian coast and -5° beyond lat. 75° N., north of Asia and North America. The January chart shows 30° off the Norwegian coast, and -60° at Verkhoyansk, in Siberia. The July chart shows 60° over the continents, to 40° in extreme north-eastern Asia.

The north polar chart of annual range of temperature shows a maximum range of about 120° in Siberia; of 80° in North America; of 75.6° at the north pole, and of 72° in Greenland. The north pole obviously has a continental climate. The minimum ranges are on the Atlantic and Pacific oceans. The mean annual isanomalies show that the line of zero anomaly passes through the pole (as it must do). The interior of Greenland has a negative anomaly in all months. The Norwegian sea area is 45° too warm in January and February. Siberia has +10.8° in summer, and -45° in January. Between Bering Strait and the pole there is a negative anomaly in all months. The influence of the Gulf Stream drift is clearly seen on this chart, as it is also on that of mean annual ranges.

The mean temperatures of the higher northern latitudes in January, July, and for the year have been determined by Mohn with the following result:

MEAN TEMPERATURES OF THE HIGHER NORTHERN LATITUDES. 60° 65° 70° 75° 80° 85° Jan. +3.0° -9.4° -15.3° -20.2° -26.0° -36.6° July 57.4 54.3 45.1 38.1 35.6 32.5 Year 30.0 21.6 12.7 5.5 -0.6 -6.2

For the north pole itself, Mohn gives the following results, obtained by graphic methods:

MEAN TEMPERATURES OF THE NORTH POLE. Jan. Feb. Mar. Apr. May June July -41.8° -41.8° -31.0° -18.4° 8.6° 28.4° 30.2°

Aug. Sept. Oct. Nov. Dec. Year 26.6° 8.6° -11.2° -27.4° -36.4° -8.9°

It appears that the region about the north pole is the coldest place in the northern hemisphere for the mean of the year, and that the interior ice desert of Greenland, together with the inner polar area, are together the coldest parts of the northern hemisphere in July. In January, however, Verkhoyansk, in north-eastern Siberia, just within the Arctic circle, has a mean temperature of about -60°, while the inner polar area and the northern interior of Greenland have only -40°. Future exploration in the immediate vicinity of the north pole may show a lower January mean temperature there than at present appears. Such exploration will, moreover, certainly necessitate readjustment of the isothermal lines as now drawn for this polar area. It may be noted that the isotherm of 32° in January crosses the Arctic circle in the north-eastern Atlantic. Elsewhere it is south of this line. By December all land within the Arctic circle is below the freezing point. Thus far no minima as low as those of north-eastern Siberia have been recorded in the Arctic, and the Arctic maxima are much lower than those of Siberia. During the last Peary expedition, the winter of 1905-06 was distinguished by “comparatively high” temperatures.

Until about twenty years ago, the available observations of temperature in the Antarctic had been taken almost altogether in the summer months. Now we know a good deal about the winter temperatures around the margins of the southern circumpolar area as well. The mean temperatures of the higher latitudes of the southern hemisphere have been worked out by Meinardus, with the following results:

MEAN TEMPERATURES OF HIGH SOUTHERN LATITUDES

S. Lat. 60° 70° 80° 90°

January 37.0° 29.7° 24.3° 21.2° July 12.9° -7.6° -19.7° -27.9° Year 25.7° 9.0° -5.1° -13.0°

It is interesting to compare these temperatures with those given on page 159 for the corresponding latitudes of the northern hemisphere. The southern hemisphere is obviously colder than the northern in these higher latitudes. The climate of the south polar zone is thus distinctly continental in character, and has the additional unique feature of a remarkably cool summer, decidedly cooler than the summer of the Arctic polar area. Important factors in lowering the Antarctic temperatures are the Antarctic continent, with its great permanent cover of snow and ice, and the surrounding cold oceans, greatly chilled by the icebergs and floating ice-masses which have broken off from the margins of the south polar land mass. In the north polar area, on the other hand, warm ocean currents, flowing northward, keep much of the water at a relatively high temperature. There is no great permanently covered ice-mass, with the exception of Greenland. And warm winds from more southerly latitudes are able to carry their warmth far into the north polar zone.

The January isotherm of 32° roughly coincides with the Antarctic circle, or, speaking in a general way, the mean temperature of the warmest month is below freezing everywhere inside the Antarctic circle. At Framheim (lat. 78° S.) the mean temperature of the warmest month was 20°.8. Within the Arctic circle, on the other hand, mean temperatures above freezing are found, except, perhaps, in the interior of Greenland and around the north pole. R. C. Mossman has contrasted the summer temperatures of the northern and southern polar areas between longs. 10° E. and 50° W. in the following striking way. In the southern hemisphere, the isotherm of 35° F. nowhere extends beyond lat. 58° S.; in the northern, it reaches as far as lat. 81° N., in Spitzbergen. At long. 10° E. the summer isotherm of 34° F. is only about 550 miles from the north pole; on the same meridian in the southern hemisphere the distance is 2300 miles. The low mean and minimum temperatures of the south polar summer are responsible for much of the difficulty and danger of Antarctic exploration. They prevent much melting of snow and ice, and are monotonous and depressing.

The temperatures at the south pole itself furnish an interesting subject for speculation. Meinardus has determined them to be -13° for the mean annual; 21.2° for January and -27.9° for July. The corresponding figures for the north pole (Mohn) are -8.9°, -41.8° and 30.2°. On the basis of these figures, the south pole averages 9° cooler in midsummer; 13.9° warmer in midwinter, and 4.1° cooler for the whole year, than the north pole. It is not yet clear exactly where the real “cold pole” of the Antarctic is. It is probably not at the south pole itself.

The mean annual temperatures at regular stations in the Antarctic have as a whole averaged somewhere between about 25° and zero. At Framheim (lat. 78° S.) the mean annual was -13.4° (Hann), the lowest hitherto recorded anywhere in the world. The maxima at regular stations approach 50° and the minima have generally been in the vicinity of -40° to 45°. Lower readings have been made at some stations, and on sledge journeys they have been as low as -60°, -70°, and nearly to -75° (Amundsen). Even in midsummer, minima of -50°, and lower, have been observed. During his sojourn at the south pole, Amundsen found a mean temperature of between 8 and 9° F.

N. Z., Novaya Zemlya. F. J., Franz Joseph’s Land. G. L. Grinnell Land.]

The annual march of temperature in the north polar zone is peculiar in having a much retarded minimum, in February or even in March—the result of the long, cold winter. The temperature rises rapidly towards summer, and reaches a maximum in July. Autumn is warmer than spring. Winter comes on gradually, the summer slowly “falling asleep.” The north polar type of annual march of temperature is illustrated in the accompanying curves (see Fig. 33). In the Antarctic area, the mean temperatures of the winter months (April to September) are surprisingly uniform. The annual temperature curve is flattened. The coldest month is not nearly as emphatically so as is the case in the Arctic. Spring is apt to be warmer than autumn. The continents do not penetrate far enough into the Arctic zone to develop a pure continental climate in the highest latitudes. Verkhoyansk, in lat. 67° 6′ N., almost on the Arctic circle, furnishes an excellent example of 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. The diurnal period of temperature is noted during the time when the sun is visible, but is hardly, or not at all, perceptible during the dark season. During the latter, according to the _Fram_ observations, the day hours are usually colder than the night hours. This appears to be an effect of winds, for colder, northerly winds prevailed during the hours of daytime, and milder, southerly winds by night. Polar climate as a whole has large annual and small diurnal ranges, but sudden changes of wind may cause marked irregular temperature changes within twenty-four hours, especially in winter. The small ranges are associated with greater cloudiness, and _vice versa_. The mean diurnal variability is very small in summer, and reaches its maximum in winter, about 7° in February, according to Mohn.

_Pressure and Winds._ Owing to the more symmetrical distribution of land and water in the southern than in the northern polar area, the pressures and winds have a simpler arrangement in the former, and may be first considered. Recent Antarctic exploration has considerably modified some of the views which have been held regarding the general winds of the south polar area, and their controlling pressures. The rapid southward decrease of pressure, which is so marked a feature of the higher latitudes of the southern hemisphere on the isobaric charts of the world, does not continue all the way to the south pole. Nor do the prevailing westerly winds, constituting the “circumpolar whirl,” which are so well developed over the southern portions of the southern hemisphere oceans, blow all the way home to the south pole. The steep poleward pressure gradients of these southern oceans end in a trough of low pressure, girdling the earth between lats. 60° and 70° S. From here, the pressure increases again towards the south pole, where a permanent inner polar anticyclonic area is found. This low pressure trough is an important wind divide. Strong, damp, cloudy, and snow-bearing easterly winds have been reported from stations along the coasts of the Antarctic continent. These, according to Meinardus, are cyclonic winds and blow from lower latitudes into the cyclonic depressions whose paths are along the sub-Antarctic low pressure trough. The relatively high temperature and humidity of these winds are thus explained by their coming from warmer latitudes, and from the ocean. They are not dry, as they should be if they were an anticyclonic outflow immediately from a south polar barometric maximum over the interior of the Antarctic continent. The limits between the “prevailing westerlies” of the southern hemisphere and the easterly winds flowing out from the pole vary with the longitude and with the season. The change in passing from one wind régime to the other has been reported upon by several Antarctic observers. The prevalence of outflowing winds from the inner south polar area is evidenced by the direction of the ice ridges on the polar plateau.

Meinardus has shown that the Antarctic anticyclone is shallow, and probably does not extend upward even to the height of the land itself. Above about 6500 feet (2000 metres), over this anticyclone, with its outflowing surface winds, there is a polar cyclone, with an inflowing system of air currents from north and west, _i. e._, a westerly circumpolar whirl. Observations of the smoke-drift from Mt. Erebus, and of the upper clouds, confirm this view. These upper currents, coming from the vast ocean area surrounding the Antarctic continent, bring in an abundance of moisture which feeds the continental ice-cap, supplying the loss due to the formation of icebergs, and to evaporation. The effect of the Antarctic anticyclone is thus limited to the inner and lower portions of the Antarctic continent. The upper portions are in a zone of lower pressure and of westerly winds. Within the area occupied by the anticyclone the winds, according to Meinardus, are prevailingly easterly with southerly components. Were there no elevated land masses around the south pole, winds blowing out from the pole, and from the east, would probably prevail throughout the high pressure area. Under certain conditions the winds from the Antarctic continent may develop high velocity and take on typical foehn characteristics, raising the temperature to an unusually high degree. These foehn winds have been a conspicuous feature at several Antarctic stations, especially in winter, when the high temperatures are most noticeable. They are also known on both coasts of Greenland, when a passing cyclonic depression draws the air down from the icy interior. These Greenland foehn winds are important climatic elements, for they blow down warm and dry, raising the temperature even 30° or 40° above the winter mean, and melting the snow.

In the Arctic area the wind systems are less clearly defined, and the pressure distribution is much less regular on account of the irregular distribution of land and water. The mean pressure as a whole increases from lat. 60° N. toward the north pole. North-easterly, _i. e._, outflowing, surface winds occur in the vicinity of the pole itself. The isobaric charts published in the report of the Nansen Expedition show that the North Atlantic low pressure area is more or less well developed in all months. In winter, it greatly extends its limits east into the inner Arctic Ocean, to the north of Russia and Siberia. Between May and August it is much less well developed. The Pacific minimum of pressure is found south of Bering Strait and in Alaska. Between these two regions of lower pressure, the divide extends from North America to eastern Siberia. This divide has been called by Supan the “_Arktische Windscheide_.” High pressures are found in North America and in Siberia from September to March, the maxima being in Asia. The belt of somewhat lower pressure connecting these two maxima is situated between Bering Strait and the north pole. In July and August the maximum pressure is between Greenland and somewhat east of Spitzbergen. The pressure gradients are steepest in winter. At the pole itself, pressure seems to be highest (about 30.079 ins.) in April, and lowest (29.882 ins.) from June to September. The annual range is therefore only about 0.20 in.

The prevailing westerlies, which in the high southern latitudes are so symmetrically developed, are interfered with to such an extent by the varying pressure controls over the northern continents and oceans, in summer and winter, that they are often hardly recognisable on the wind maps. The isobaric and wind charts prepared by Buchan show that on the whole the winds blow out from the inner polar basin, especially in winter and spring. During his last expedition, in the winter of 1905-06, Peary reports “every few days we had violent winds from the south—sometimes in the shape of squalls of a few hours’ duration, sometimes continuing as furious gales for two or three days.” During a westerly gale of six days’ duration (lat. 85° 12′ N.) Peary and members of his party drifted some seventy miles to the eastward on the ice.

In the European and North American polar areas the annual march of pressure gives a spring maximum, in April and May, and a minimum in January or February. The daily fluctuations in pressure in these circumpolar latitudes are about twice as large in winter as in summer.

_Rain and Snow._ Rainfall on the whole decreases steadily from equator to poles. The amount of precipitation must of necessity be comparatively slight in the polar zones (15-10 ins., and less), chiefly because of the small capacity of the air for water vapour at the low temperatures there prevailing; partly also because of the decrease, or absence, of local convectional storms and thunder-showers.[25] Even cyclonic storms cannot yield much precipitation. The polar zones, therefore, have a permanent deficiency of precipitation. Their deserts of snow and ice are climatic deserts in more senses than one. These extended snow and ice fields naturally 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 snow fields, ice sheets, and glaciers are produced. Nansen is of the opinion that the amount of condensed vapour, much of it being in the form of “frozen fog” (hoar frost) and not readily measurable, exceeds evaporation in the polar districts.

[25] Locally, under exceptional conditions, as in the case of the western coast of Norway, the rainfall is a good deal heavier.

The commonest form of precipitation is naturally snow, the summer limit of which, in the northern hemisphere, is near the Arctic circle, with the exception of Norway. In lat. 70° N., at Boothia Felix, 40 per cent. of the precipitation from June to August comes in the form of snow. So far as exploration has yet gone into north polar latitudes, rain seems to fall in summer. It is doubtful whether there are places near sea-level where _all_ the precipitation is snow. It is also uncertain whether any mountains reach a height where nothing but snow falls. Von Drygalski believes that the inland ice-cap of Greenland, over 2600 feet above sea-level, meets these conditions. The interior of the Antarctic continent probably never has rain. The snow of the polar regions is characteristically fine and dry. Schwatka has pointed out that the snow huts of the Eskimos could not be built with the kind of snow that falls in the United States. At low polar temperatures _flakes_ of snow are not found, but precipitation is in the form of ice spicules. The finest glittering ice-needles (“diamond dust”) often fill the air, even on clear days and in calm weather, and, gradually descending to the surface, slowly add to the depth of snow on the ground. Dry snow is also blown up from the snowfields on windy days, interfering with the transparency of the air. Snowfalls at temperatures of -40°, and even below, have been reported from eastern Siberian and Arctic stations. It is probable that under these conditions the air is warmer aloft.

_Humidity, Cloudiness, and Fog_. The absolute humidity must be low in polar latitudes, especially in winter, on account of the low temperatures. 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 much cloudiness in the higher latitudes of the temperate zones (see table, p. 116). In polar climates, the summer generally has the maximum cloudiness; the winter is clearer. The data and curve given below illustrate these conditions (see Fig. 34). In the Antarctic the stations at the margins of the polar anticyclone have the clearest skies. The summer maximum is largely due to fogs, which are produced where relatively warm, damp air is chilled by coming in contact with colder air or with ice. Open water in the midst of an ice-covered sea is often detected at a distance by means of the “steam fog” which rises from it. The amount of cloudiness naturally varies considerably with the location of the station, and with the prevailing winds, as is the case in warmer latitudes. Thus the coasts and islands bathed by the warm waters of the Gulf Stream drift usually have a higher cloudiness in winter than in summer. The place of fog is in winter taken by the 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.

ANNUAL MARCH OF CLOUDINESS IN POLAR LATITUDES. MARINE TYPE. (Seven stations. Lat. 70° N.)

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year 7.1 6.9 6.4 7.0 7.7 8.3 8.5 8.2 8.0 8.0 6.8 6.6 7.5

ZONE: MARINE TYPE]

Our knowledge concerning cloud forms and movements in the polar zones has increased rapidly during recent years, especially for the Antarctic. The records of the Nansen expedition show a greater cloudiness by day, and with stronger winds. Cumulus forms are rare, even in summer, and it is doubtful whether this cloud occurs at all in its most typical development. Clearly defined cloud forms have been reported by some observers to be very rare indeed in the Arctic, especially in the winter sky. On the other hand, Lieutenant Royds, of the _Discovery_, reports that he never saw such striking and beautiful examples of every kind of cloud as within the Antarctic circle. At Griffith Island, in the north polar zone, two months passed without clouds. And “day after day, with glorious clear skies and continuous sunshine” is reported by the _Discovery_ in the Antarctic. Stratus is probably the commonest cloud of high latitudes, often covering the sky for days without a break. In place of well-developed cloud forms, the air is filled with fog in summer, which often grows into poorly defined stratus clouds. Cirrus forms follow stratus in frequency. Nansen’s results give an average cloud movement from W. N. W. and N. W. In the Antarctic, the upper clouds come from westerly directions. The _Belgica_ expedition noted cirrus from the east in summer only. At the South Orkneys, cirrus was observed at altitudes of 6000 to 8000 feet.

_Cyclones and Weather._ The prevailing westerlies continue up into the margins of the polar zones. Many of their cyclonic storms—the weather controls of temperate latitudes—also continue on to the polar zones, giving sudden and irregular pressure and weather changes. The inner polar areas seem to be beyond the reach of frequent and violent cyclonic disturbance. Calms are more common; the weather is quieter and fairer; precipitation is less. Mostof the observations thus far obtained from both polar zones naturally come from this marginal zone of greater cyclonic activity, violent winds, and snowy, disagreeable, inhospitable weather, and therefore do not show the features of the actual polar climate.

A very thorough study of cyclonic movements in the highest latitudes was made in connection with the Nansen expedition in the _Fram_. During the three years of her drift, depressions passed on all sides of her, with a preponderance on the west. The direction of progression averaged nearly due east, and the hourly velocity twenty-seven to thirty-four miles, which is about that in the United States. The rainy winds were usually S. and S.E., while N.E. and N.W. were least likely to bring rain or snow. For the higher latitudes, most of the cyclones must pass by on the equatorial side of the observer, giving “backing” winds in the northern hemisphere. The main cyclonic tracks are such that the wind characteristically backs in Iceland, and still more so in Jan Mayen and on the eastern coast of Greenland, these districts lying on the north and west of the path of progression. Frightful winter storms occasionally occur along the east coast of Greenland and off Spitzbergen. During the drift of the _Fram_ the southerly winds were the warmest in winter and the northerly the coldest, showing that, at the 82d parallel of latitude, the Siberian cold pole ceases to have much influence.

For much of the year in the polar zones the diurnal control is weak or absent. The successive spells of stormy or of fine weather are wholly cyclonically controlled. Extraordinary records of storm and gale have been brought back from the far south and the far north. The Swedish Antarctic expedition, for example, under Nordenskjöld, in 1902-03, experienced for five months, beginning in May, a period of storms with short intermissions never exceeding three days, and during all of this period the average wind velocity was twenty-three miles an hour, and for a fortnight it averaged forty-five miles. The _Discovery_ reported a gale on July 19, 1902, which lasted ten hours with a velocity of eighty-five miles an hour. The Mawson expedition brought back from the coast of Wilkes Land a record of an average wind velocity throughout the year of nearly fifty miles an hour. Hourly velocities of one hundred miles were not uncommon. This particular locality seems at present to possess the unenviable reputation of being the windiest region in the world. It is truly the “home of the blizzard.” Over the marginal area between the Antarctic continent and its surrounding oceans is a storm belt which, for frequency and intensity of its cyclones, is unequalled. Gales, and snows, and cold: these are the dominant climatic features.

During a northern polar winter the average thickness of ice formed over the oceans, where no storms or strong tides interfere, reaches six feet and more. Nansen found a thickness of over eight feet in one year. During the long summer days the temperature rises well above the winter mean, and under favourable conditions certain phenomena, such as the diurnal variation in wind velocity, for example, give evidence of the diurnal control. But the irregular cyclonic weather changes continue, in a modified form. There is no really warm season. Snow still falls. The summer is essentially only a modified winter. Its clear spells are relatively warm, and winds bring lower temperatures. In spite of its lack of high temperatures, the northern polar summer, near the margins of the zone, has many attractive qualities in its clean, pure, crisp, dry air, free from dust and impurities; its strong insolation; its slight precipitation. The prevalent summer fogs are a serious disadvantage.

_Twilight and Optical Phenomena._ The monotony and darkness of the polar night are decreased a good deal by the long twilight, due to the high degree of refraction at low temperatures. The sun actually appears and disappears some days before and after the times which are geometrically set. Light from moon and stars, and from the aurora, also relieves the darkness. Optical phenomena of great variety, beauty, and complexity are common. Solar and lunar haloes and coronas, and mock suns and moons are often seen. Auroras seem to be less common and less brilliant in the Antarctic than in the Arctic. Sunset and sunrise colours within the polar zones are described as being extraordinarily brilliant and impressive.