ARCTIC AND ANTARCTIC WEATHER
Geographically, the arctic zone is north of theArctic Circle (66.5°N) and the antarctic zone is south of Antarctic Circle (66.5°S). The Arctic is extremely important to the military defense of Canada and the United States and is the subject of ever-increasing military opera-tions. Therefore, Aerographer’s Mates must familiarize themselves with the prevailing weather and peculiarities of these regions.
The Arctic is the aerial crossroads of theworld. This is not only due to the shorter arctic routes between some of the major cities of the world, but also because flying weather over the Arctic is generally better than that encountered over the familiar ocean routes.
To understand some of the important weatherand problems of the Arctic, you must understand the broad underlying causes of the arctic climate.
SEASONAL TEMPERATURE VARIA-TIONS.— From our previous discussion of climatic controls, we have seen that the most important factor that determines the climate of an area is the amount of energy it receives fromthe Sun. During the winter much of the Arctic receives little or no direct heat from the Sun. The cold winter temperatures common in the Arctic result from a lack of the Sun’s energy.
The Sun is not the only factor responsible forthe arctic climate. Two other factors, the land-sea- ice distribution and mountain barriers, con-tribute to the tremendous variation in climate at different points of similar latitude.
1. Land-sea-ice features. In the NorthernHemisphere, the water features include the Arctic, North Atlantic, and North Pacific oceans. These bodies of water act as temperature moderators since they do not have large temperature variations. A major exception occurs when large areas are covered by ice in winter. The land features are the northern continents of Eurasia, North America, the island of Greenland, and the Canadian Archipelago. As opposed to the water areas, the land areas tend to show the direct results of the extremes of seasonal heating and cooling by their seasonal temperature variations.
2. Mountains. The arctic mountain ranges ofSiberia and North America are factors which con-tribute to the climate and air-mass characteristics of the regions. These mountain barriers, as in mid-latitudes, restrict the movement of air from west to east. During periods of weak circulation, the air is blocked by the ranges and remains more or less stagnant over the area. It is during these periods that the air acquires the temperature and moisture characteristics of the underlying surface. Thus, these areas are air-mass source regions, and they are particularly effective as source regions during the winter when the surface is covered with snow and ice.
The Greenland ice cap is essentially a moun-tainrange more than 10,000 feet above mean sea level. It restricts the movement of weather systems, often causing low-pressure centers to move northward along the west coast of Greenland. Some of the largest rates of falling pressure in the world (other than hurricanes and tornadoes) are recorded here. The deep, low centers that move along the west coast of Greenland are primarily responsible for the high winds that are recorded occasionally in that area. At times, winter temperatures in the Attic are unusually high. This situation is brought about by deep, low centers moving into the Arctic, coupled with compression of air (the Foehn effect) as it often blows down off the sloping edges of the ice caps, primarily the Greenland ice cap.
ARCTIC AIR MASSES.—The moisture con-tent of air masses that originate over land is low at all altitudes in the winter. The distinction between air masses almost disappears during the summer because of the nearly uniform surface conditions over the arctic and subpolar regions. The frozen surface thaws under the influence of lengthened or continual daylight, the snow melts from the glaciers and pack ice, the ice melts in the lake areas in the Arctic, and the water areas of the polar basin increase markedly. Thus, the polar area becomes mild, humid, and semimaritime in character. Temperatures are usually between freezing and 50°F. Occa-sionally, strong disturbances from the south in-crease the temperature for short periods. Daily extremes, horizontal differences, and day-to-day variabilities are slight.
During the winter months, air masses areformed over areas that are completely covered by ice and snow. The air masses are characterized by very cold surface air and a large temperature inversion in the lowest few thousand feet. Since the amount of moisture the air can hold depends on the air temperature, the cold arctic air is very dry (low absolute humidity). The air mass that originates over oceans does not have a surface temperature inversion in the winter, the surface air temperature is warmer, and there is a cor-responding increase in the moisture content of the air. It is during movement inland of moist air from the warmer waters that most of the rather infrequent arctic cloudiness and precipita-tion occurs during this season.
During the summer months, the large expanse of open water and warmer temperatures in the Arctic result in increased moisture. Conse-quently, the largest amount of cloudiness and precipitation occurs during these summer months.
ARCTIC FRONTS.—The weather associated with fronts in the Arctic has much the same cloud structure as with polar fronts, except that the middle and high cloud types are generally much lower, and the precipitation is usually in the form of snow.
Periods of maximum surface wind usuallyoccur during and just after a frontal passage. This strong wind flow often creates hazards, such as blowing snow and turbulence, which make opera-tional flying difficult.
The best flying weather in the Arctic over landusually occurs in midsummer and midwinter; the worst (low ceilings and risibilities) is during the transitional periods between the two seasons. Winter is characterized by frequent storms and well-defined frontal passages, but because of the dryness of the air, cloudiness and precipitation are at a minimum. In the summer, there are fewer storm passages and fronts are weaker; however, the increased moisture in the air results in more widespread clouds and precipitation. Over the sea areas the summer weather is very foggy, but winds are of lower speeds than in the winter.
During the transitional periods of spring andfall, operational flying conditions are usually the worst. Frontal systems are usually well defined, active, and turbulent. Icing may extend to high levels.