Polar Circle

The latitude line at 66 degrees 33 minutes, and the area on the pole side of this latitude line, is called the Arctic Circle in the Northern Hemisphere and the Antarctic Circle in the Southern Hemisphere. This area centered on the pole is also called the polar region. The area from the polar circle to high latitudes is also called the cold zone in mathematical climate zones. Because of the high latitude, the altitude of the sun is low and the amount of direct solar radiation reaching the earth's surface is small, so it is cold. In the Köppen climate classification, it can refer to an area where the average temperature of the warmest month is below 0°C, or to an area where the average temperature of the warmest month is below 10°C, which is a climate where trees cannot grow. There is night all day for half the year in winter, and radiation cooling continues. Therefore, the annual temperature difference between the daytime half of the year in summer and the nighttime half of the year in winter is large, while the daily temperature difference is small. Generally, low and dry high pressure is formed in the lower atmosphere due to radiation cooling, but low pressure is formed throughout the year in the middle and upper troposphere, and is called the polar vortex. The phenomenon of surface pressure fluctuating in the opposite direction between the Arctic and the mid-latitude areas is called the Arctic Oscillation. It has been pointed out that the Arctic Oscillation is related to cold waves, heavy snowfall, and abnormally high temperatures in mid- and high-latitude regions, as well as the destruction of the stratospheric ozone layer in the polar regions. Most of the occurrences are in anecmene (uninhabited areas), but their importance is increasing in terms of underground resources, military, and air routes. In addition, it was discovered that the cause of the ozone hole, which causes the ozone layer to thin in the stratosphere above, is chlorofluorocarbons released as a result of human activity, which led to the conclusion of an international treaty to reduce greenhouse gases such as chlorofluorocarbons.

[Shuji Yamashita]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

緯度66度33分の緯線、またこの緯線から極側の地域をいい、北半球では北極圏、南半球では南極圏とよぶ。極を中心としたこの地域は極地という呼び方もされる。極圏から高緯度地域を数理的気候帯では寒帯ともいう。高緯度のため太陽高度は低く地表に到達する直達日射量は少なく、低温である。ケッペンの気候区分では、最暖月平均気温が0℃以下の地域をさす場合と、樹木が生育できない気候である最暖月の平均気温10℃未満とする場合がある。冬半年は終日夜であり、放射冷却の状態が続く。したがって夏の昼半年と冬の夜半年の気温の年較差は大きく、一方、気温の日較差は小さい。一般に大気下層では放射冷却によって低温で乾燥した高気圧が形成されているが、対流圏中・上層では年間を通じて低気圧ができ、極渦(きょくうず)とよばれている。北極圏とそれより中緯度側の地上気圧が逆に変動する現象があり、北極振動という。北極振動は中・高緯度地域の寒波、大雪、異常高温や、極域の成層圏オゾン層の破壊との関連が指摘されている。ほとんどがアネクメーネ（非居住地域）であるが、地下資源や軍事、航空路の面から重要性が増大している。また、上空の成層圏でオゾン層が希薄となるオゾンホール（オゾンの穴）の原因が人間活動の結果放出されるフロンガスであると判明し、フロン類などの温室効果ガスを削減するための国際条約締結の端緒となった。

［山下脩二］

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