Temperate lake - Ontaiko

This is one of the thermal (climatic) classifications of lakes and ponds, and refers to lakes and ponds whose surface or entire water temperature is above 4°C in summer and below 4°C in winter, with the boundary being 4°C, where the water is at its maximum density. They are mainly found in temperate and cold zones (the cold zone refers to the D climate in the Köppen climate classification. In Japan, Hokkaido has a D climate).

In temperate and cold zones, the surface water temperature rises considerably in summer, but the deep layers remain cool, especially at 4°C in deep lakes. Meanwhile, in winter the surface cools down to below 4°C. At this time, the middle and deeper layers are made up of heavy 4°C water, so winter stratification is created by the light surface water and ice at 0-4°C. In such lakes, circulation (convection) occurs twice a year, at the beginning and end of winter when the entire layer is at 4°C, so temperate lakes are sometimes called doubly circulating lakes. If the lake is deep and sufficient heat is stored in the deep layers during summer, heat to compensate for the cooling of the surface is carried from below by convection in winter, and the surface layer does not drop below 4°C. Therefore, depending on the shape of the lake basin, some lakes in temperate zones can become tropical lakes.

[Tadashi Arai]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

湖沼の熱的（気候的）分類の一つで、水が最大密度を示す4℃を境にして、夏は湖沼の表層あるいは全層の水温が4℃以上に、冬は4℃以下になる湖沼をいう。主として温帯と冷帯に分布する（冷帯とは、ケッペン気候区分のD気候のこと。日本では北海道がD気候である）。

　温帯と冷帯では、夏に表層水温はかなり昇温するが、深層は低温で、とくに深い湖沼では4℃に保たれる。一方、冬には表面は冷却されて4℃以下になる。このとき、中層以深は重い4℃の水なので、表層の軽い0～4℃の水や氷により冬の成層がつくられる。このような湖沼では、全層が4℃になる冬の初めと終りに年間2回の循環（対流）が生ずるので、温帯湖のことを二回循環湖とよぶこともある。もし水深が大きく、夏の間に深層に十分に熱が蓄えられていれば、冬にも表面の冷却を補う熱が対流により下方より運ばれ、表層も4℃以下にはならない。したがって湖盆の形態によっては、温帯湖の地域にあっても、熱帯湖になる湖沼がある。

［新井　正］

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