極圈

極圈南北緯66.5度的緯線,其緯度是黃赤交角的余角。因緯度甚高,接收太陽輻射量不足,終年寒冷、白覆蓋。永晝和永夜
極圈生物必須忍受這種嚴寒的氣候,所以大多擁有極厚的皮下脂肪與極密的毛髮來禦寒。例如:北極熊企鵝

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