《黄帝内经·素问·八正神明论》
所以，气候温和，日色晴朗时，则人的血液运行流畅，而卫气充盈浮于肌表，血容易泻，气容易行；天气寒冷，日色阴霾时，则人的血液运行凝涩，而卫气沉潜于里。
是故天温日明，则人血淖液而卫气浮，故血易泻，气易行；天寒日阴，则人血凝泣而卫气沉。
Therefore, when the climate is mild and the sun is clear, the blood flows smoothly, the衛氣 is abundant and floats on the skin, and the blood is easily discharged and the qi is easily circulated; when the weather is cold and the sun is阴霾, the blood flow is stagnant and the衛氣 sinks into the interior.