An important reason why caves are warm in winter and cool in summer is that they have a thick "roof". On the top of the cave in northern Shaanxi, pedestrians can walk or even take a truck. Even brick kilns and earthen kilns should be covered with a thick layer of soil to keep the cave warm, just like air conditioning. "

In addition, the top and wall of the cave can neither directly absorb heat from the atmosphere nor directly dissipate heat to the atmosphere, only the hole can directly contact with the outside world. Therefore, the temperature change in the cave always lags behind the external temperature change, and the temperature difference changes little.