Air has a characteristic: hot air rises and cold air sinks. At night, the ground and air dissipate heat, and the air temperature at the top of the mountain drops faster than that at the foot of the mountain, so the warm air below runs and the cold air at the top of the mountain slides down the hillside. After cold air reaches the valley or depression, it can't go to other ground, so it has to stay in the valley and depression honestly. Therefore, the more cold air in valleys and depressions at night, the lower the temperature. Generally speaking, the temperature of depressions is 4-5℃ lower than that of the surrounding plains. Therefore, when there is no frost on the hillside and peacefully, there will be frost in the valley and low-lying land, so there is a saying in the Ming Dynasty that "frost hits low-lying land". More knowledge: If there is frost on a peaceful hillside, the frost in valleys and depressions will be heavier.