Why is Chengde very cold in winter?

Chengde area is characterized by temperate continental climate and temperate monsoon climate, with long, cold and dry winters, which are greatly influenced by Siberian cold air. Cold waves occur from time to time, and the northwest wind blows for a long time, and the frost-free period is 140~ 160 days, which is not found in European countries with temperate maritime climate with latitude (4 1 degree north latitude) higher than Chengde. It seems that the Great Wall built by the ancients is really the dividing line between meteorology and hydrology.