Why did it suddenly cool down?

In winter, the land temperature is lower than the ocean temperature, which forms a strong cold high pressure in the mainland, and the air flow diverges in all directions from the high pressure center. The air mass moving to low latitudes belongs to cold air mass in nature, and when it meets warm air mass, it forms a cold front. When the cold front crosses the border, it becomes rainy (snowy), windy and cool. After crossing the border, the temperature and humidity dropped sharply and the air pressure rose. In winter, the cold air mass moves southward rapidly (in the northern hemisphere, but in the southern hemisphere), which often leads to disastrous gale, cooling weather and even cold wave.

In this way, the snowfall area in winter (the same as the rainfall area in summer) is distributed near the front, that is, the intersection of cold and warm air masses. This position is not fixed, but is determined by the strength of the two air masses. The cold air mass is weak, the front and precipitation areas have high latitudes, the cold air mass is strong, and the front and precipitation areas have low latitudes.

The land cold high that affects China in winter is the Asian high centered on Mongolia and Siberia (also known as the Mongolian Siberian high), which has a very large sphere of influence and has a great impact on the weather in China, especially in the northern region.