Why is it easy to clear up when the northwest wind blows in winter?

In winter, the northwest wind blowing in the southeast of China generally comes from northern China, Russian Siberia and Mongolia, where it is very cold in winter. According to the records of historical meteorological data, in June 1955+ 10/5, the temperature in Yansk, Vilho, Siberia dropped to minus 68℃. Why are these areas particularly cold? The main reason is that the ground in these areas is covered with snow and ice, and the cold air stays for a long time in winter, and receives less sunlight and heat during the day, while the heat emitted into the air at night is much more than the heat absorbed during the day. This long-term imbalance of low heat income and high expenditure makes these areas contain a lot of cold air. Meteorologists call these areas the birthplace of cold air.

Cold air is characterized by being heavy and dry (containing less water vapor). Because of its heavy weight, it sinks into the ground, forming a huge high-pressure area, which often spreads in all directions. If the main force of cold air diffusion this time is from northwest to southeast, affecting the southeast of China, this is what we call blowing northwest wind.

The cold air going south is often fierce, which will crowd out the warm air that once stayed in the southeast of China and supplement a lot of dry and cold cold air. We know that the main factor causing rainfall is water vapor. If there is too much steam in the air, it will rain easily. If there is less water vapor in the air, it may clear up. So in winter, the cold air in the north goes south and the northwest wind blows tightly, so it is easy to clear up. The same is true of the agricultural proverb that "the west wind blows the rain and hits the feet."