What is the main reason for the sudden cold weather?

In winter in the northern hemisphere, the direct sunlight point is located in the southern hemisphere. The altitude angle of the sun is small in various places in the northern hemisphere. The atmospheric path of the same amount of solar radiation is relatively long. The solar radiation weakened by the atmosphere is more, and the solar radiation reaching the ground is relatively small. few. At the same time, due to the small altitude angle of the sun, the same amount of solar radiation spreads over a large area, and the ground receives less solar radiation per unit area. The temperature is relatively low.

Also, in the northern hemisphere winter, the days are short and the nights are long, so the heat income is less and the expenditure is more.

In addition, during winter in the northern hemisphere, cold high pressure forms over land, and the high pressure center continuously transports cold air masses to low latitudes, intensifying the coldness.

Similarly, you can know the winter conditions in the Southern Hemisphere.