Why is it so cold in winter?

The cold air activity on the earth is the direct cause of the cold weather.

At the same time, because the earth revolves around the sun, when the direct point of the sun in the northern hemisphere moves south, these places in the northern hemisphere will get cold. Similarly, in the southern hemisphere of the earth, when the direct point of the sun moves northward, these places in the southern hemisphere will get cold. It is not the reason why the earth revolves to perihelion and aphelion.

In the northern hemisphere, Eurasia is greatly influenced by the warm westerly winds of the North Atlantic in winter. When the warm west wind in the North Atlantic is stable, it can prevent the polar cold air from going south, thus weakening the cold wave. But this year's La Nina phenomenon broke the stability of the warm west wind in the North Atlantic.

La Nina phenomenon is that the southeast trade winds blow away the sea water heated by the sun near the equator, which makes the lower cold sea water turn up and the pressure increase, which makes the subtropical high in the northern hemisphere stronger and prevents the warm westerly wind from coming to Eurasia, thus helping the polar cold air to continue southward.