The weather in winter is really cold! Why is this?

In winter, the sun shines obliquely on the earth, and the sun must penetrate through a thicker atmosphere to shine on the earth. There is too much energy loss along the way. Less heat is accumulated during the day and more heat is lost at night, which makes the temperature drop repeatedly. This is like the reason that it is cool in summer morning and hot at noon. The distance between the sun and the earth is not the main reason. At the same time on the earth, the temperature is different everywhere. It's tens of degrees below zero in the north of China, but it's still more than twenty degrees below zero in Hainan. Coupled with the cold air moving south near the North Pole, the winter is even colder.