Why is it cold?

The transfer of energy is divided into three aspects: one is radiation, the other is heat transfer, and the third is doing work.

It's about 273 degrees below zero, but the temperature in the universe is absolutely zero. The reason why the temperature of the earth in the universe is so high is that the living things on the ground, including the living things on the ground, absorb the radiation of the sun and convert it into heat energy, thus maintaining the temperature of the earth. In summer, the sun shines almost vertically, so the radiation of the sun is very strong, and the heat absorbed by the ground is greater than the primary radiation of the ground, so the temperature is getting higher and higher. As the sun moves south, its radiation becomes more and more dispersed. The radiation on the ground is greater than the radiation absorbed by the ground, so the weather will get colder and colder. But in the southern hemisphere, it is just the opposite of us. It's winter here, summer there, summer here and winter there!

Of course, this is only a simple explanation, ignoring the role of atmosphere, but the basic principle is like this!