Why does the temperature change a lot in winter?

The coldest time of the day is before the sun comes out.

Because the heat brought by the sun the day before was stored in the atmosphere, at night, the sun will not continue to provide heat, and the heat in the atmosphere will be slowly released and reduced. It gets cold at night. When the sun came out the next morning, the temperature was the lowest, because the sun came out immediately (the sun shone on our heads on cloudy days, which also played a role), providing new heat and the temperature rose.

Most of the frost on the ground in winter comes out at that time.