Why is it sometimes hot in winter?

Because of the intersection of yellow and red.

The intersection of yellow and red is the fundamental reason for the change of four seasons and the division of five zones on the earth. Because of the existence of the ecliptic angle, the direct point of the sun moves back and forth between the north and south latitudes of the earth, which changes every year, resulting in the seasonal changes of the height of the sun at noon, the length of day and night, and the seasonal changes of the solar energy obtained in various places, thus forming the replacement of the four seasons.

Climate:

"beginning of winter" refers to wind and rain, humidity, light, temperature, etc. At the turning point, the climate is gradually changing from autumn to winter. In winter, the direct point of the sun moves south to the tropic of Capricorn, and then turns back to the north.

In early winter, the sun reaches the longitude of 225 degrees, and the earth is at declination-16 19'. The height of the sun in the northern hemisphere becomes smaller, the daylight time becomes shorter, and the amount of solar radiation obtained in the northern hemisphere becomes less and less. However, there is still some energy in the heat stored on the surface in the second half of this year, so it is not very cold.