Why is there white fog in cold weather (winter), but sometimes it doesn't?

When the temperature is low, the gas exhaled by people will liquefy and become small water droplets, which look like white fog. If the temperature is lower, the white fog will be more obvious. The concentration of white fog in the weather is related to the humidity of the air. How else will the fog disperse after the sun comes out? After the sun came out, the temperature rose, white fog droplets evaporated and the weather was fine.