The answer is that air and water also have the property of saturation, which means that air can only absorb a certain amount of water vapor. When it is cold, the ability of air to absorb water vapor is much worse than when it is hot. Therefore, in the cold winter, the water vapor in the air exhaled by humans and animals will no longer be absorbed by the outside air; this series of water vapors, when exposed to cold, will condense into tiny water droplets and turn into white fog. In particular, the water vapor exhaled by humans has a much higher temperature than the air, so it is easier to condense. Just like throwing a cup of hot water high in subzero weather, all of this water will quickly condense and evaporate.