大雾到底是怎样形成的呢？让我们来看看生活中的一个现象：在做饭的时候，大家肯定能看到锅上方总是有雾气产生，大雾的形成原理与此相同，只是雾的范围大，维持的时间长而已。空气中能容纳一定量的水汽，气温愈高，空气中所能容纳的水汽愈多，反之愈少。白昼的温度一般比较高，空气中可容纳较多的水汽；但是到了夜晚，地面热量的散失使地面温度下降，同时影响接近地面的空气层，使空气的温度也降低。当温度低到不能容纳原先所有的水汽时，过剩的水汽便凝结成小水滴或小冰晶，浮游在近地面的大气中，雾就形成了。凌晨时分气温最低，所以大雾经常出现在早晨。当太阳升起后，温度逐渐升高，小水滴或小冰晶又变成水汽或降落到地面，雾就渐渐散去。
Then, how on earth does fog come into being? Let’s look at a phenomenon in our daily life: when cooking, we can always see steam above the pot. Fog forms in the same way, only that its extent is larger and its duration, longer. The air can hold a certain amount of water vapor. The higher the temperature, the more vapor can be contained in the air. Conversely, cooler air holds less vapor. Generally speaking, the temperature is higher in the daytime and the air can hold more vapor. At night, however, the dissipation of heat will lower the temperature of the earth’s surface, which, in turn, will affect the air close to the ground and make it cooler. When the temperature is too low for the air to hold the vapor previously contained in it, the superfluous vapor will condense into tiny water droplets or ice crystals, floating in the lower atmosphere. This is what we call fog. The air temperature is lowest before daybreak, so heavy fog usually occurs in the morning. After sunrise, as the temperature becomes higher, the tiny water droplets or ice crystals will turn into vapor again or fall to the ground. And, little by little, the fog will disappear.
