Clouds and fog are the same thing - billions of tiny water droplets (or sometimes ice crystals) suspended in air. The only difference is altitude. A cloud is floating up in the sky; fog is one that happens to be at ground level. When you walk through fog, you’re actually walking through a cloud.
Fog forms when humid air cools to its dew point - the temperature at which water vapor starts condensing into liquid. The most common kind is radiation fog, which forms on cool clear nights when the ground rapidly loses heat into the sky. The air directly above cools and forms a low ground-hugging layer of fog, usually burning off in the morning sun.
Other types include advection fog (when warm humid air drifts over a cold surface - common at sea and on coastlines), valley fog (cool air pooling in low spots), and steam fog (when cold air sits over warm water, like the “steam” you see rising off lakes on cold mornings). Officially, weather scientists only call air “foggy” when visibility drops below 1 kilometer (about 0.6 miles). Above that threshold, the same droplet-filled air is just called mist.
