ELi5: “Why do clouds fall?” i.e. Fog


My four year old asked this morning from out on our deck “why do clouds fall”? It was quite foggy out.

In: Earth Science

Clouds don’t fall. Clouds form at the elevation where the temperature/humidity dew point of water changes so that water vapor begins to precipitate out of air and form into tiny droplets. That can happen at ground level, fog. Or it can happen up higher, clouds.

Fog is actually usually clouds that form just above the water or ground. It’s because air can only hold so much water vapor. The water evaporates from the surface of the water where the air is warmer, but if there’s a sharp drop in temperature in the air as it rises it can reduce how much water the air can hold. That water vapor condenses into a cloud we call fog.