ELI5: Why does fog disappear when the sun comes out?

Imagine you're breathing on a cold window. See that steamy stuff? That's like fog! Fog is just tiny, tiny water droplets hanging in the air.

Think about when you take a shower. The bathroom gets all steamy, right? That's because the hot water turns into water vapor (which is water in gas form), and it’s all mixed in with the air. When the air cools down, like when you open the window, that water vapor turns back into tiny water droplets – that's the steam you see.

Fog is the same thing! The air near the ground gets full of water vapor (maybe from a lake, river, or even just damp ground). When the air cools down at night, the water vapor turns into those tiny water droplets we call fog.

Now, what happens when the sun comes out?

• The sun is like a giant, warm hairdryer.
• It heats up the air and the ground.
• This heat gives the tiny water droplets in the fog enough energy to turn back into water vapor.
• Since water vapor is invisible, the fog disappears! It hasn't gone anywhere; it's just changed its form.