ELI5: Why is fog sometimes thick and sometimes light?

Imagine you're making soup. Sometimes you add a little bit of noodles, and sometimes you add a whole bunch! Fog is like that, but instead of noodles, it's tiny, tiny water droplets floating in the air.

• Light fog is like soup with just a few noodles. You can still see through it pretty well. There aren't many water droplets blocking your view.

• Thick fog is like soup packed with noodles! It's hard to see through because there are so many water droplets bumping into each other and blocking the light from reaching your eyes.

• More water: If the air is full of water vapor (like after a rain), and then it cools down, more water vapor will turn into those tiny droplets, making the fog thicker. Think of it like adding more noodles to the soup.

• Colder temperature: Cold air can't hold as much water vapor as warm air. So, when the air cools down, the water vapor has to go somewhere – and it turns into those tiny droplets that make fog. The colder it gets, the more water turns into droplets, and the thicker the fog becomes.

So, basically, the more water droplets there are in the air, the thicker the fog will be, just like the more noodles you put in the soup, the thicker it gets!