ELI5: What is radiation fog?

Imagine you have a warm cup of cocoa on a cold night. If you leave it outside, it will cool down, right? Radiation fog is kind of like that happening to the ground!

• The ground gets warm during the day from the sun.
• At night, when the sun goes down, the ground starts to cool down. It gives off its warmth into the air, like your cocoa. This warmth leaving is called radiation.
• If the air is still and there isn't much wind to mix things up, the ground gets really cold.
• This cold ground then cools the air right above it.
• Cool air can't hold as much water vapor (the stuff that makes clouds) as warm air.
• So, the water vapor turns into tiny water droplets that float in the air. We see these droplets as fog!

Radiation fog is more common on clear nights because clouds can trap some of the warmth from the ground. Without clouds, the ground cools down much faster, making the air colder and the fog thicker. You'll often see it in valleys or low-lying areas where the cold air can settle. It usually burns off (disappears) when the sun comes out and warms the ground again.