ELI5: What is advection fog?

Imagine you're holding a warm cup of hot chocolate on a cold winter day. See the steam rising from it? That's like a little bit of water vapor being added to the air.

Now, imagine that warm, moist air from over a lake (like the hot chocolate steam) is blowing towards a cold area (like your cold winter day). This blowing is called advection.

Advection fog happens when that warm, moist air travels over a cold surface. Here's what happens:

• The warm, moist air cools down very quickly when it touches the cold ground or water.
• As the air cools, it can't hold as much water vapor anymore.
• The extra water vapor turns into tiny droplets of water, floating in the air.

Think of it like this: you bring a warm glass of water into a very cold room. The glass gets foggy, right? That's similar to advection fog. The warm, moist air (the warm water) is moving (advecting) into a cold place (the room), and the water vapor condenses into fog (on the glass).

So, advection fog is basically fog that forms when warm, moist air moves over a cold surface and cools down, causing the water vapor in the air to condense into tiny water droplets. You might see it near the coast where warm air from the ocean blows over cold land, or near lakes when warm, moist air blows over the cold water.