Have you ever looked outside, only to see that the air was thick and gray, and you could barely see outside?

Squeaks and I looked out of the Fort window this morning, and that’s exactly what we found. [Squeaks squeaks excitedly]. It was such a strange and different thing to see!

Normally, we can see a few trees outside, and even a road, but today, we could barely see the closest tree. It was a little bit spooky, and the news put out a warning to be careful when driving on the roads. All of this was because of the gray mist covering everything, called fog.

Fog can make it hard to see outside, and walking through it can make you feel a bit damp or cold. When I walk through fog, I always imagine that I’m walking through a big rain cloud. [Squeaks nods and squeaks excitedly]. That’s right, Squeaks — it seems that way because fog does look a lot like clouds, except really close to the ground!

In fact, clouds and fog are made of the same thing: tiny droplets of water. From far away, clouds might sometimes look like thick fluffy pillows, or even cotton candy. But really, walking through a cloud wouldn’t feel fluffy or comfortable at all!

You’d get very wet. There’s always some water in the air, except it’s not a liquid. It’s usually a gas — the same type of thing as the air itself, which is why we can’t see it.

We call that type of water, water vapor. But when there’s too much water in the air, it condenses, or pulls together really close, to become a bunch of super tiny droplets of liquid. When this happens high up in the sky, all those little droplets form what we call a cloud.

This can happen much closer to the ground, too, which is when we call it fog. It just doesn’t happen as often. For fog to form, there needs to be a lot of water that’s just reached the ground. [Squeaks mentions a time when this happens].

Just after a rainstorm is the perfect time for fog to happen, Squeaks. There’s all that water on the ground, plus there’s a lot of water vapor that’s still in the air. Next, the water vapor has to get pretty cold, very quickly.

If the temperature drops, the water vapor in the air starts to collect into those little liquid droplets. You can see this happen in real life if you have some cold water in a glass! Pretty soon the outside of the glass will be wet, but that water doesn’t come from inside the glass … it comes from the air.

The cold glass makes water vapor in the air collect into liquid drops. When this happens in the air outside, the little droplets are so small that they can float through the air, and we can see them all spread out. That’s fog!

It really is just like a cloud on the ground, isn’t it? There are lots of different types of fog, and each happens in different places, with different amounts of cold air. The most common kind is called radiation fog.

This is the kind of fog that sets in overnight, when air with a lot of water vapor in it cools down. In the morning, when the sunlight warms the air again, the radiation fog disappears. [Squeaks asks a question — how does fog go away?] Good question, Squeaks. Where does the water go when the fog disappears?

When the water in the fog starts to heat up, it begins to evaporate, or to turn back into water vapor and spread out. When the water vapor is spread out again in the air, we can’t see it. It’s too small for our eyes.

But just you wait, Squeaks. I hear it’s supposed to get pretty cold tonight, and I bet we’ll get some more fog! [Squeaks squeaks excitedly]. Let’s bring our weather journals outside so we can observe what we find.

Is there fog where you live? Have you ever walked through fog and not been able to see well? Do you have any other questions about the weather?

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