Why is it so foggy across so much of the US?

Fog has been blanketing communities from the Plains to New England this week, but why? AccuWeather's Bernie Rayno explains.

Transcript

00:00 As we continue to see fog across the central and eastern US, you may be wondering what is causing this to happen.

00:06 We kind of touched on a little bit of this, but for more details, we head to Bernie Raynaud.

00:10 And in tonight's WeatherWise segment, where we take something complex and try to make it easy to understand,

00:17 let's talk about why we have all the fog.

00:22 Well, it starts with snow cover.

00:24 Yeah, I said it, snow cover.

00:26 Look at the extensive snow cover right now.

00:28 Now, here's the thing. Snow cover means the ground is cold.

00:33 And when you think about it, to form clouds, what do you need?

00:36 A temperature contrast between warm and cold.

00:38 And if you can get a collision of those two air masses, you get upward motion, you get clouds, and sometimes precipitation.

00:44 Well, here we go. A lot of cold ground going on right now.

00:47 But when you look at the water vapor loop, where's the flow coming from now across the central US?

00:53 With our upper level low moving to the north, you're getting this southerly flow, bringing in warm, moist air.

01:00 And it's that warm, moist air that's doing what? It's colliding with that cold ground.

01:06 The warm, moist air, because the cold air is more dense, stays on the ground.

01:10 You get the upward motion.

01:11 But the temperature contrast, instead of being about 3,000, 4,000 feet in the air, it's right at the ground.

01:17 So the cloud's not forming aloft, it's forming right on the ground.

01:21 And that's where you're getting the fog.

01:23 And unlike most times, where fog is most persistent and most dense in the morning,

01:29 because of the way this is setting up, the fog and the low clouds linger all day.

01:35 And that's why we're having so many problems with the fog.

01:39 And now you know.

01:41 All right, Bernie, good stuff there.