• 10 months ago
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.

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