What is Fog?

17:54 Monday 21st November 2011
Drive BBC Radio Cambridgeshire

ANDY BURROWS: I got into work around about eleven o’clock this morning and it was really thick on the A14 and the A1. It got better as I got towards Cambridge, but it was pretty horrible, that’s for sure. Now I’m rather intrigued about fog. I don’t know if anybody else is, but let’s speak to Liz Bentley, who’s the Head of the Weather Club. She’s on the line. Hello to you Liz.
LIZ BENTLEY: Hi. How are you?
ANDY BURROWS: I’m OK thank you very much. It never really lifted where I live yesterday. And then I went outside round about eight nine o’clock last night and it had gone. And I was thinking, that’s weird, isn’t it? You’d think it would get thicker at night-time.
LIZ BENTLEY: At night-time. Absolutely. No. It’s been an unusual few days, because we haven’t seen any fog, and traditionally we do get fog at this time of year. But it’s been so mild. And I think just the slight change in the temperature, things allowed to drop away on Saturday, allowed that fog to form. And it just didn’t go yesterday. And I think what you’re noticing with the variation in fog is just subtleties in the wind. So if the wind picks up enough it just lifts that fog off the ground. And places where the wind just stays pretty much flat, calm, then the fog will just stay. And that’s really, it’s the subtleties from one place to another.
ANDY BURROWS: I asked a couple of people this, over the weekend, and no-one really gave me a satisfactory answer. I feel like a small child, but I was just intrigued by it. What is fog?
LIZ BENTLEY: OK. Very simply, fog is just a cloud that has developed at the surface. So people who dream of floating around in the clouds in the sky, when you walk through fog,, that’s effectively what you’re doing, but on the ground. And it forms because the air becomes far too saturated to hold any moisture any more, and the water in gas form condenses out into little water droplets that just remain suspended in the air. If you walk through fog, you’ll notice that you do pick up a little bit of moisture on your skin or your hair or the coat that you’re wearing.
ANDY BURROWS: Brilliant. Yes. I was just driving through it this morning, and I was thinking, actually I was expecting to have to put the wipers on at some stage, but I didn’t. But I did notice there was a little bit of condensation on the windscreen as I was driving through it. I’m right in thinking the sun usually burns it off, or effectively the wind just blows it away normally?
LIZ BENTLEY: Yes. If the sun can get to work, it will burn it back, and then it usually reforms again at night-time. But the wind will be a key factor, and it did today certainly across most of the country. Once the wind started to pick up a little bit, it lifted it up into low cloud. So visibility improved at the surface, but there was still a lot of cloud around up in the sky. And the wind direction is going to change over the next couple of days, and that will see the fog just clearing away as well. So the wind certainly has an influence on where and when we’ll see fog.
ANDY BURROWS: Sure. What I was intrigued about yesterday as well, and why it got us talking, is because flights were delayed, weren’t they, at various airports across the country. I think it was the same early on this morning as well. So is it difficult to predict?
LIZ BENTLEY: Well it is and it isn’t. We kind of know when fog’s likely to happen. But it’s like a thunderstorm. You’ll have the right conditions for this thing to develop or form, but it’s exactly when and where it’s going to happen is the real tricky thing for forecasting. So I’m sure, looking at the forecasters, they would have known fog was likely, but exactly how dense, exactly what time it’s going to come in, that kind of thing, is very very difficult. There was talk that it wasn’t going to be as bad, the fog last night, but I woke up this morning to very thick fog again. So it’s slightly unpredictable I guess you would say.
ANDY BURROWS: Right. All you ever wanted to know about fog. That’s all I wanted from this item. And I think we know everything we ever need to know about fog. I just want to ask you a couple more things. Are we all going to pay for this relatively glorious weather? The forecast yesterday, we had one on this programme as well, temperatures of 13, 14, 15 degrees. Is a real harsh winter going to come as a real shock over the next couple of weeks?
LIZ BENTLEY: Well there were some forecasts of that. The news headlines about a month ago were saying we were going to see some really snowy conditions again at the end of October and into November. Well they haven’t materialised yet. And looking ahead at say the monthly forecast, which takes us well into December, I think things will change this week. You’ll really start to notice a more autumnal feel to the weather, the chillier nights. They forecast snow for the Highlands of Scotland by the end of the week. So we’ll start to see a little bit of that wintry stuff. But there’s no sign at the moment that we’re going to see the kind of snowmageddon that we saw last winter. And so at the moment I guess the answer to your question is no really. Just enjoy these mild conditions. For me, I guess you’re the same, I’ve been on and off with my heating at home, because you just don’t know what to do, do you?
ANDY BURROWS: No. It is odd. But we should enjoy it, because I feel a surprise is round the corner at some stage. Always great to speak to you. Thank you very much, Dr Liz Bentley, that was, from the Weather Club.