Paris Alston: This is GBH’s Morning Edition. When trying to make sense of the weather and meteorological patterns, there are a lot of helpful tools out there. One of them is isobars, the curved lines on weather maps that connect points with the same average sea level air pressure.

Jeremy Siegel: To learn more about these isobars, we are joined by GBH meteorologist Dave Epstein for our weekly science Deep dive. Good morning Dave.

Dave Epstein: Good morning Jeremy. Good morning Paris. Good to be here.

Siegel: So first let’s break things down a little bit more. What exactly is an isobar and how are they determined?

Epstein: Right. So what’s interesting is that there’s not the same amount of air pressure all across the planet at any given time, but you can find areas that have the same amount of air pressure at any given time. And where the pressure is equal, we call that an isobar. So for example, if the pressure in Boston were let’s make it 29.92 inches, that’s the average sea level pressure. And it was also in Worcester and also in Springfield, you’d have a little isobars that would be 29.92 inches running along basically the Mass Pike.

Alston: So this week’s topic we should mention, Dave is in part inspired by a listener question. Barbara from Douglas is a boater and wrote in: I want to get better at reading isobar maps and understanding how moving air masses change weather patterns. So I’m so impressed that Barbara, who I don’t believe is a meteorologist, is so interested in this and I’m curious how best to make sense of this for the average person.

Epstein: Yeah. So isobar maps are important for the general public. You know, if you want to look at a weather map, you’ll see those black lines, that they often have between the Hs and the Ls. That’s high pressure and lower pressure. So the isobars will go increasingly higher toward the higher pressure, increasing lower toward the lower pressure. If the isobars are close together, and as you look at weather maps, you’re sort of like, well, what’s close together? If you have a big pressure change, say, from New York City all the way up to Caribou, Maine, then the air has to rebalance itself so you have more wind. So isobars that are closer together give more wind. If you’re a hiker, topographical maps as you’re hiking, there’s lines on there. And those are basically lines of how high up you are. And the closer together those lines are, the steeper the climb is going to be. So it’s the same sort of principle closer to the other lines, more of a bigger change. Same thing with isotherms. If they’re close together, you have a bigger temperature change between point A to point B.

Alston: Hmm, okay. And are there any other types of weather maps that we should be geeking out on, Dave, and looking into? When we’re trying to figure out how to use them for our average daily lives?

Epstein: Yeah. I mean there’s like dew point lines, right? So isodrotherms would be the lines of equal dew point. And we can use those to see, hey, where’s there dry air right now? Where is there moist air? You can have isotachs. So those are lines of equal wind. So if the isotachs are really close together, it’s very windy. And you can also see sort of where the wind is going to be, you know, is it 25 miles an hour or is it 30 miles an hour? So you get an idea of how strong the winds are in a particular location. So meteorologists use iso maps all the time. We look at them, especially isotherms and isobars. Those are probably the two more common ones, but we look at isotach maps as well.

Siegel: How much do these isobars figure into a prediction of the weather when you’re making a forecast? Like how much do you use this in comparison to the other stuff that you’re tracking when you’re trying to figure out how windy, how much it’s going to rain, what the temperature is going to be like around Boston?

Epstein: Yeah. The good question. So I mean, it’s not — you use a map of surface sea level pressure or you use a map where you have isoheight lines above the atmosphere. So you go up to 10,000ft, 18,000ft, 30,000ft, because you want to get an idea of how big changes are across an area. And the bigger the change with those iso maps, the more sort of weather you’re going to have. And what I mean by that, the stormier the weather is going to be. So when I look at maps, if I see like a low pressure area that’s forecast to move into the area, let’s take it in the winter. And those isobars look like they’re going to be really tight together, then I know that that’s going to be a more intense storm. So we definitely look at them. They’re an important component of forecasting.

Alston: Well, that is GBH meteorologist Dave Epstein. And a reminder that if you ever have a question for him, you can always text that to (617) 300-2008 or email thewakeup@wgbh.org. Dave, thank you so much.

Epstein: Good to be here.

Siegel: You’re listening to GBH news.

 

Meteorologist Dave Epstein is our go-to person for pressing weather questions on everything from winter blizzards to summer droughts. He’s also a horticulturist, meaning he’s an expert in anything that grows leaves and flowers. GBH’s Morning Edition asked our audience for weather and gardening questions, and Epstein graciously answered them on the air.

Have a gardening or weather question for meteorologist Dave Epstein? Email us at   thewakeup@wgbh.org or text 617-300-2008.

I want to get better at reading isobar maps and understanding how moving air masses change weather patterns. ... Do you have any teaching guides that you recommend? — Barbara from Douglas

Isobars are the curved lines on weather maps that connect points with the same average sea level air pressure.

“What’s interesting is that there’s not the same amount of air pressure all across the planet at any given time, but you can find areas that have the same amount of air pressure at any given time. And where the pressure is equal, we call that an isobar,” Epstein said.

So if the pressure in Boston is identical to the air pressure in Worcester and Springfield, for instance, an isobar map would show a straight line connecting the three cities.

On the ground or at sea, following that straight line usually looks like calm, steady weather, Epstein said.

Isobars that are closer together mean the air is shifting between different pressure areas. Hikers will recognize them as similar to topographical maps, in which more closely clustered lines indicate a steeper climb.

“If you have a big pressure change, say, from New York City all the way up to Caribou, Maine, then the air has to rebalance itself so you have more wind,” Epstein said.

There are other kinds of isobars, too:

  • Isotherms indicate areas of equal temperatures.
  • Isodrosotherms are dew point lines.
  • And isotachs are lines of equal wind speeds.

“If the isotachs are really close together, it’s very windy,” Epstein said. “And you can also see sort of where the wind is going to be, you know, is it 25 miles an hour or is it 30 miles an hour? So you get an idea of how strong the winds are in a particular location.”
As a meteorologist, Epstein looks at isobar maps both at sea level and in the atmosphere.

“You go up to 10,000 feet, 18,000 feet, 30,000 feet, because you want to get an idea of how big changes are across an area,” he said. “When I look at maps, if I see like a low pressure area that’s forecast to move into the area … and those isobars look like they’re going to be really tight together, then I know that that’s going to be a more intense storm.”