Paris Alston: This is GBH’s Morning Edition. We are in another heat wave and while hot temperatures are expected in the summer, the humidity can make it feel like it’s dragging. But what is it about the combination of warmth and stickiness that makes things feel so much worse? To discuss this, we are joined by meteorologist Dave Epstein. Good morning Dave. Thanks for being here.

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

Alston: So what’s the latest on the forecast here? When will we see some relief?

Epstein: Yeah, that’s a great question. So in terms of the humidity, I was just looking at some maps here. It does look to me like we’re going to stay with these humid conditions for the foreseeable future. So does that mean that we won’t see a day where it breaks a little bit? Yeah, it’s definitely possible, but I think that we are definitely in a humid pattern. The humid pattern has kicked in a little bit earlier. This is, you know, when you get to late July and especially the middle of August, tend to get what we call, or what’s referred to as those dog days of August. But it looks to me like this humid pattern is just going to continue again for the foreseeable future.

Alston: Now, Dave, whenever we talk about humidity, we always mention the dew point. Right now I’m seeing it’s about 71 degrees. Remind us why the dew point matters and what is generally thought to be a comfortable one.

Epstein: Yeah. So the dew point is a temperature. It’s actual temperature. And it says what temperature would the air have to be in order to be completely full of moisture? So the air at every single different temperature, the air can have a certain amount of moisture in it. If it’s really cold, the air can only hold a certain amount of moisture. So think about it this way: Cold air is going to be like a 16 ounce glass. Warm air is going to be like, an Olympic swimming pool. So when the 16 ounce glass is full, you have 16 ounces of water. The relative humidity is 100%. When the swimming pool is full, you have a lot more water. So 50% relative humidity in the summer is going to be more humid even than 100% relative humidity in that cold air, because 100% of a small amount, it’s still a small amount, 50% of a huge amount is still a big amount.

Alston: Oh, interesting. Okay, so now I’m thinking about taking a dip in an Olympic sized swimming pool, but also like the thought of, like jumping in a cold shower, how that can feel good or bad on a hot day, depending on what your comfort level is. So when we talk about humidity, Dave, how does that all translate into the discomfort that we feel on our bodies as humans?

Epstein: Yeah. So when you’re outside and it’s hot out, you would start to sweat. And if you were in Arizona or let’s say Las Vegas yesterday or the day before when it hit 120, the relative humidity was 4%. That low. The dew point was near zero. So the second that water comes out of your pores, it evaporates. And that process is a cooling process. And so that’s why the saying 'It’s not the heat, it’s the humidity.’ So if you were in that kind of heat, yes, it would have been not great, but you would have at least felt a little cooling if you went in the water. Now let’s go to the East Coast. Lots of humidity, high dew points, dew point 71. You go outside, you sweat. Guess what? No evaporation. The air already has so much moisture in it that it’s really hard to evaporate off your skin. And so you stick with that very hot feeling, and your body’s ability to cool is lessened tremendously. So when the dew point is over 60, you start to feel it. Over 65, it’s uncomfortable. Over 70 for most people, it’s oppressive and it’s not really enjoyable.

Alston: And that also helps explain that difference when we talk about dry heat, right? When it’s hot, but you don’t necessarily feel all that stickiness.

Epstein: Exactly.

Alston: So Dave, one thing that happens that I always think about when it’s really humid outside is my hair, because as soon as all that moisture hits it, it shrivels up. And it’s amazing because it’s shrinkage with my kind of hair. And it gets really, really curly and it draws in, right? And even though that could be undesirable from an aesthetic perspective, in some cases it’s actually healthy because the steam adds moisture. Are there any of those type of other benefits, be it for plants, humans or animals?

Epstein: Yeah. I mean, the high humidity is good for a few things. With less evaporation, if we are in a drier pattern, we don’t see a lot of rain, then the ground also doesn’t evaporate and so the ground holds more moisture. Also, certain plants do like the tropical weather. So if you’re putting out your tropical houseplants for the summer, they’re really going to love this weather as long as you give them enough water. Some plants with high humidity don’t do as well: Like this can promote disease for tomato plants, for example. But eggplants seem to do really well. Peppers seem to do really well in the high humidity. And basil also loves the high humidity. And I know there’s some folks out there that just love this absolutely steam heat. For me, I’m outside till about 9 o’clock and then forget about it. Give me the air conditioning.

Alston: Yeah, I’m with you there, Dave. Well, that is GBH meteorologist Dave Epstein. A reminder if you ever have a plant or weather related question for him, you can text that to 617-300-2008. Dave, thank you so much.

Epstein: Thank you.

Alston: 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.

How can I tell how humid it’s going to be today? And why does humidity feel so awful?

Weather forecasts often include two humidity-related numbers: A percentage of relative humidity and a dew point.

The more informative number in this case is the dew point, Epstein said.

“It says: What temperature would the air have to be in order to be completely full of moisture?” Epstein said. “At every single different temperature, the air can have a certain amount of moisture in it.”

People’s comfort levels will vary, but as a general rule, dew points under 60 mean the air is drier. Dew points over 60 mean humidity is noticeable. Once they climb over 65, they’re uncomfortable. And over 70, for most people, is “oppressive and it’s not really enjoyable,” Epstein said.

Dew point cheat sheet

Under 60: Dry or comfortable

60-64: Noticeable humidity

65-69: Uncomfortable

Above 70: Feels oppressive

Tuesday’s dewpoint hit 72 at Logan Airport, with relative humidity at 67%.

The relative humidity number can also be good to know, Epstein said, but it’s important to understand that two days can have the same relative humidity percentage and feel very different.

Think of the amount of humidity in the air as a water filling up a vessel, Epstein said. A low dew point is like a small glass of water. In the winter, when temperatures are colder and dew points generally lower, even 100% relative humidity won’t feel so bad.

But in the summer, when dew points creep into the 60s and 70s, even a lower percentage of relative humidity means a lot of water is in the air.

“Cold air is going to be like a 16 ounce glass. Warm air is going to be like, an Olympic swimming pool,” Epstein said. “So when the 16 ounce glass is full, you have 16 ounces of water. The relative humidity is 100%. When the swimming pool is full, you have a lot more water. So 50% relative humidity in the summer is going to be more humid even than 100% relative humidity in that cold air, because 100% of a small amount, it’s still a small amount; 50% of a huge amount is still a big amount.”

Arid climates can experience dangerously high temperatures but very low humidity, Epstein said. That means our bodies’ sweat mechanisms are more effective: When we sweat, the moisture evaporates quickly into the dry air, cooling our skin.

In high humidity, that sweat takes longer to evaporate and we stay overheated, Epstein said.

“The air already has so much moisture in it that it’s really hard to evaporate off your skin,” Epstein said. “You stick with that very hot feeling, and your body’s ability to cool is lessened tremendously.”

Humidity can be beneficial for some tropical plants, Epstein said. Crops like basil, eggplant and peppers tend to do well in humid conditions. People with curly, coily, or dry hair can also see some benefits with the increased moisture.

“Some plants with high humidity don’t do as well: This can promote disease for tomato plants, for example,” he said. “I know there’s some folks out there that just love this absolutely steam heat. For me, I’m outside till about 9 [a.m.] and then forget about it. Give me the air conditioning.”