Why Dew Point Is This Summer's 'It Girl'

Happy Independence Day, Short Wavers! Do you have plans outdoors this weekend and want to figure out just how swampy it's gonna feel? For that kind of mental preparation, we're revisiting an episode in which some meteorologists are telling us to pay more attention to dew point temperature, not relative humidity. Interested in more weather episodes? Email us your question at [email protected] mailto:[email protected]. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:16Emily Kwong?

  You're listening to Short Wave from NPR.

• 0:22Emily Kwong?

  Hey, Short Wavers. Emily Kwong here. OK, I know summer just got here, but she is making her feelings known. There have been so many 90 and 100 degree days and afternoon thunderstorms. And then there was last month's heat dome, which brought oppressive heat to the eastern half of the US. Experts forecast that heat waves will only become more intense and more frequent with climate change. So how can we proceed with summer safely? Well, I'll let you in on a little secret. It's called the dew point. Former Short Wave producer Thomas Lu reported this episode out a few years ago. You'll hear him chatting with my former co-host, Matty Safia, about what the dew point is

• 1:04Emily Kwong?

  and why some meteorologists say it's the key to understanding how truly swampy it is outside. Stay cool and keep listening to Short Wave, the science podcast from NPR.

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• 2:44Speaker 03

  Okay, Thomas Lu, summer hater. Here's a scenario. I wake up in the morning. I'm preparing to go on an afternoon bike ride on my hog around the city, check my weather app for the forecast. It's hot, you know, say about 85, but manageable. And the humidity is like 57%, let's say. Does that mean I'm still going to have like a nice bike ride or?

• 3:05Speaker 04

  Well, maybe, sort of. It's a little hard to tell with just humidity.

• 3:10Speaker 03

  Okay, but humidity is telling us how much water is in the air, right?

• 3:14Speaker 04

  Right, right, right. So, yes, here's where it gets a little bit tricky. To understand this, we need to consider a couple of things. Water in the air, temperature, and how these two interact with one another.

• 3:25Speaker 03

  Okay.

• 3:26Speaker 04

  So I called up someone I thought might have some answers.

• 3:29Speaker 05

  Hi, I'm Greg Jenkins. I'm a professor in the Department of Meteorology and Atmospheric Sciences at Penn State University.

• 3:37Speaker 04

  And Greg explained relative humidity like this.

• 3:40Speaker 05

  Relative humidity is this ratio or percentage of water vapor over a term that is related to water vapor in a saturated state.

• 3:54Speaker 03

  Okay, okay. So I'm going to oversimplify here, but relative humidity is the moisture content in the air compared to the maximum moisture content that could be in the air.

• 4:06Speaker 04

  Totes, that's why it's called relative humidity.

• 4:10Speaker 05

  It's not an absolute measure of moisture.

• 4:12Speaker 04

  Greg says a key factor in relative humidity is air temperature. You know, the number we usually look at when describing if it's going to be hot or cold out.

• 4:20Speaker 03

  Right, right.

• 4:21Speaker 04

  Warmer air can contain more moisture, while cooler air can contain less moisture. So over the course of a day.

• 4:29Speaker 05

  If you just had the amount of water vapor in the atmosphere fixed, and you let the temperature run its normal course, the relative humidity would go up and down just based on temperature.

• 4:42Speaker 03

  OK, so you can have the same amount of moisture all day, but the relative humidity will change as the day gets hotter or colder.

• 4:49Speaker 04

  Yep, indeed. So here's another way to think about it. Picture two drinking glasses, a pint glass and a juice glass.

• 4:57Speaker 03

  OK.

• 4:58Speaker 04

  Since we know the warm air can hold more moisture, we'll use the larger pint glass to represent air that's 90 degrees Fahrenheit, and the smaller juice glass to represent air that's 70 degrees Fahrenheit.

• 5:10Speaker 03

  Got it.

• 5:11Speaker 04

  If you fill each glass with water about halfway, you'll have two different amounts of water in each glass.

• 5:18Speaker 03

  Oh, but you can still describe both glasses as having 50% relative humidity because they're both half-full, even though one has much more water.

• 5:28Speaker 04

  Yes, exactly. That's exactly right.

• 5:30Speaker 03

  But I'm guessing that the 90-degree day, or the bigger glass, will probably feel a lot muggier?

• 5:36Speaker 04

  You'd probably be right, Maddie. But just seeing that percentage alone, you'll need to know a little more to help you decide how to dress for maximum comfort. Here, this is what Greg says.

• 5:47Speaker 05

  There are other terms that can be used to get an absolute sense of moisture in the atmosphere at any given time.

• 5:53Speaker 04

  Like dew point temperature, which can be a little more helpful, if not a little more accurate, to gauge how much moisture is actually in the air at any given time.

• 6:03Speaker 05

  Yeah, dew point is the temperature at which any parcel of air would become saturated.

• 6:10Speaker 04

  So at this point, the amount of water vapor is at its max. You're really on this edge where physical droplets of water will kind of start to form.

• 6:19Speaker 03

  Oh, the dew. Dew in dew point.

• 6:21Speaker 04

  Yes, exactly. The dew in dew point.

• 6:23Speaker 03

  I'm getting it.

• 6:24Speaker 04

  Basically, what's happening is that air has cooled to a point, the dew point, where it can no longer hold water in its gas form. The water molecules are slowing down and forced out of the air, condensing around things like dust to form clouds. Or...

• 6:42Speaker 05

  Your car window will have dew all over it, or the grass will have dew sitting on top of it.

• 6:48Speaker 03

  Condensation.

• 6:49Speaker 04

  Condensation it is.

• 6:51Speaker 03

  OK, Thomas. So dew point sounds, I don't know, more fun and a little bit more accurate. I'll say it.

• 6:57Speaker 04

  Oh, yeah, yeah, totally, Maddie. So, you know, to get you a little more jazzed up,

• 7:03Speaker 04

  you know, like I am right now, you can totally find the dew point in the same place you're looking at your daily forecasts, on your phone or on your weather app.

• 7:13Speaker 05

  So it's not something that we really need to calculate. In fact, it's reported from all of our observations at most of the weather stations around the country.

• 7:26Speaker 04

  The dew point is more of a concrete number. So let's go back to our drinking glass metaphor. Both days had a relative humidity of 50%. That doesn't really tell you much, right?

• 7:36Speaker 03

  Right.

• 7:37Speaker 04

  But when we look at dew point, for the 90-degree day, the dew point was at 69 degrees. For the 75-degree day, the dew point was 51.

• 7:47Speaker 03

  Yeah, that's easier to compare. I mean, that feels like a lot more helpful.

• 7:50Speaker 04

  Right, exactly. And that's the dew point of it all.

• 7:52Speaker 03

  OK.

• 7:53Speaker 04

  Here's a scale you can use based on the dew point to help you gauge how comfortable you're feeling. Anything 55 and below is often described as dry or comfortable. Between 55 and 65, you'll start hearing words like sticky or damp. Dew points at 65 or above, people will start describing feeling like they're walking through a swamp.

• 8:18Speaker 05

  When the dew point temperature starts to rise, it's telling us that the moisture in the environment is increasing. There's no doubt about that. And the closer, the higher it gets to the actual temperature, the worse it's going to be in terms of how the body feels.

• 8:34Speaker 03

  OK, Tealoo, so we've chatted about relative humidity, the air temperature, and why dew point is a more absolute gauge of water in the atmosphere. But like, when it comes to how I feel, like, why exactly does it matter? Like, talk to me about some biology.

• 8:48Speaker 04

  Yeah, in high heat like the days of summer, our bodies are constantly trying to keep cool.

• 8:54Speaker 03

  Oh, yeah, through sweat. It's been a real sweaty summer out here, Thomas.

• 8:58Speaker 04

  Oh, it's been gross, Matty. I have been drenched all summer. But you see, sweat is one way our body cools itself off. And on hot, humid days, the air is already pretty saturated with water, which unfortunately means sweat evaporates more slowly. So this leaves our skin feeling damp and sticky. And, you know, it collects on our skin and keeps our body warmer.

• 9:23Speaker 03

  And moist and gross.

• 9:25Speaker 04

  You know what? Yep, that's exactly it, truly. But on a more serious note, Matty, Greg told me it actually ties into climate change.

• 9:35Speaker 05

  So as the temperature rises, we can estimate what the heat stress is on the body just by looking at temperature and dew point on the heat index and on the human body.

• 9:47Speaker 04

  And that heat index he's referencing, it's a chart put out by the National Weather Service. We'll link to it in our episode notes. The chart takes into account temperature and moisture in the air to indicate how hot the weather feels to our body.

• 10:00Speaker 05

  In general, when the heat index goes above 90 degrees, you start to see health impacts, OK? When the heat index goes beyond 105, the risk of heat exhaustion and cramping and sunstroke, those go up and people start to show up in emergency rooms. If you go to heat index beyond 130 degrees, you are likely going to see large numbers of fatalities.

• 10:29Speaker 04

  It's a little daunting to, you know, think about how much heat can affect our bodies.

• 10:34Speaker 03

  Right.

• 10:35Speaker 04

  So on a day-to-day level, air temperatures, dew point, and relative humidity might help us decide what to wear or if we should go on a run or how much lotion to put on our bodies. But from a big-picture level, it also helps us measure and predict the impact of heat, which is super important as the climate heats up.

• 10:54Speaker 03

  OK, Thomas Lu. I had no idea about dew point. I am a convert. I appreciate you. This was awesome. Thank you so much.

• 11:01Speaker 04

  Thank you.

• 11:03Speaker 03

  This episode was reported and produced by Thomas Lu, edited by Gisele Grayson and fact-checked by Indy Cara. The audio engineer for this episode was Kwasi Lee. I'm Maddy Safiya. Thanks for listening to Short Wave from NPR.

• 11:29Speaker 00

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