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

Summary

Short Wave Podcast Summary: "Why Dew Point Is This Summer's 'It Girl'"

Release Date: July 4, 2025 | Host: Emily Kwong | Produced by NPR's Short Wave

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Introduction: Navigating the Scorching Summer

In the July 4th episode of NPR's "Short Wave," titled "Why Dew Point Is This Summer's 'It Girl'," host Emily Kwong tackles the often-overlooked meteorological concept of dew point and its significance in understanding and enduring the sweltering summer months. As the United States grapples with unprecedented heat waves and high humidity levels exacerbated by climate change, understanding dew point becomes essential for both personal comfort and public health.

Relative Humidity vs. Dew Point: Decoding the Moisture Metrics

The episode opens with Thomas Lu presenting a common summer scenario: preparing for an afternoon bike ride with a forecast of 85°F and 57% humidity ([02:33]). He questions whether these numbers indicate a pleasant outing or potential discomfort. Maddie Sofia responds by acknowledging the complexity of relying solely on humidity percentages to gauge how muggy it feels.

To clarify, Thomas brings in Greg Jenkins, a professor in the Department of Meteorology and Atmospheric Sciences at Penn State University. Greg defines relative humidity succinctly:

“Relative humidity is this ratio or percentage of water vapor over a term that is related to water vapor in a saturated state.” ([03:27], [03:30])

Thomas simplifies this by explaining,

“Relative humidity is the moisture content in the air compared to the maximum moisture content that could be in the air at its current temperature.” ([03:56], [03:59])

Using an illustrative metaphor, Maddie compares relative humidity to two drinking glasses—one representing warm air (90°F) and the other cooler air (70°F)—both filled halfway. While both glasses have 50% relative humidity, the larger glass holds more water, highlighting how relative humidity can be misleading without considering temperature ([04:39], [04:47]).

Understanding Dew Point: The More Reliable Moisture Gauge

Transitioning from relative humidity, Maddie introduces dew point as a more accurate measure of atmospheric moisture:

“Dew point is the temperature at which any parcel of air would become saturated.” ([05:43], [05:52])

Greg further elaborates:

“The dew point is the temperature... where physical droplets of water will start to form.” ([06:00])

Maddie reinforces this concept with the earlier drinking glass analogy, demonstrating that while both days have the same relative humidity, the dew points of 69°F and 51°F provide a clearer picture of actual moisture content ([07:15], [07:36]). This makes dew point a more tangible indicator for everyday use, helping individuals better assess how humid it truly feels outside.

Impact on Human Comfort and Health: Beyond Just Feeling Sticky

Delving deeper, the conversation shifts to the biological implications of high dew points. Maddie explains the body's cooling mechanism through sweating:

“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.” ([08:38], [08:44])

This slow evaporation leaves the skin feeling damp and sticky, impeding the body's ability to cool down effectively. Greg connects this physiological response to broader climate trends:

“When the heat index goes beyond 105, the risk of heat exhaustion and cramping and sunstroke... goes up and people start to show up in emergency rooms.” ([09:25], [09:36])

He emphasizes that as climate change drives temperatures higher, understanding and monitoring dew point becomes crucial for predicting and mitigating heat-related health risks.

Practical Applications: Making Sense of the Numbers

To make dew point more relatable, Maddie outlines a dew point scale to help listeners assess their comfort levels:

• 55°F and below: Described as dry or comfortable.
• 55°F to 65°F: Felt as sticky or damp.
• 65°F and above: Comparable to "walking through a swamp."

This scale aids individuals in making informed decisions about daily activities, such as dressing appropriately, planning outdoor exercises, or taking preventive measures against excessive heat.

Conclusion: Embracing Dew Point as the Summer's "It Girl"

The episode concludes with Thomas expressing his newfound appreciation for dew point:

“I had no idea about dew point. I am a convert. I appreciate you. This was awesome. Thank you so much.” ([10:18], [10:50])

Maddie highlights the dual importance of dew point: on a personal level, it helps individuals manage their daily comfort and health; on a societal level, it serves as a vital metric for assessing the broader impacts of rising temperatures due to climate change. By bringing dew point into the spotlight, "Short Wave" empowers listeners with the knowledge to better navigate and respond to the challenges of a hotter, more humid summer.

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Notable Quotes:

• Greg Jenkins on Relative Humidity:
  “Relative humidity is this ratio or percentage of water vapor over a term that is related to water vapor in a saturated state.” ([03:27])

• Maddie Sofia on Dew Point:
  “Dew point temperature... to gauge how much moisture is actually in the air at any given time.” ([05:36])

• Greg Jenkins on Heat Index and Health:
  “When the heat index goes beyond 105, the risk of heat exhaustion and cramping and sunstroke... goes up and people start to show up in emergency rooms.” ([09:25])

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This comprehensive exploration of dew point not only demystifies a critical meteorological term but also underscores its practical significance in our daily lives, especially as we confront increasingly extreme weather patterns.

Transcript

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[Emily Kwong]

You're listening to Shortwave from npr. Hey shortwavers, Emily Kwong here. Okay, I know Summer like 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 Shortwave producer Thomas Lu reported this episode out a few years ago. You'll hear him chatting with my former co host Maddie Sofia about what the dew point is and why. Some meteorologists say it's the key to understanding how truly swampy it is outside. Stay cool and keep listening to Shorewave, the science podcast from NPR Foreign.

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[Thomas Lu]

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.

[Maddie Sofia]

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

[Thomas Lu]

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

[Maddie Sofia]

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. Watering the air temperature, and how these two interact with one another. So I called up someone I thought might have some answers.

[Greg Jenkins]

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

[Maddie Sofia]

And Greg explained relative humidity, like this.

[Greg Jenkins]

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

[Thomas Lu]

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 totes.

[Maddie Sofia]

That's why it's called relative humidity.

[Greg Jenkins]

It's not an absolute measure of moisture.

[Maddie Sofia]

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.

[Greg Jenkins]

Right, Right.

[Maddie Sofia]

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

[Greg Jenkins]

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.

[Thomas Lu]

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

[Maddie Sofia]

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

[Charles Schwab]

Okay.

[Maddie Sofia]

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.

[Thomas Lu]

Got it.

[Maddie Sofia]

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

[Thomas Lu]

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.

[Maddie Sofia]

Yes, exactly. That's exactly right.

[Thomas Lu]

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

[Maddie Sofia]

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.

[Greg Jenkins]

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

[Maddie Sofia]

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.

[Greg Jenkins]

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

[Maddie Sofia]

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.

[Thomas Lu]

Oh, the dew in dew point.

[Maddie Sofia]

Yes, exactly. The dew in dew point.

[Thomas Lu]

I'm getting it.

[Maddie Sofia]

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 vapor form. The water molecules are slowing down and forced out of the air, condensing around things like dust to form clouds.

[Greg Jenkins]

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

[Thomas Lu]

Condensation.

[Maddie Sofia]

Condensation it is.

[Thomas Lu]

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

[Maddie Sofia]

Oh, yeah, yeah, Totally, Maddie. So, you know, to get you a little more jazzed up, you know, like I am, right. I 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.

[Greg Jenkins]

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.

[Maddie Sofia]

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. 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.

[Thomas Lu]

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

[Maddie Sofia]

Exactly. And that's the dew point of it all. Okay, 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, 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.

[Greg Jenkins]

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 okay, Tilu.

[Thomas Lu]

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.

[Maddie Sofia]

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

[Thomas Lu]

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

[Maddie Sofia]

Oh, it's been gross, Maddie. 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 and moist and gross. You know what? Yep, that's exactly it. Truly. But on a serious note, Maddie, Greg told me it actually ties into climate change.

[Greg Jenkins]

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.

[Maddie Sofia]

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.

[Greg Jenkins]

In general, when the heat index goes above 90 degrees, you start to see health impacts. Okay. When a 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 a heat index beyond 130 degrees, you are likely going to see large numbers of fatalities.

[Maddie Sofia]

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

[Thomas Lu]

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

[Maddie Sofia]

Thank you.

[Thomas Lu]

This episode was reported and produced by Thomas Liu, edited by Gisele Grayson and fact checked by Indi Kara. The audio engineer for this episode was Kwesi Lee. I'm Maddie Sofia. Thanks for listening to Short Wave from N.

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