Many people feel hotter when it's humid outside. Here's the science behind it

LAUREN GILGER: The monsoon season is in full swing here in Arizona. There’s rain outside across the Valley today, and earlier this week, we saw our first real dust storm sweep across the city. Parts of the city got hit with a pretty big downpour, including at my house.

And while it’s really cooling things off, the high today is just 93°, according to the National Weather Service, it’s also gotten things pretty sticky outside there this week. The humidity today, at last check, was 65%, and here in Arizona, we’re really used to a dry heat. So, what do we all need to know about this humidity?

Here to tell us more is Erinanne Saffell, Arizona’s state climatologist emerita. Good morning, Erinanne. Thanks so much for coming on.

ERINANNE SAFFELL: Good morning.

LAUREN GILGER: OK, so I hear you like to watch the dew point more than the humidity levels. Tell us why.

ERINANNE SAFFELL: I absolutely do. And some folks out there probably are aware of the dew point, because that’s been talked about in Phoenix for decades. It’s one of the ways that we figure out how much moisture is in the atmosphere. So we use that dew point. It’s actually a temperature, and it’s measured all the time. So you can have dew point temperature measured on your weather station or on the Arizona weather stations. But it’s a way to figure out how much moisture there is in the atmosphere.

LAUREN GILGER: OK, so you like to look at that more than that humidity percentage, I read. OK. ... There’s also this measurement that I’ll see on my phone, right? That that says the actual temperature and then it’ll say like "feels like," this measurement, right? And that is probably very impacted by humidity, right?

ERINANNE SAFFELL: Yeah, so you’re probably getting the heat index, which is one way to kind of figure out how hot it is in the atmosphere. But the heat index doesn’t really work well for Arizona, because one of those things with that heat index is considering dew point temperature, and we’re usually pretty dry. So you can actually see sometimes that that heat index or that feels like might be even cooler than the actual air temperature, so it can be confusing.

LAUREN GILGER: Yeah. OK, so sometimes you might have, you know, a hot temperature outside and then it goes down with the rain, but the humidity goes up and it still feels hot, maybe just in a different way.

ERINANNE SAFFELL: Yeah, that’s kind of the way it is. Especially when we’re looking at what we’re experiencing this morning, isn’t it beautiful out there? ... So those clouds, one of the things with a thunderstorm is it cools temperatures down. That’s — it drops down that cold air from aloft, where those clouds were, and it brings that cooler air down where we are, so we’re all enjoying that right now.

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LAUREN GILGER: Yeah. OK, so tell us the science behind this. Like, why do our bodies feel hotter when there’s more moisture in the air?

ERINANNE SAFFELL: Right. Yeah, that is one of the things that we want to assess when we’re dealing with, how safe people are, what’s the temperature outside, what does it feel like to your body. And that’s because your body wants to cool off by evaporation. And so if it’s really moist and sticky outside, or if you have a high dew point temperature, then your body can’t cool off as effectively. That’s one of the considerations that we want to take into account and help keep people safe.

LAUREN GILGER: Right. So when we think about heat safety, and you know, we’ve seen this rise in heat deaths here over the years, is it more, less dangerous when there’s humidity outside than when it’s dry and 110 or 115?

ERINANNE SAFFELL: It’s a mix, isn’t it? And so when we have those really hot temperatures, when we get the 110, 115 degrees, it is hot. It doesn’t matter what the dew point temperature is, it doesn’t matter what the humidity is, your body is still going to be hot.

And if your body can’t cool off effectively, then you’re at risk. So we want people to understand, go inside, make sure you watch for folks as well — older folks, younger folks, even your pets, we want to make sure that everyone has a safe place to be that’s cooler. Because being outside in our summer can be very, very hot.

LAUREN GILGER: Yeah. OK, so how does humidity affect the storms that we’re seeing right now and throughout this monsoon season? Is that is that something you watch for?

ERINANNE SAFFELL: Oh, absolutely. Well let’s go back to the dew point temperature, if you don’t mind. ... A little meteorology 101, if we will. So we’re measuring dew point temperature all the time when we’re launching the weather balloons. That’s telling us the dew point temperature all the way up in the atmosphere. So it’s giving us how much moisture we have in the atmosphere.

And so when we see those clouds, what they’re actually at is they’re reaching, or what we would say, they’re cooling to the dew point temperature. They’re saturating, they’re condensing. And so when we see that, we’ve reached that condensation point, then hopefully we can get those droplets to come out of that thunderstorm.

LAUREN GILGER: OK, OK. What else are you watching for when you’re watching these storms come in? I mean, obviously, we’re we’re seeing rain right now, there’s a flash flood warning in effect, lots of rain in the forecast. What’s on your radar?

ERINANNE SAFFELL: Right. Yeah ... looking at things to see whether or not we can get a storm. So we look at the moisture, which is how much water’s in the atmosphere, how high the dew point temperature is. We look at the instability, is that air going to rise and cool to the dew point temperature?

And then when it comes out of the sky, we have to pay attention to what it’s been like with our soils. For example, if you look outside today, you probably got some some rain today, and you look at your backyard and it’s probably saturated if you got some of that storm, which means the next storm that comes in, that water is not going to go into the ground, and instead, it’s going to run off.

So we want to pay attention to that because that’s very common when we start getting moisture, when we start getting thunderstorms, we can have more of that that flooding that can happen. So we want people to understand that.

The other thing that I think is important is those storms that we get can happen upstream from where we’re driving, and all of a sudden, here’s water in a channel that we may not even have known was going to happen. So we want everyone to be safe from that.

LAUREN GILGER: Yeah, OK, lots to watch for as we have a pretty effective monsoon season this time around. That is Erinanne Saffell, Arizona’s state climatologist emerita joining us. Erinanne, thank you so much for coming on the show, as always, appreciate it.

ERINANNE SAFFELL: Happy to help.

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