It’s a dry heat in Boise, right? Here’s how humidity affects you in extreme weather


This week’s blistering temperatures are enough to make it feel as if you’re melting. But the reading on your thermometer — or your phone — doesn’t always reflect accurately how it feels to be outside.

Temperatures had already topped 100 degrees this year before Monday, when it was 100 by 3 p.m. and 103 shortly after that. On June 3, it was also 103, breaking the record for that day by 5 degrees.

On Tuesday, it’s expected to be at least 103 again, which doctors warn is dangerously hot. But how much of a difference do a few degrees here or there make, really? Is 103 much different from 98?

“If you just look at the temperature, there’s probably not a lot of difference in danger,” Michael Roach, executive medical director for primary care at Saint Alphonsus Medical Group, told the Idaho Statesman by phone.

But a measurement called the “heat index,” Roach said, takes into account more than just the raw temperature and comes closer to measuring what the experience of being outside is like on a given day.

And to understand why the heat index matters, you first have to understand our bodies’ sweating, which is what helps us cool.

“When the environmental temperature reaches pretty close to what your core temperature is, the only way that you can lose heat is through evaporation. And that’s essentially what sweat does,” David Paul, an expert in exercise physiology at the University of Idaho, told the Statesman by phone.

And this week, the outside temps are easily exceeding the human core temperature of 98.6 degrees.

Though sweating is effective at cooling you down, it becomes less effective when it’s humid.

“When you go somewhere where it’s really high humidity, there’s already so much water vapor in the air, that the sweat on your skin has nowhere to go,” Paul said.

Since the sweat can’t evaporate as quickly, it makes it feel hotter out than what the temperature actually is.

To account for this effect, the National Weather Service (NWS) uses the heat index, which employs the humidity and the temperature.

And Boise’s low humidity can be helpful.

For example, NWS predicted that at 5 p.m. Tuesday around the airport, it will be 103 degrees with only 12% relative humidity. That corresponds to a 98-degree heat index and means that there’s just about one tablespoon of water in the air in a 100-square-foot space.

If the relative humidity was, say, 75% instead, there would be seven tablespoons of water in that same amount of space and the heat index would be a whopping 165 degrees, as calculated with an online tool.

The heat index indicates how it feels to be in a shady location with a light wind, so being in the sunlight can increase how hot it seems by “up to 15” degrees, according to the weather service.

The measure is important because of how it relates to people’s ability to regulate their internal temperatures, Paul said. Once the humidity rises to 40% or so, it begins to feel hotter than the temperature reading indicates.

“That’s not a huge difference, but it is a difference,” Roach said.

Fortunately for the Treasure Valley, this week’s heat won’t produce an incredibly high heat index. The flip side is that it’s going to be so hot anyway, there’s not much comfort.

“If you’re in Boise, and it’s very, very dry, the heat is getting so great that your body probably cannot evaporate enough water to keep your core temperature reasonably normal,” Paul explained.

“There’s a lot of things where heat index really matters, but (when) we’re getting into extreme heat, like we are now, even dry heat is not going to protect you.”

One reason why this week’s temperatures could be more dangerous is that it’s this hot so early in the summer, and our bodies have not adjusted yet.

“There are some adaptations that take a few days versus there’s other ones that take several weeks,” Paul said.

For example, people who often work out in the heat might be better at adapting.

“Even a fully acclimated person in 100-and-something-degree heat, they’re not replacing the water they’ve lost. They still have to be really cautious,” Paul said.

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