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It’s close to 120 degrees in Phoenix. Does the ‘dry heat’ matter?

When temperatures are this high, the heat is dangerous and potentially lethal, regardless of humidity levels.

Camelback Mountain is visible through hazy, hot air at sunset in Phoenix on June 15. (Caitlin O'Hara/Getty Images)
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We’ve all heard the expression “dry heat” before. Maybe your next-door neighbors decided to retire in Tucson, or your work colleague grew up in Phoenix. They might refer to the blistering 110 degree plus temperatures common in the Desert Southwest as “dry heat,” but what does that really mean?

And does it really make any difference if it is nearly 120 degrees outside?

It is a sensible question to ask, especially as 40 million Americans in the West have been enduring temperatures over 100 degrees this week.

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To be sure, in many areas of the West the relative humidity levels are indeed quite low, ranging between 5 and 15 percent during the afternoon. But officials at the National Weather Service say it is so hot that the dry air does not make the heat less dangerous.

“The temperatures here are so hot it doesn’t really matter,” said Marvin Percha, a meteorologist at the Weather Service in Phoenix. “Here in Arizona, we hear about it being a dry heat. It’s true that the body can cool itself more efficiently with drier air, but we’re looking at temperatures between 115 and 117 … [the body] can just get overwhelmed.”

Hot air can affect humans in two main ways: by affecting our ability to regulate body temperature and by dehydrating us.

For the former, meteorologists have crafted a metric known as the heat index, which is also expressed in degrees, to gauge how much strain the ambient environment places on the body. It takes into consideration air temperature and the relative humidity, but requires making a number of assumptions that dictate how the human body absorbs and emits heat.

That means the formula constructs an “average human,” in this case 5-feet-7-inches tall and 147 pounds, who wears clothing on 84 percent of their body and walks at a leisurely 3.1 mph. It also assumes that a person sweats uniformly and loses 2 to 12 percent of their heat through breathing.

The higher the humidity, the tougher it is for sweat to evaporate, and subsequently the more difficult it is to cool down. If the air mass is completely saturated, sweat will not evaporate, meaning we cannot cool ourselves through perspiration alone.

The Weather Service has created a table that conveys how heat and humidity interact — and, admittedly, “dry heat” is a thing. For instance, a temperature of 90 with 40 percent relative humidity will net a heat index of only 91 degrees, but it spikes to 105 degrees at relative humidity of 70 percent.

Humid or not, heat becomes increasingly hazardous once the absolute temperature reaches the mid-90s and higher. And, with highs in the 115 to 120 range across Arizona and nearing 110 in California’s Central Valley, the heat could quickly become deadly. Heat is the leading cause of weather-related deaths in the United States in a typical year.

Heat and humidity also influence how easily we can become dehydrated. At hotter temperatures, the air can hold more water, speeding up evaporation. This effect is amplified if it is dry outside. That makes for a Catch-22 ― if it is humid out, your sweat evaporates more slowly and you struggle to cool down, but you retain more water. In dry environments, your sweat vanishes quickly, but in doing so depletes the body of moisture. In “dry heat” environments, like across the Desert Southwest, dehydration can hit fast as the atmosphere works to desiccate the body.

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That could even be a problem at night, too, for those without air conditioning.

“We’re looking at lows near 90, and the homeless community is greatly impacted by the heat,” Percha said. “Many emergency management agencies or government groups have opened up cooling shelters, but the majority close their doors during the overnight.”

All told, dry heat is a thing — but it does not matter at temperatures this extreme.

The National Weather Service’s recommendations are simple: “Stay indoors and seek air-conditioned buildings.”

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