Swimming pools are supposed to offer relief. Do they actually make the air more humid?

That old cliché about the Southwest—“It’s hot, but it’s a dry heat”—is so firmly rooted in Arizona that when it’s muggy outside, locals are wont to demand an explanation. In Phoenix, they have been known to blame artificial lakes and swimming pools for unwanted humidity.

“Like folk wisdom everywhere, this line of reasoning is widely accepted as true because the evidence seems overwhelming,” the Arizona Republic wrote in 1985. “In short, so the theory goes, excessive use of water has ruined the quality of desert living.”

The idea is more than a little paradoxical. Pools and lakes are supposed to offer some relief from summer heat. Could they actually be making Phoenix more miserable? It’s an apt question during the city’s hottest summer on record. Just on Friday, the mercury there soared to 117, tying the highest temperature on record in August.

For decades, scientists have investigated how land-use affects Phoenix’s blistering desert climate.

In the 1980s, scientists at Arizona State University looked into the matter, charting measures of humidity over time. If pools were changing the air’s moisture content, the city should have grown more humid as it developed. But their analysis found that humidity hadn’t actually risen at all.

“Many residents thought atmospheric moisture was increasing due to the increase in golf courses, man-made lakes, and swimming pools,” Sandra Wardwell, lead author of the 1986 analysis, said in an email. But that idea was wrong, she said. If anything, absolute humidity had actually declined.

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