Psychedelic images reveal what your weather app isn't telling you

My weather app says it’s 80 degrees, but that’s a lie. I’m standing on a football field at Portland State University with professor Vivek Shandas, and we can feel intense heat radiating off the grass. Instead of being made of cellulose, these blades are plastic, because this field is artificial turf. Whereas natural vegetation has a cooling effect on a landscape — it “sweats” water vapor — this stuff absorbs and radiates the sun’s energy, making the ambient temperature especially uncomfortable.

Shandas, who’s founder and director of the Sustaining Urban Places Research Lab , can prove it. He hands me a Flir (forward-looking infrared) camera that I attach to my phone and control through an app. I point it at the ground and it gives me a color-coded view of the temperature at any given spot — approaching 150 degrees Fahrenheit in parts of the ground beneath my feet, compared to as low as 100 degrees in a patch of real grass in the sun next to it. (In the video below comparing the two, brighter oranges and yellows indicate higher temps, with purple showing lower readings.) We’re actually picking up significant differences even within the field: The darker greens are about 10 degrees hotter than white lines, which are bright enough to reflect some energy away — the same reason you feel cooler wearing lighter colors on a hot day.

At 1:45 in the afternoon, conditions are relatively mild so far. “If you think about a 100-degree day on a soccer game or a football game turf like this, I mean, those ankles are going to blow out, those knees are going to blow out,” Shandas says while pointing his own Flir camera at bits of turf. “Those kinds of temperatures are not good for the joints or physiology generally.”

We walk up a small hill at the edge of the field to get an overview of the I-405 highway. These multilane thoroughfares are consistently among the hottest parts of a city, because the surface soaks up the sun and vehicles generate still more heat with the friction of their tires. In addition, an internal-combustion engine produces a substantial amount of waste heat. You can think of them as thousands upon thousands of little heaters zooming past the Portland State campus every hour. I point the Flir camera at the lanes and from a few hundred feet away get readings approaching 120 degrees — still far cooler than the artificial turf — and pan down to the shaded vegetation in front of us, where it’s in the mid-80s.

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For pedestrians, shade is especially important on sidewalks. Making our way along the side of the field, pavement in the sun is in the mid-130s, dropping 50 degrees in adjacent shaded areas (shown below).

Back in his car, Shandas attaches a small magnetic sensor to the roof. A different app feeds me a constant stream of readings. Here in Portland’s downtown, with its tall, shade-bestowing buildings, it’s 84 degrees. As we move east across the Willamette River, it dips to 82 degrees before steadily climbing — 85, 86, 87, 88, and finally topping out at 89 F.

There are many reasons for these disparities. The more trees a neighborhood has, the more shade it gets, and the more of that “sweating” cooling effect. (That low reading of 82 came as we drove down a tree-lined street in a residential neighborhood.) Lower-income areas tend to have much less tree cover, and therefore more open pavement that absorbs heat and releases it at night: The highest readings we recorded were in areas with wide roads, few trees, and low buildings, which provide minimal shade. So not only do these areas suffer higher highs during the day, but their residents don’t enjoy as much of a respite in the evening, compounding the stress on their bodies. This is especially dangerous for people who can’t help but be outside, like construction workers and the unhoused.

Depending on where you live in a city, this heat unfolds in unique ways, hour after hour. “Places with the most buildings and infrastructure tend to be the hottest in the morning,” Shandas says, “but often some of the taller buildings keep it much cooler relatively in the daytime.” Areas with shorter buildings and few trees get blasted all day, taking a heavy toll on residents.

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The hotter the planet gets, the wider these disparities grow. On this 80-degree day we’re finding differences of up to 7 degrees between neighborhoods. But during a previous heat wave, Shandas recorded 125 degrees in one notoriously hot area on the city’s east side, compared to 99 in a hilly, verdant neighborhood. “On a 90-degree day, the difference is about 15 degrees to 18 degrees,” Shandas says. “On a 110-degree day, it goes up to 20 to 25 degrees, and on. I don’t know what that limit is. I don’t think I want to find out.”