Hot in the city

Anyone who lives in a big city knows that on hot days some areas are less unpleasant than others.  But an American researcher has found that on a typical summer day, urban temperatures can vary by as much as 11.5 degrees Celsius across gradients as small as a few hundred metres.

A few years ago, Vivek Shandas, an urban climatologist at Portland State University in Oregon, realised that he could map urban temperatures by equipping cars with inexpensive sensors. Measurements could then be taken as drivers crisscrossed the city. 

The instruments cost about $US350 to $US400 each, plus a small amount for a few modifications, including 3D printing supplies to make mounting brackets. 

GPS then recorded the sensors’ positions as volunteers drove through a dozen or so assigned areas 6am, 3pm, and 7pm, collecting tens of thousands of measurements in an hour.{%recommended 7740%}

“We [have] a little citizen-science brigade,” Shandas says. “They go through parks and residential areas and industrial areas, and really try to capture as diverse an area as possible.”

He then compared the measurements to maps of the areas in which they were made, looking at how urban landscapes – including buildings, surface reflectivity, trees, and wind patterns – affected the results. From that and “some fancy computations” he filled in the gaps to create a map so detailed that people can easily spot features of their own neighbourhoods. 

“We create a model that tries to predict why each temperature reading is what it is, based on the surroundings,” he says. 

The results are not only stunningly detailed, but startling. Not only is there a marked difference between hot and cool spots, but the spots themselves aren’t always where one might expect them to be.

It was no surprise, for example, that wooded parks were cool spots. But so too was downtown Portland, an urban area where trees are in short supply. The city’s skyscrapers, it turns out, produce a lot of shade, but aren’t so tightly spaced that they block the wind. 

Parks, on the other hand, aren’t always cool oases. “Open spaces were hotter than I would have anticipated,” Shandas says, especially if their grass wasn’t watered in the summer. 

Part of the cooling effect of vegetation, he says, is due to evapotranspiration — the process by which plants use water for photosynthesis. “If you don’t have water, you don’t have that cooling,” he says.

In other cities, Shandas explains, hot spots can be produced by waterfront development projects. “Big buildings along waterways block the wind,” he says. “Areas adjacent to or downwind from those areas end up becoming very hot.”

To date, he adds, his team has studied 10 cities, including Houston, Texas; Doha, Qatar; Villahermosa, Mexico; and Vancouver, Canada. 

Some of the data has been published, including a 2017 paper about Portland in the journal Climate

But mostly, Shandas is interested in providing data for local-level decision-makers. His own city of Portland, for example, is currently in the middle of a summer that has already produced the largest number of days with temperatures above 32.2 degrees Celsius (the old 90 degrees Fahrenheit) in history. One of the city’s concerns has been to offer “cooling centres” for people who lack their own air-conditioning. But the process might be improved, he says, because some communities that register as hot spots on his maps are “very far” from the nearest such centre. 

In the longer term, he adds, data such as his is useful for urban planning. “It’s making its way into policies that cities are using as they think about what the future may bring,” he explains. 

For example, a small segment of one of Portland’s main streets that is mostly noted for strip malls and automobile lots recently planted 247 trees in an effort to improve its liveability. At the moment, the trees are only about an inch in diameter, Shandas says, “but we will model them 20 years out.” 

In the long run, he says, he wants to see cities actively integrate heat into their urban planning, especially for a climate-changing future. 

“What can be done to mitigate the increasing intensity and duration of heat waves in the future?” he asks. “A big goal is to get developers to think about how they would include climate in their design specifications.”

Not that comfort is the only possible outcome of efforts to reduce urban hot spots. Pouya Vahmani, an urban climate modeller at Lawrence Berkeley National Laboratory in California, has found that “cool roofs” — meaning ones with light-coloured surfaces designed to reflect sunlight back to space — are also boons to water conservation, an increasingly important issue in the climate-warmed future.

In a study of 18 California counties published in the journal Nature Communications, he found that such roofs could lower the average metropolitan temperature by as much as 1.2 degrees Celsius — enough to lessen yard-watering needs by a whopping 9%. 

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