On one of Baltimore's hottest days this past summer, the city baked at 98 degrees — according to the official reading at the Inner Harbor.

But at the same time, it was 87 degrees in leafy Leakin Park. And the mercury rose to 103 degrees in neighborhoods such as East Baltimore's McElderry Park and Middle East.

A team of volunteers gathered the measurements Aug. 29 as part of a research project studying urban “heat islands.”

Jeremy Hoffman, a scientist who helped organize the effort, called the findings alarming.

“That’s almost as much as the total temperature change throughout a day,” said Hoffman, climatologist at the Science Museum of Virginia in Richmond. “At the same time, two areas of the same city can be experiencing a wildly different heat wave.”

The federally funded research was intended to spur discussions about how to address the effects of urban heat islands — blocks of sprawling blacktop and few trees where higher temperatures can compound public health threats from factors such as poor air quality and a lack of healthy food options.

Heat can worsen underlying health conditions such as heart or lung disease, and can sicken or kill people by causing heat exhaustion or heat stroke. Heat was a factor in 28 deaths in Maryland this year, the most since 2012. Half of the deaths occurred in Baltimore.

Some cities have addressed the problem by opening more public air-conditioned spaces, removing or whitewashing pavement and planting more trees. Baltimore already has set a goal of growing its tree canopy to cover 40 percent of the city by 2037.

John Bullock, a city councilman who represents parts of West and Southwest Baltimore, said all options should be on the table to lessen the heat island effect.

“It definitely should be a priority,” he said. “It should be something we look at from a public health perspective, but also an equity perspective.”

The temperature data was collected via thermocouples — sensitive instruments that measure the electric voltage created by temperature differences — attached to the windows of cars driven by more than a dozen volunteers sent across the city. The researchers were instructed to zig-zag across a specific corner of the city from 3 p.m. to 4 p.m. Aug. 29, again from 7 p.m. to 8 p.m. that night, and then from 6 a.m. to 7 a.m. the next day.

Data being released today by the National Oceanic and Atmospheric Administration show that during that first time window, typically the hottest portion of the day, temperatures can vary widely by different categories of land use. Parks, not unexpectedly, were the coolest, with extensive tree cover and limited pavement. Also unsurprisingly, the city’s urban core was its hottest, especially neighborhoods along the city’s east side.

The highest temperatures were recorded in the Penn Fallsway, Middle East and Milton-Montford neighborhoods. Herring Run Park, Druid Hill Park and Leakin Park had some of the coolest spots.

Above-average heat was common throughout central Baltimore, and also stretched into the Reisterstown Road corridor of Northwest Baltimore and parts of Southwest Baltimore.

Many urban neighborhoods close to the water, including Fells Point and Locust Point, as well as relatively suburban parts of North Baltimore, were roughly 7 degrees cooler than the hottest spots, the researchers found.

The researchers have not yet analyzed data from the second and third hours of data collection. Information from those periods, revealing which areas cooled off faster than others, could provide even more insight into the heat island effect, Hoffman said.

The additional data should “tell us more about the structures and the human landscapes in those areas and how they interact with temperature throughout the day,” he said.

The experiment follows similar ones around the country and the world that show similar heat island extremes. Researchers also found a 17-degree temperature range in Washington in August, and a 15-degree variation in Richmond, Va., last summer.

The Baltimore and Washington studies, conducted with $30,000 of NOAA money, are meant to serve as models for other cities to conduct similar research, perhaps even more cheaply. The researchers said they fear the public health effects of heat islands could get worse, with heat waves expected to become more extreme and frequent as Earth’s climate changes.

“We’re hoping these results inspire other cities to think about this issue and this climate stressor,” Hoffman said.

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