Researchers zero in on Twin Cities neighborhoods hit hardest by heat island effect

The last Saturday in July brought a heavy, miserable heatwave to the Twin Cities as temperatures hovered above 90 degrees. But it felt much cooler on a shady block in south Minneapolis’ Kingfield neighborhood.

That is the essence of the urban heat island effect: shadier sections in predominantly white neighborhoods have more tree cover to help cool the impact of the summer sun than other parts of urban areas where asphalt, concrete and industrial parks dominate the landscape.

The leafy section of south Minneapolis on Pillsbury Avenue is where Anna Berglund and Seiko Shastri started their route collecting data to help Hennepin County understand which areas are hotter and the differences between various blocks.

Berglund and Shastri were two of more than 120 volunteers who spent July 27 driving across the Twin Cities using window-mounted sensors to collect street-level data on the heat island effect. Their route took them through Minneapolis’ South Side, through forested parkland near Lake Nokomis and past industrial parks off Hiawatha Avenue as the temperature gauge in their Evie Carshare Nissan Leaf stayed in the lower 90s.

“This is the perfect day to do this,” Shastri said.

The National Oceanic and Atmospheric Administration (NOAA) selected Hennepin and Ramsey counties to participate in the urban heat island mapping project , which has studied the heat island effect in cities around the world for the past eight years. Urban areas experiencing a heat island effect can have up to a 20 degree temperature difference depending on the amount of shade.

A 2020 study revealed that Minneapolis’ formerly red-lined neighborhoods, areas where discriminatory housing practices took place from the 1930s through 1960s, were almost 11 degrees hotter than the city’s coolest neighborhoods .

The data will be used by Hennepin and Ramsey counties to allocate climate investments aimed at reducing the disparities between wealthier, whiter neighborhoods and formerly redlined areas that are home to more immigrants and people of color.

“We see a strong correlation between areas that are hotter and areas that are formerly redlined,” said Cliff Mountjoy-Venning, a senior planning analyst on Hennepin County’s climate and resilience team.

Updating the map

Hennepin and Ramsey counties submitted a joint application for the NOAA project, with support from the University of Minnesota and the cities of Minneapolis and St. Paul. The project has financial backing from the U.S. Departments of Health and Human Services and Housing and Urban Development.

The project typically selects individual cities, but climate staff at Hennepin and Ramsey counties decided they would have a better chance with a joint application. Both counties say reducing the heat island effect is part of racial equity goals baked into their respective climate plans.

Areas that are socially and economically vulnerable in Ramsey County also feel hotter in the summer, according to climate and health supervisor Abigail Phillips.

“It’s an unfortunate pattern you see in any map you look at,” Phillips said.

The Metropolitan Council mapped the urban heat island effect across the seven-county metro area in 2016 and 2022. That data is solid, but was gathered by satellite imaging instead of on the ground.

The NOAA project uses sensors from CAPA Strategies, a company that analyzes the data for the federal government. The data from past studies is available online at Heat.gov. Data collected by volunteers in Hennepin and Ramsey counties is expected to be available this fall.

“It’s a different kind of data, it’s going to have similar trends, but it’s going to be a little more targeted and focused,” Mountjoy-Venning said.

Heat islands in the Twin Cities are concentrated in downtown Minneapolis and St. Paul, along major highways, and in industrialized areas like St. Paul’s Midway.

Ramsey County sent data collectors across all 152 square miles, Phillips said. But county officials still placed an emphasis on areas deemed socially vulnerable and points of interest like schools, libraries and light-rail stations, she said.

Hennepin County is 554 square miles, much of which is farmland or leafy suburbia to the west, so the county decided on a mix of routes that emphasized the urban core. Routes went through north Minneapolis and northwest suburbs, northeast Minneapolis and Fridley, south Minneapolis, Edina, Richfield and Bloomington.

“Heat doesn’t stop at city borders, so we want to make sure we’re providing this information for as much of the county as we can,” Mountjoy-Venning said.

Short- and long-term strategies

The easiest way to reduce the heat island effect is to plant trees. But benefits from tree planting take up to 20 years to be fully realized. Both Hennepin and Ramsey counties plan to use the data to inform future tree planting, but want to offer more immediate benefits to residents living in those areas.

Ramsey County and St. Paul public health departments intend to use the data to conduct targeted outreach about extreme heat and health risks, Phillips said. Living in hotter environments is linked to cardiovascular and respiratory issues. Heat waves can be deadly, especially for the very young or old, pregnant women and people who work outdoors. Hot, stagnant air can also increase air pollution.

One short-term fix is helping residents near heat islands invest in weatherization and home air filtration improvements, Phillips said. Those can be fairly simple, like a Corsi-Rosenthal box , a common at home air purifier that uses box fans and purifiers.

“Things like that that we can do in the short term,” Phillips said.

‘Important data’

Both counties said they were impressed by the number of volunteers willing to devote a summer Saturday to data collection. Volunteers attended a virtual training and met up to have their vehicles outfitted with the sensors just after dawn on the collection day. They were instructed to stick to their routes and stay under 35 mph to ensure the sensors could take an accurate temperature reading every second.

The volunteers drove their routes three times that hot Saturday, once early in the morning, in the heat of mid-afternoon and in the evening in an attempt to capture the heat island effect across a day. Berglund and Shastri live in the Lowry Hill East neighborhood.

The two diligently drove their route Saturday afternoon, and made specific time notes in a message to administrators when they briefly made a wrong turn. Those notes are important to the integrity of the data collected, according to Phillips, and a reason that both counties were so impressed with their volunteers.

Berglund and Shastri are excited to see the data when it comes out and hope it results in tangible benefits for residents.

“I think it’s cool,” Shastri said. “It’s a fun, sort of unusual engagement opportunity.”

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