Hanging 2 meters above the ground in backyards and parks, more than 200 sensors are measuring the air temperature around the Twin Cities metro area.

University of Minnesota assistant climate professors Peter Snyder and Tracy Twine are hoping the data gathered from these sensors will help them better understand why urban areas generate and hold more heat than surrounding areas — a phenomenon known as the urban heat island effect.

“Most of us live in urban areas,” Twine said. “It’s our daily weather. It’s what we’re experiencing, so it has a lot to do with our lives.”

Since 2011, the urban climate project Islands in the Sun has been collecting and recording temperature data from around the metro. The University’s Institute on the Environment and College of Food, Agricultural, and Natural Resource Sciences are funding the $400,000, four-year project that could be the densest network of temperature sensors in the country.

So far, the data is already revealing a few things about how heat is distributed across the Twin Cities, Twine said, including the hottest and coolest spots in the cities.

The annual air temperature of a city with more than 1 million people can be about 2 to 5 degrees Fahrenheit warmer than its surroundings, and as much as 22 degrees warmer during the night, according to the U.S. Environmental Protection Agency.

Three months of data from Islands in the Sun showed that both Minneapolis and St. Paul had an average temperature difference of about 3 degrees compared to surrounding areas.

Downtown Minneapolis is the hottest area in the metro, Twine said, followed by downtown St. Paul. The coolest spots in the cities are around lakes and riverbanks.

Postdoctoral researcher Brian Smoliak, who’s working on the Islands project, said there are 82 sensors in Minneapolis and St. Paul alone. From those sensors, researchers have learned exactly how far out the heat island stretches from the two downtowns.

In downtown Minneapolis, the heat island resides north of Franklin Avenue, south of Broadway Avenue, west of Highway 280 and east of Lake Harriet, Smoliak said.

Heat waves are typically amplified inside cities, Twine said, and excessive heat is also linked to air quality issues, like the formation of poisonous ozone gas. The dark surfaces from pavement, rooftops and asphalt absorb more heat from the sun than vegetation; tall buildings often block wind and trap in warmth; and cars and buildings generate extra heat, she said.

Handling extreme temperatures

Pamela Blixt, preparedness manager for the Minneapolis Health Department, said the city experiences two to three extreme heat events each year.

In 1995, Chicago experienced a heat wave that resulted in nearly 500 deaths. While the Twin Cities hasn’t experienced a heat wave that extreme, Blixt said, she recalls seeing a spike in hospital visits for heat-related illness after last summer’s record heat waves.

Over the last decade, the state has experienced an increase in heat and extreme heat events, said Kristin Raab, climate and health program director for the Minnesota Department of Health. The urban heat island effect aggravates these trends.

In 2011, there were more than 1,200 emergency room visits due to heat-related illness. Since 2000, there have been 44 heat-related deaths, with six in 2012 alone.

“It’s certainly something to be concerned about,” Raab said. “We’re working with the pollution control agency and with other agencies to look at how to mitigate the heat urban island effect.”

Earlier this month, Minneapolis set a goal to reduce carbon dioxide pollution by about 80 percent by 2050.

Minneapolis Sustainability Program Coordinator Brendon Slotterback said the city is also working on developing more immediate plans to reduce heat around the city, such as planting more trees and possibly incorporating green or white rooftops and surfaces in future infrastructure.

Over the past four years, Minneapolis has planted more than 6,000 new trees every year, he said. But with increased heat concerns and the spread of emerald ash borer, he said, the city will likely increase that number soon. Trees provide shade and reflect heat that would otherwise be absorbed by materials like pavement, he said.

“From a public health standpoint and an infrastructure standpoint, I think we are very worried about the impact of extreme heat on the people of the city,” he said.

Smoliak said having so many sensors collecting data within the cities is providing a much more nuanced picture of urban heat islands in the metro area.

Once they finish collecting data next year, Twine said, the researchers plan to share it with the Georgia Institute of Technology and the University of Georgia, which are collecting similar information on urban heat islands. Together with a $750,000 grant from the National Science Foundation, they’ll create a network of sensors to measure urban heat islands across the country.

“The urban heat island is a perfect example of something we can actually do something about,” Smoliak said. “Now that we understand the heat island better through this project, it’s going to help us evaluate these strategies for reducing it.”