Leah Day, owner of Lighthouse Bikes in South Portland, said the Knightville/Mill Creek area can become a real oven in the summer, when the sun beats down on the streets and the parking lots. There isn’t enough tree shade to provide relief, she said. She said her bike rental business takes a hit during high heat days because people from away come to Maine to escape the heat and are surprised to discover just how hot it can be during the summer. Penelope Overton/Staff Writer

The doors to Leah Day’s South Portland bike shop were thrown open Tuesday in hopes of catching a stray southwesterly breeze coming off nearby Casco Bay, but there was no relief to be found from the midafternoon sun and humidity.

The heat was coming from all angles: the sun, the air-conditioning units poking out of second-story windows, passing cars and delivery trucks, the half-block stretches of unshaded sidewalks, and even Ocean Street. On a summer day, the neighborhood can feel like an oven, Day said.

“That’s what all pavement and not enough trees will do for you,” Day said. “A lot of our rental business comes from tourists, and they come to Maine to get away from the heat. They are really surprised at how hot it gets. When it’s like this, our business takes a hit.”

The Knightville/Mill Creek section of South Portland is an urban heat island. The manmade environment of this roughly 600-acre area increases the air temperature for the 3,700 people who call it home by a state-leading 11.1 degrees, according to a nationwide heat index analysis released Wednesday.

According to Climate Central, a New Jersey nonprofit of scientists and communicators, this seaside cluster of asphalt and cement is 11.1 degrees hotter because of its plethora of pavement and heat-absorbing buildings than if it were one big sprawling field of grass.

Global temperatures are rising everywhere because of human-caused climate change, but the built environment is amplifying the temperatures in many cities, even in temperate climates like Maine where most people do not think of extreme heat as a major threat to the population.


According to the latest science update to the state climate action plan, the average daily temperature in Maine will rise 2-4 degrees by 2050 and up to 10 degrees by 2100. While winters are warming fastest, our summers will be hotter and marked by more extreme heat days: the average summer high in turn-of-the-century Portland will feel like Scranton, Pennsylvania, does now, or about 8.9 degrees hotter.

Extreme heat is the world’s deadliest weather and climate hazard, according to the Environmental Protection Agency. During heatwaves, the urban heat island effect worsens heat-related illness and puts vulnerable groups like the elderly, outdoor workers and children at greater risk.

It also leads to higher energy bills and strained power grids as public demand for cooling spikes.

City arborist Mark Reiland evaluates the root structure of a tree about to be planted in the Bayside neighborhood of Portland in June. It is one step the city is taking to reduce the heat island effect in a neighborhood that lacks an extensive tree canopy. Photo courtesy of the City of Portland

The most concentrated heat island in Maine can be found in Portland, where the built environment amplifies the base “field of green” temperature anywhere from a low of 6.4 degrees in the East End and 7.3 degrees in Deering Center to 10.5 in the Old Port and 10.8 degrees in East Bayside.

“A lot of our older U.S. cities were built to be warm, built to keep heat in, built to keep ourselves warm, but now we’re in a new paradigm where we want to keep heat out,” said Jen Brady, Climate Central’s senior data analyst. “It’s not something we can shift overnight, but there are things we can do.”

That includes planting more trees, especially along paved streets; planting rooftop gardens for shade and temperature reduction; replacing dark roofs with roofs made of highly reflective materials; and replacing hot concrete or asphalt pavements with pavements of reflective or permeable materials.


Portland is taking steps to mitigate the heat island effect, Sustainability Director Troy Moon said.

“Addressing urban heat is an important part of our climate adaptation strategy,” Moon said. “Our parks department is currently implementing a program to plant 120 trees in the Bayside neighborhood, which lacks the extensive tree canopy found in other parts of the city.”

The city is also proposing a citywide overlay zone to address heat by requiring new developments to incorporate more trees and green space, incorporate pavement that reflects rather than absorbs heat, and have roofs that reflect heat or have solar panels installed, Moon said.

The urban heat island index developed by Climate Central uses a model developed by three Italian architects that was published in the October 2020 edition of Scientific Reports that quantifies the factors that cause developed areas to heat up.

The model uses data on the distribution of different land cover types in each census tract, from green space to paved areas, along with data on building height, population density and industry to estimate urban heat island intensity.

It does not account for the cooling impact of sea breezes, which can make a significant difference in coastal communities at certain times of the day, Brady said. At other times of the day, however, when there is no wind, the ocean will reflect sunlight and raise the temperature.

The Knightville neighborhood of South Portland, seen hear from the Casco Bay Bridge, is the state’s hottest urban heat island, according to a study of development density and other factors. Penelope Overton/Staff Writer

Back at Lighthouse Bikes, Day longs for a Knightville-Mill Creek tree-planting campaign that could provide some respite to her shade-starved neighborhood. But the extreme heat does make for a compelling climate change anecdote for her shop’s guided bike tours.

When leading tours, the Peaks Island resident likes to point out areas along the nearby coastal bike path that illustrate Maine’s changing climate, like where king tides boosted by sea level rise have overrun the pavement or a public art installation naming local species at risk from rising seas.

“But on the really hot days, when the people on our tours can’t believe it’s this hot even in a cold state like Maine, the cyclists can feel the heat,” Day said. “It’s not good for business, but it certainly makes its point. It’s not supposed to be this hot.”

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