A new heat mapping project from Multnomah, Washington and Clackamas counties found a significant disparity in the temperatures experienced in different parts of the region.
Canby and Molalla had the highest temperatures among Clackamas County cities. Canby also ranked among the lowest for tree canopy and highest for impervious land. Just 17% of the city is shielded by tree canopy, which lowers temperatures through shade and evapotranspiration. More than half of the land in Canby and Molalla is impervious, which often leads to higher temperatures.
Throughout the three counties, suburban cities near highways comprised most of the hottest areas. Those areas generally have fewer trees, more hard surfaces like roads and rooftops, and “sprawling development patterns,” according to the study . Parks and rural forested areas had the lowest temperatures on hot summer days.
In Clackamas County, areas zoned for multi-family residential and mixed-use residential had slightly higher temperatures than the county-wide average.
“I want people who find their homes and workplaces in hotter areas to feel empowered to use this information to prepare and adapt,” Clackamas County Health Officer Dr. Sarah Present said in a press release. “People can better understand their personal risks and make a plan for hot summer days. People can also take small steps with their neighborhoods, employers, and local government to cool these areas down over time. Fortunately, we still have time to make changes in our communities today to help us adapt to a warmer future.”
The maps are one way of determining where resources should be focused, both in preparation for severe heat events and during heat waves.
Green roofs, which have vegetation growing on them, are one way to reduce temperatures and energy use.
“We can design our buildings, especially the ones that we build new, to take advantage of passive cooling and provide shading to nearby places,” Brendon Haggerty, Multnomah County’s Healthy Homes and Communities program manager, said at an April 3 press conference announcing the release of the heat maps. “Some cities around the country are starting to experiment with ensuring that we have shaded pathways to get to transit stops, for example. And we can also do things like varying the orientation and the height and scale of our buildings to help with airflow.”
Paired with data about where vulnerable populations like seniors and people with with disabilities live, the heat index can also help determine where local governments locate cooling centers during heat waves.
The study data was collected on July 22, 2023, with 125 volunteers collecting almost 270,000 temperature and humidity measurements through sensors on their cars.
The Portland Metro heat mapping project covered more than 419 square miles, making it the largest in the world to date.
The heat map “is a tool to help us prepare and do a better job next time. It helps us understand vulnerability. It helps lots of organizations from across the metro area take action to reduce risks,” Haggerty said.
Though weather data for the region is often reported with measurements from the Portland airport, those figures don’t capture the variation between cities and even neighborhoods.
The National Weather Service reported a high of 89 F at PDX on July 22, 2023, but a volunteer in the Tualatin area reported a high nearly six degrees warmer.
The lowest recorded temperatures were near Forest Park, Lake Oswego and the Lewis & Clark College campus. The lowest afternoon temperature was 77.4 F, while the highest afternoon temperature in the study was 94.6 F — more than a 17-degree difference.
The three counties “are working together, along with health care systems, to help areas with high heat become safer over time,” the report summary stated.
Clackamas County plans to use the heat map data to inform a Climate Adaptation plan, Natural Hazard Mitigation Plan “and other emergency response plans to reduce impacts of extreme heat on employees, community members, and visitors.”
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