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Results of heat mapping project show inequitable impact of hotter summers, will inform actions by King County and City of Seattle


King County Executive
Dow Constantine

Results of heat mapping project show inequitable impact of hotter summers, will inform actions by King County and City of Seattle


The results of a heat mapping project that King County and the City of Seattle conducted last summer quantifies the harmful impact that hotter summers are having on the region, exacerbating inequities. The scientific study will inform both immediate and long-term actions.


King County and City of Seattle today announced the results of a groundbreaking heat mapping project that quantifies the harmful, inequitable impact that hotter summers are having on the region. The data show that surface-level temperatures in areas with paved landscapes, less tree canopy, and industrial activity are substantially  higher during summer heat events compared to less urbanized areas.

King County Executive Dow Constantine outlined how the study results will inform work across departments and initiatives to mitigate the impact of hotter summer temperatures, which is exacerbating inequities.

“Our experts and volunteers have provided the scientific research we need to better prepare our region for the extreme heat events that are occurring more and more often,” said Executive Constantine. “The harmful and inequitable impacts of climate change demand both immediate action and structural changes to create more resilient communities. The data from the heat mapping project will help us achieve both.”

Volunteers helped King County and City of Seattle record more than 110,000 temperature measurements at three different times of the day during a one-day extreme heat event in July 2020. Temperature data were collected at one-second intervals while driving designated routes in different locations around the county.

Applying data to reduce heat impacts

Heat is evenly distributed during the afternoon

A map showing afternoon temperatures in King County.

Areas with more natural landscapes retain less heat

A map showing evening temperatures in King County.

Data analysis shows that areas with hard landscapes held on to heat longer than areas that have more natural landscapes, increasing the potential for heat-related health risks in those areas. More urbanized areas were as much as 20 degrees hotter than less urbanized areas, due in large part to differences in land use and land cover.  

Urban areas are especially prone to higher temperatures due to the abundance of hard surfaces – such as parking lots, rooftop, and roads – that absorb heat. Limited vegetation and industrial activity can also contribute to what is known as the urban heat island effect.

Hotter temperatures increase the risk of illness and death from heat stroke and cardiovascular disease. Researchers at the University of Washington found an increase in emergency medical service calls, hospitalizations, and mortality in King County during hotter temperatures.

Heat-related health risks in King County are exacerbated by existing inequities in housing, access to healthcare, and health outcomes. Those most at risk are older adults, the very young, pregnant women, people who work outdoors, people who are unhoused, and people who have chronic medical conditions. Many of the areas affected by higher temperatures are the same areas that have been disproportionately affected by COVID-19 and other health inequities.

Applying current data to reduce harmful effects of hotter summers

The results of the scientific study will inform both immediate and long-term actions by King County and City of Seattle, including actions related to emergency management, public health, homelessness, human services, land use, urban planning, open space conservation, and infrastructure design.

For example:

  • King County Metro Transit is using the data to inform bus stop design and amenities to account for more extreme weather events, particularly at stops serving communities disproportionately impacted by climate change.
  • The 3 Million Trees initiative that Executive Constantine kicked off on Earth Day will work with communities where there is the greatest need for urban tree canopy, aligning with King County’s 30-Year Forest Plan.
  • City of Seattle’s Trees for Neighborhoods has an immediate application in the Rainier Valley now that there is data that identifies specific blocks that need shade.
  • King County’s Land Conservation Initiative is already prioritizing the permanent protection of urban greenspace, which provides relief during extreme heat events and prevents an increase in hard landscapes that retain heat. The heat mapping project creates opportunities to align equitable access to natural landscapes with heat impact mitigation.
  • King County Parks will use the data to inform park design and improvements to account for hotter temperatures during peak summer months.
  • The 2020 Strategic Climate Action Plan includes strategies to make ownership of energy-efficient green homes – which cost less to heat and cool – more equitable
  • Other applications include updating land-use and urban planning policies to prioritize greenspace and other cooling strategies, including providing more equitable access to energy-efficient green homes that are designed to stay cooler during increasingly hot summers.

King County and City of Seattle in January applied for a FEMA hazard mitigation grant that would help fund development of a countywide urban heat island mitigation strategy. If the grant application is successful, it would provide funding for urban and street tree planting, increasing public access to greenspace, offering incentives for energy efficiency retrofits and green roofs, increasing access to cooling centers, and increased outreach and education about managing heat impacts on health.

Reducing heat impacts is one of many priority actions in King County’s updated Strategic Climate Action Plan, which provides strategies to cut countywide greenhouse gas emissions in half by the end of this decade, prepare for the impacts of climate change, and build more sustainable and resilient frontline communities.

King County and City of Seattle – with the help of 17 volunteers – collected the data on July 27, 2020, when the National Weather Service recorded a high of 94 degrees at Sea-Tac International Airport. The highest temperature recorded was 98.8 degrees, which was recorded in a few locations. The biggest difference in temperature between two locations was 23.3 degrees.

King County and City of Seattle conducted the study with CAPA Strategies, a climate consulting firm that works with municipal governments, non-profits, universities, and businesses to describe hyper-local hazards, such as extreme heat, and develop solutions the build resilience and adaptive capacity with and for communities.

Relevant links

Doug Williams, Department of Natural Resources and Parks, 206-477-4543

King County Executive
Dow Constantine
Dow constantine portrait

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