New maps of King County, Seattle show how some communities are harder hit by heat waves

If this weekend’s heat wave sends temperatures soaring well above 90 degrees in King County as meteorologists expect, some communities are likely to suffer much worse than neighbors mere miles away. 

That take-away comes from a new map of temperature data throughout King County collected during a scorching day last July. The map, which was publicized by the county Wednesday, shows how the impacts of heat waves and the effects of climate change depend — even at a small scale — on where you live.

And some areas — particularly those with fewer trees and more pavement — boil hotter during heat waves like the one forecast for this weekend. 

The difference can be enormous. Temperatures recorded at the warmest and coolest comparable locations last July differed by more than 23 degrees, according to the data.   

“Everybody’s affected by heat, but not everybody is affected equally,” said Lara Whitely Binder, a climate preparedness program manager with King County. 

As the climate changes, heat waves are projected to increase in frequency and intensity across the country, according to the national climate assessment published in 2018. The Pacific Northwest, which remains poorly adapted to extreme heat, is forecast to see rising heat illnesses and deaths as temperatures warm, making understanding these microclimates important to public health. 

The new map could help leaders prepare for extreme events and create a more tolerable landscape for a hotter future. 

“This does provide us with real data across the region and shows where the biggest problems are with daytime heat,” said King County Executive Dow Constantine in a Wednesday interview. “With the obvious changes we’re seeing in climate and extreme weather events, understanding this phenomenon becomes more urgent.” 

The new map includes data from some 110,000 temperature readings collected across the county by volunteers, who on July 27 of last year attached sensors to their vehicles and drove predetermined courses throughout the county. The highest temperature recorded that day was nearly 99 degrees. 

The 17 volunteers collected readings during three hourly time periods: morning (6-7 a.m.), afternoon (3-4 p.m.) and night (7-8 p.m.). Every second, the sensors, which were extended above the vehicle with a three-foot tube, collected temperature, humidity, speed, longitude and latitude data as the volunteers drove. Readings were later excluded from the data if the vehicle was moving less than 5 mph or over 35 mph.

The project was designed and analyzed by CAPA Strategies, a consulting firm paid about $39,000 by the city of Seattle and King County for its work, according to Whitely Binder.

Analysis of the data found that industrial areas and places with dense buildings concentrated heat, while tree canopy mitigated the warmth. 

Throughout the county, most areas experienced an intense afternoon heat, but some neighborhoods warmed up more slowly in the morning or cooled down more quickly in the evening, the data suggests. 

Addison Houston, an environmental health mitigation & response planner with Public Health Seattle & King County, said the hottest areas on the map correlate with the Washington state health department’s mapping of environmental health disparaties.

“The areas that have the highest temperature are associated with other health disparities,” Houston said, meaning that many of the people most at risk from high heat are often those who live where temperatures remain hottest. 

Areas of south King County in Auburn, Kent, Renton and Burien stand out as hot spots. 

“The burden of that falls on Black and indigenous and people of color communities and is largely oriented with the south end of King County, where there’s a higher concentration of industrial activities, dense roadways and airports,” Houston said. 

Whitely Binder said cities have already used the map to decide where to establish cooling centers ahead of the weekend heat wave. 

Long term, it could help decide where the county plants trees, inform building and zoning code changes and help target energy efficiency campaigns. 

Whitely Binder said the county plans to do additional comparison of the temperature data and equity measures. The county also has applied for federal funding to develop an urban heat mitigation strategy.

She said this weekend is a reminder that the county must be prepared for heat waves that are expected to be more frequent and severe as the climate changes. 

“We’ve taken advantage of having a very temperate, mild climate,” Houston said. “That’s not going to be the conditions coming in the future.”