How heat islands are impacting Seattle temperatures

As Seattle heads into its 16th day of temperatures above 80° — a record, according to NWS Seattle — a new analysis finds urban heat islands expose some residents to elevated heat risks.

Why it matters: Urban heat islands (UHI) are boosting temperatures within 65 major U.S. cities that are home to 50 million people, or 15% of the total U.S. population, according to Climate Central's analysis of 2024 temperatures .

• Nearly 34 million people nationally live in environments where UHIs can raise temperatures by at least 8°.

Zoom in : In Seattle 58% of residents live in islands with temperatures that feel at least 8° warmer than surrounding areas.

• That's up slightly from the 56% living in heat islands last year, according to Climate Central's 2023 analysis .

How it works: Heat islands — urban hotspots with higher temperatures compared to surrounding areas — are created when roads, buildings, parking lots and so on "trap" heat.

• Often such neighborhoods, which tend to be poorer, lack trees and plants that would otherwise help cool the area and.
• Seattle's heat index is also impacted by its proximity to bodies of dark water, which absorb more heat than lighter colored surfaces, according to NOAA.

Driving the news: Contributing factors driving the UHI index are built environments, such as roads, buildings and parking lots, a reduced percentage of green space with not enough plants to help cool the air and dense populations.

• "The entire planet is warming due to human-caused climate change, but the built environment further amplifies both average temperatures and extreme heat in cities," per a Climate Central statement announcing the new data.

What they're saying: "Urban heat islands already expose residents to disproportionate heat risks and cooling costs, which will only climb as long as carbon pollution drives up global temperatures," said Climate Central's Jennifer Brady in a statement.

Caveat: Scientists factor the heat island effect into their climate change monitoring and projections, and it does not explain the sharp uptick in global average temperatures , melting glaciers , record ocean warmth and more.

The bottom line: Cities are increasingly understanding the need to combat the heat island effect by adding more green space, using reflective paint on roads and buildings, opening cooling centers and more.

• In Seattle, $69 million of a recently approved $1.55 billion transportation levy that will go to voters this fall is earmarked for climate change solutions, such as increasing the city's tree canopy.
• Last year, the city joined the nation's first statewide tree canopy collaboration aimed at expanding urban tree cover evenly in neighborhoods.

What's next: NWS says Seattle temperatures will continue to rise over the weekend, hitting around 90° Saturday and Sunday, before dropping down into the 70s and low 80s next week.