Everything you need to know about the urban heat island effect

With 80% of Americans living in urban areas, understanding the heat island effect is crucial when reporting on environmental health as our climate gets hotter.

The heat island effect occurs in urban areas where buildings, roads and other infrastructure absorb and re-emit heat from the sun. City temperatures can be 1-7°F higher than in greener outlying areas; the effect is especially extreme in heavily urbanized or industrial areas with sparse greenery.

The relentlessness of heat islands from day to night can be dangerous. Even after sunset, the heat can cause dehydration and heat exhaustion. Since man-made structures hold onto heat, especially if they have dark surfaces, nighttime temperatures in cities remain higher by about 2-5°F. These temperature averages can seem relatively small, but there can be enormous differences felt in cities like Las Cruces, NM, where there was a 44.5 °F difference between shaded grass and exposed pavement on a hot day.

The heat experienced in a heat island is often described as atmospheric or surface urban heat islands. The former varies less and describes the warmer air felt in cities compared to its surroundings. The latter changes more significantly when the sun is shining and describes the heat felt from roads, buildings, and non-green surfaces absorbing heat and re-emitting it over time.

Research is limited on chronic exposure to high temperatures and humidity. But, the acute effects of a heat wave exacerbated by the heat island effect can be the difference between life and death. Portland, Oregon, experienced a record high temperature of 116°F during a heat dome in the summer of 2021, a 42°F spike above average. As a result of the event, 69 people died. According to the county's official report, the heat dome accounted for 61% of all heat-related visits to emergency departments and urgent care centers.

Research has shown that low-income communities of color often experience significantly higher heat conditions than surrounding areas. Low-income communities are also at greater risk of heat-related illness and death, which can be due to inadequate housing conditions without working air conditioning. Also, low-income communities don't have proper resources to find alternative shelter during heat waves.

In formerly redlined neighborhoods, a discriminatory practice used to deny housing loans to minorities, research has shown that these neighborhoods were 4.6°F warmer than non-redlined areas. Running air conditioning during high temperatures can be a financial burden, and between a quarter and one-third of US households experience some form of energy insecurity.

Here are some health equity-related resources to inform your reporting on urban heat islands:

• Mapping Inequality is an online resource for understanding the history of racial and ethnic discrimination in housing policy.
• The National League of Cities created a tool to mitigate heat wave risk by visualizing tree canopy data and bus stops.
• The EPA created a webpage that helps communities identify and assess heat islands.

Satellite imaging and redline maps

Extreme heat is a silent killer and can be challenging to visualize compared to other disasters like flooding or wildfires. However, satellite imaging of heat maps can help readers visualize and understand heat islands when layered with vegetation and redlined maps.

A city's redlining history can illuminate health disparities related to extreme heat. Journalists can help readers understand the causes of the heat island effect in redlined areas by covering the inequality of urban tree cover, the percentage of cityscape covered by tree canopy, and how vegetation can be used to reduce heat-related health impacts.

While the urban heat island effect doesn't contribute significantly to climate change by increasing global temperatures, the phenomenon is affected by it.

The summer of 2023 was 0.41°F warmer than any other summer in NASA records, and NOAA's latest projections gave 2024 a 61% chance of beating 2023. The earth is warming quickly, and people in low-income urban neighborhoods are disproportionately affected.

Additional resources

• Dangerous urban heat exposure has tripled since the 1980s, with the poor most at risk - The Conversation.
• As Rising Heat Bakes U.S. Cities, The Poor Often Feel It Most - NPR.
• NOAA is crowdsourcing a national urban heat map - Grist.
• The danger of urban 'heat islands' - High Country News.
• The surprisingly simple way cities could save people from extreme heat - Grist.