With 80% of Americans living in urban areas, understanding the heat island effect is critical to reporting on environmental health in our warming climate.
The heat island effect occurs in urban areas where buildings, roads, and other infrastructure absorb and release heat from the sun. In the city, temperatures can be 1–7 °F higher than in greener outlying areas; the effect is particularly extreme in heavily urbanized or industrial areas with sparse greenery.
The relentless heat islands from day to night can be dangerous. Even after sunset, the heat can cause dehydration and heat exhaustion. Because man-made structures retain heat, especially if they have dark surfaces, nighttime temperatures in cities remain about 2-5°F higher. These temperature averages may seem relatively small, but enormous differences can be felt in cities like Las Cruces, NM, where on one hot day there was a 44.5°F difference between shaded grassy areas and exposed sidewalks.
The heat experienced in a heat island is often described as an atmospheric or surface urban heat island. The former varies less and describes the warmer air felt in cities compared to its surroundings. The latter varies more when the sun is shining and describes the heat felt by roads, buildings and non-green surfaces, which absorb heat and release it over time.
Research on chronic exposure to high temperatures and humidity is limited. But the acute effects of a heat wave, exacerbated by the heat island effect, can make the difference between life and death. Portland, Oregon, experienced a record high temperature of 115°F (46°C) during a heat dome in the summer of 2021, an increase of 10°F (5°C) above average. As a result of the event, 69 people died. According to the county’s official report, 61% of all heat-related visits to emergency departments and urgent care centers were due to the heat dome.
Research has shown that low-income communities of color often experience significantly higher heat conditions than surrounding areas. Low-income communities also face a higher risk of heat-related illness and death, which may be due to inadequate housing conditions without functioning air conditioning. Additionally, low-income communities lack the resources needed to find alternative housing during heat waves.
In former “redlined” neighborhoods, a discriminatory practice used to deny home loans to minorities, studies have found that these neighborhoods were 2°C warmer than non-redlined areas. Running air conditioning in high temperatures can be a financial burden, and between a quarter and a third of U.S. households experience some form of energy insecurity.
Here are some health equity resources to support your reporting on urban heat islands:
Satellite images and redline maps
Extreme heat is a silent killer and can be difficult to visualize compared to other disasters like floods or wildfires. However, satellite imagery of heat maps can help readers visualize and understand heat islands when overlaid with vegetation and red-bordered maps.
A city’s redlining history can reveal health disparities related to extreme heat. Journalists can help readers understand the causes of the heat island effect in redlined areas by covering inequality in urban tree cover, the percentage of the urban landscape covered by tree canopy, and how vegetation can be used to reduce heat-related health impacts.
Although the urban heat island effect does not contribute significantly to climate change through rising global temperatures, the phenomenon is influenced by it.
The summer of 2023 was 0.41°F warmer than any other summer in NASA’s records, and NOAA’s latest projections give 2024 a 61 percent chance of being warmer than 2023. The Earth is warming rapidly, and people in low-income urban neighborhoods are disproportionately affected.
