Urban heat islands are areas in cities that experience significantly higher temperatures than their rural surroundings. This urban heat island effect is driven by the built environment: pavement, buildings, and infrastructure absorb and retain heat, causing city neighborhoods to stay warmer, especially during summer and even more so at night. As climate change leads to more frequent heatwaves in cities, the UHI effect exacerbates extreme heat, posing growing risks to public health, infrastructure, and energy system.
What is an Urban Heat Island?
Urban heat islands are a microclimate phenomenon where urban areas are notably warmer than nearby rural areas. In simple terms, a city forms an “island” of heat in the landscape. The temperature difference can range from a couple of degrees to more than 5°C on hot days or nights, depending on city size, layout, and weather conditions. For example, scientists at Lawrence Berkeley National Lab have found that on a hot sunny afternoon, a densely developed city can be 1–3°C hotter than the surrounding countryside.
Surface vs. air temperature: There are two aspects to the urban heat island effect –surface temperature and air temperature.
Surface urban heat islands refer to the higher temperatures of city surfaces like asphalt roads and rooftops, as seen in thermal images. Land surface temperatures in a city on a summer afternoon can be scorching, as dark roofs and pavement may reach 50–60°C, much hotter than vegetated rural land. In contrast, atmospheric urban heat islands refer to the air temperature in the urban canopy layer (the air between the ground and tree/building tops). This is the heat people actually feel.
Surface vs. air temperature: There are two aspects to the urban heat island effect –surface temperature and air temperature.
Surface urban heat islands refer to the higher temperatures of city surfaces like asphalt roads and rooftops, as seen in thermal images. Land surface temperatures in a city on a summer afternoon can be scorching, as dark roofs and pavement may reach 50–60°C, much hotter than vegetated rural land. In contrast, atmospheric urban heat islands refer to the air temperature in the urban canopy layer (the air between the ground and tree/building tops). This is the heat people actually feel.
The heat island is caused by:
- Heat absorption: Concrete and asphalt surfaces in cities absorb and retain more solar radiation than vegetation and natural surfaces in rural areas.
- Lack of vegetation and water: In urban areas, there are fewer trees and plants to provide shade and cool the air through evapotranspiration (the process by which plants evaporate water).
- Urban geometry (“canyon effect”): Tall buildings closely packed together create urban canyons that block wind flow and trap heat. Reduced ventilation means hot air is not dispersed.
- Human activity (anthropogenic heat): Heat is also produced by transportation, industry, and air conditioning, contributing to the overall temperature increase.
- Air pollution and climate change: Emissions from vehicles and industry increase the concentration of greenhouse gases and other pollutants that trap heat in the atmosphere.
Impacts on cities
It increases cooling energy demand. Higher ambient temperatures mean buildings consume more electricity for air conditioning, straining power grids on hot days. It worsens air quality (heat accelerates smog formation). Most importantly, urban heat can be a serious public health hazard. Heatwaves already kill more people than any other weather-related hazard, and this danger is magnified in cities where nighttime relief is lacking. High inner-city temperatures contribute to heat exhaustion, heat stroke, and increased mortality, especially for vulnerable groups like the elderly and those without air conditioning. During the historic European heatwaves (e.g. 2003 and 2022), cities like Paris saw thousands of excess deaths, partly due to urban heat island effects keeping nighttime temperatures dangerously high.
How urban heat islands are measured and mapped
Understanding the extent and intensity of a city’s UHI is the first step in managing heat risks. Key approaches include:
- Weather stations and sensors: Many cities rely on networks of weather stations and air temperature sensors to monitor urban heat.
- Satellite remote sensing (surface temperature maps): A powerful way to map urban heat islands is by using satellite thermal imagery to measure land surface temperatures (LST). Satellites like NASA’s Landsat and ESA’s Sentinel carry thermal infrared sensors that can detect the heat emitted by surfaces across an entire city in one snapshot.
- Urban climate models and simulations: To fill in gaps in observations, researchers use computer models to simulate urban climate and project UHI intensity.
Urban heat islands and greenery: the cooling effect of trees
One of the most powerful tools to counter urban heat islands is urban greenery, especially trees. Trees, parks, and vegetated areas function as the city’s cooling system, harnessing natural processes to reduce temperatures. The cooling effect of trees in a city can be substantial, thanks to two main mechanisms:
- Shade: Trees block and reflect sunlight, preventing solar radiation from heating up pavement and building surfaces. On a hot day, the surface under a tree’s shade can be 20–45°C cooler than nearby unshaded asphalt.
- Evapotranspiration: Trees and plants release moisture from their leaves (transpiration) which evaporates and cools the air, similar to how sweating cools our skin. This evaporative cooling can absorb a tremendous amount of heat from the environment.
5–12°C
A recent global meta-study found that urban trees can reduce daytime air temperatures by up to 5–12°C in hot climates under ideal conditions. n temperate European cities, the cooling is a bit more modest but still very significant. Trees were found to cool local daytime temperatures by as much as 4–6°C in some cases.
Beyond direct temperature reductions, urban greenery provides numerous co-benefits that help mitigate the urban heat island and improve comfort: trees humidify the air (in dry heat conditions, this is beneficial for comfort), they reduce wind speed at ground level (which can be positive or negative for cooling depending on context), and they improve air quality by filtering pollutants. During heatwaves, shaded, green areas can serve as cool refuges for people. Urban trees and plants also help lower surface temperatures, which can reduce the radiative heat that pedestrians feel (the difference between walking on a sunlit concrete plaza vs a tree-lined path is dramatic).
Crucially, urban greenery mitigates heat inequalities. Wealthier neighborhoods often have more tree cover, while low-income areas suffer from “green disparity” and worse urban heat islands.
Crucially, urban greenery mitigates heat inequalities. Wealthier neighborhoods often have more tree cover, while low-income areas suffer from “green disparity” and worse urban heat islands.
Urban heat island mitigation through Nature-based Solutions (NbS)
Nature-based solutions refer to using natural systems and green infrastructure to address environmental challenges. In this case, using vegetation, water, and natural processes to cool cities and buffer against heat.
Urban forests and tree planting: As discussed, expanding the urban tree canopy is one of the most direct ways to reduce urban heat. Many cities have set ambitious tree planting goals specifically aimed at UHI mitigation.
Urban parks and green spaces: Creating new parks and preserving open green space within cities directly combats the heat island effect. Parks act as “cool islands” – not only are they cooler internally, but they create breezes and a cooling influence that can extend into nearby urban areas. Green corridors or green belts that weave through the city (like river parkways or urban trails) also help ventilate and cool the urban landscape.
Green roofs and green walls: Green roofs (rooftop gardens or vegetation layers) and green walls/vertical gardens cover built surfaces with plants, which cools the building and the surrounding air. Green walls covered in vines or modular planted systems can similarly cool and shade building facades, plus they humidify the air and make street canyons cooler and more pleasant.
Urban parks and green spaces: Creating new parks and preserving open green space within cities directly combats the heat island effect. Parks act as “cool islands” – not only are they cooler internally, but they create breezes and a cooling influence that can extend into nearby urban areas. Green corridors or green belts that weave through the city (like river parkways or urban trails) also help ventilate and cool the urban landscape.
Green roofs and green walls: Green roofs (rooftop gardens or vegetation layers) and green walls/vertical gardens cover built surfaces with plants, which cools the building and the surrounding air. Green walls covered in vines or modular planted systems can similarly cool and shade building facades, plus they humidify the air and make street canyons cooler and more pleasant.
Water-based solutions (blue infrastructure): Water is another natural coolant. Cities are reviving fountains, ponds, and water features as heat mitigation measures. Open water bodies in a city (lakes, rivers, or even large fountains) can have a cooling effect, especially if used in combination with greenery (e.g. a fountain in a shaded plaza). Some cities are experimenting with spray mists or fine water sprays in public squares during heatwaves to mimic the cooling of a sea breeze.
Cool materials and surfaces: While not “nature-based” in the sense of living systems, using cool roofing and paving materials that reflect more solar energy (higher albedo) is often considered alongside NbS as part of a comprehensive UHI mitigation strategy.
Cool materials and surfaces: While not “nature-based” in the sense of living systems, using cool roofing and paving materials that reflect more solar energy (higher albedo) is often considered alongside NbS as part of a comprehensive UHI mitigation strategy.
The 3-30-300 rule for trees and UHI mitigation
Urban planners and forestry experts often refer to the “3-30-300 rule” as a guiding principle for urban greening. This rule was conceived by urban forestry professor Cecil Konijnendijk as a simple formula to maximize the benefits of urban trees for residents and the environment. The rule states that every urban resident should have:
The rule states that every urban resident should have:
3
at least three trees visible from their home
30
at least 30% tree canopy cover in their neighborhood
300
a high-quality public green space (park or garden) within 300 meters of their residence
In essence, the 3-30-300 rule combines individual well-being (seeing trees from home, which research shows improves mental health and reduces stress) with neighborhood-scale greening (30% canopy for local cooling, shade and biodiversity) and city planning (no one should be more than 300 m away from a park, ensuring equitable access to cooler, restorative green spaces).
UpGreen by ASITIS: data-driven green infrastructure planning
UpGreen by ASITIS is an advanced service and analytics tool designed to help cities plan and manage urban green spaces for climate resilience. Satellite-based green mapping: UpGreen uses high-resolution satellite imagery (including data from European Space Agency satellites) to map 100% of the urban green spaces in a city. Beyond mapping location, UpGreen evaluates the vitality and density of each green space. It also quantifies ecosystem services provided by the greenery, including microclimate regulation (cooling).
For example in Copenhagen, UpGreen identified every tree and evaluated its condition and likely survival, helping prioritize which areas need planting or tree care. It also assessed tree stress factors and cooling potential in each district.
For example in Copenhagen, UpGreen identified every tree and evaluated its condition and likely survival, helping prioritize which areas need planting or tree care. It also assessed tree stress factors and cooling potential in each district.
How urban heat island data are used by cities
- Vulnerability mapping and public health: Cities overlay heat island maps with social and health data to create heat vulnerability maps. This helps answer which areas and populations are most at risk during a heatwave?
- Urban planning and zoning decisions: Urban planners use UHI data to inform long-term decisions about land use, development, and green infrastructure. If a city master plan shows certain districts as heat islands (e.g. a downtown with little greenery), planners might change zoning regulations to require green spaces, trees, or reflective materials in new developments there.
- Targeting tree planting and green investments: Perhaps the most direct use of UHI data is to guide urban forestry and greening initiatives. Many cities have ambitious tree-planting campaigns to combat heat, but budgets are limited, therefore deciding where to plant trees for maximum cooling benefit is critical.
- Public communication and engagement: Heat island maps serve as an effective communication tool to raise awareness. City agencies often publish interactive heat maps online or through open data portals so that citizens can explore how their neighborhood compares.
- Climate adaptation and resilience planning: At a higher level, UHI data is incorporated into cities’ climate adaptation plans and risk assessments. Many European cities developing Climate Adaptation Strategies or SECAPs (Sustainable Energy and Climate Action Plans) include urban heat mapping as a baseline. UHI data helps identify heat risk areas and evaluate adaptation options like cool pavements, increasing tree canopy, or creating urban cooling centers.
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