A hot city is not equally hot everywhere. A shaded street, an exposed car park and a dense commercial district can have different thermal conditions at the same time. The urban heat island effect helps explain why the places we build can retain more heat than their surroundings.
This explainer looks at what creates urban heat, how to read temperature maps, and what to ask before investing in trees or reflective roofs.
Why cities hold on to heat
Buildings and paved surfaces absorb solar energy and release heat. Replacing vegetation with hard surfaces also removes shade and moisture that would otherwise provide cooling. Closely spaced buildings can restrict airflow, while vehicles, industrial equipment and air-conditioning systems add waste heat.
The effect can persist after sunset as materials release stored energy. Its strength varies with weather, geography and urban form; there is no single temperature difference that applies to every city or neighbourhood. Source: US EPA, What Are Heat Islands?
A thermal map is not a weather forecast
The NASA image accompanying this article shows ground temperatures around Delhi on May 5, 2022, near midnight. It is a historical illustration of spatial differences, not a report of conditions today.
Surface temperature and air temperature are different measurements. Thermal instruments observe heat emitted by roofs, roads and other surfaces. Weather stations and appropriately positioned sensors measure the air. A bright-red roof on a thermal map does not mean the surrounding air has the same temperature.
EPA recommends choosing data to match the question being investigated. Satellite observations provide broad coverage, while ground measurements can reveal conditions at selected locations. Neither automatically represents every street or hour of the day. Source: US EPA, Measuring Heat Islands.
Before sharing a temperature map, check its date, time, units, colour scale and measurement type. Ask what the comparison area is. A city centre compared with a nearby irrigated field is a different comparison from two residential streets.
How trees help—and why location matters
Trees provide shade, and vegetation cools through evapotranspiration: water evaporates from plants and their surroundings, using heat in the process. Trees that shade buildings can also reduce demand for air conditioning. Source: US EPA, Benefits of Trees and Vegetation.
For a neighbourhood project, our recommendation is to start with the places people actually use: walking routes, waiting areas and shared outdoor spaces. A planting target is easier to evaluate when it identifies who will gain shade, where and at what time of day.
Planting is only the beginning. EPA’s guidance highlights species selection, available space and ongoing maintenance. A project needs responsibility for caring for the trees after the launch event. Source: US EPA, Community Member Tree Planting Efforts.
What a cool roof can change
A cool roof reflects more sunlight and can shed absorbed heat effectively. These properties reduce heat transferred into a building compared with a conventional roof. EPA describes potential benefits for indoor temperatures and cooling energy use, as well as surrounding temperatures. Source: US EPA, Using Cool Roofs to Reduce Heat Islands.
That does not make every coating an equivalent investment. Before choosing a product, ask for performance information relevant to the building, roof condition and local climate. Include installation, maintenance and replacement costs in the comparison.
For more household context, see our guide to reducing your impact at home.
Five questions for a city cooling proposal
Our suggested checklist turns a broad promise into a project that residents can assess:
- What problem is being measured? Hot surfaces, warm outdoor air, indoor discomfort or electricity use?
- Who benefits? Identify the streets, buildings and people the project is intended to serve.
- What is the baseline? Record conditions before the work and explain how comparisons will account for different weather.
- Who maintains it? Name the organisation responsible for trees, roofs or monitoring equipment.
- How will results be reported? Publish the measurement method and limitations alongside any headline improvement.
These are questions to adapt locally, not a promise that one intervention will deliver the same result everywhere. A useful first community project could be documenting gaps in shade on a regular walking route and discussing them with the relevant property owner or local authority. Explore more ways to get involved in community climate action.
The takeaway: start with a specific place and a clearly defined measure of success. A cooler roof, a shaded pavement and a lower neighbourhood air temperature are valuable outcomes—but they are not interchangeable claims.