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Why Cities Are So Hot, and How to Cool Them Down

20th July 2025
in Natural Global Resources
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In a city, a grassy park might be a place to stretch out with a book, an asphalt road your route to work, a building wall a canvas for a mural. But beyond their familiar roles, each of these surfaces plays a critical and often unseen role in shaping urban heat.

Many cities are warming at twice the global rate — a problem that’s only worsening with rapid urbanization. And while rising temperatures are a problem everywhere, some cities and neighborhoods (often the poorest and most vulnerable) swelter more than others.

The reason for this comes down to the urban environment. Built infrastructure like roads, buildings and sidewalks, as well as natural infrastructure like trees and water bodies, determines how heat moves through a city. In most cities, the abundance of dark, impervious surfaces, like asphalt, traps heat and drives temperatures up — contributing to the urban heat island effect.

But urban infrastructure can also be one of the most powerful tools to keep people cool, without relying on energy-hungry air conditioning. The key is focusing on “surface infrastructure” — the places where the physical city and the atmosphere interact. In fact, cities around the world are showing that seemingly simple changes to surfaces, like painting roofs white or planting trees, can have a surprisingly big impact on temperatures. It’s a matter of knowing how and where to use these solutions.

White roofs, like this one on Milos Island, Greece, can keep people cool inside while lowering surrounding temperatures. Photo by photopoems/Shutterstock

What Makes a City Hotter — or Cooler

Urban infrastructure interacts with energy from the sun — reflecting it, absorbing it, transforming it, storing it — in ways that shape the experience of urban heat. Understanding and harnessing these mechanisms is key to designing cooler cities.

Surfaces can absorb or reflect heat

When sunlight strikes a surface, one of two things happens: The energy is either absorbed or reflected. While energy that gets reflected back into the atmosphere doesn’t cause warming, energy that is absorbed by surfaces is re-emitted as heat. Dark surfaces tend to absorb energy and elevate temperatures; white or reflective surfaces tend to have the opposite effect.

There’s more than one way to measure heat. Looking at different metrics offers a more complete picture of how effective heat solutions are — from lowering the air temperature outside to changing how hot it feels.

Land surface temperature (LST) uses satellite data to measures the temperature of surfaces such as roofs, treetops and roads.

Air temperature is measured about two meters above the ground using combined data from various weather stations. This is the metric commonly seen in weather reports.

Heat index measures it feels like outside by adjusting air temperature based on humidity.

Wet bulb globe temperature (WBGT) is similar to heat index, but considers direct sunlight and wind speed in addition to air temperature and humidity.

Universal thermal comfort index (UTCI) accounts for the combined effects of direct sunlight, reflected radiation, air temperature, wind level and humidity to accurately measure heat’s impact on the human body.

Strategically increasing reflectivity (also known as “albedo”) — for example, by painting rooftops or pavements light colors — can significantly decrease local temperatures. Phoenix has reduced surface temperatures by over 7 degrees C (12 degrees F) in some areas by adding reflective treatments to pavements. In the Almeria region of southern Spain, after farmers whitewashed their greenhouse roofs, local air temperature fell by about 1.6 degrees C (2.9 degrees F) compared to surrounding areas.

Cool roofs have the additional benefit of lowering indoor temperatures and reducing the need for air conditioning. In Malaysia, white roof tiles have reduced annual household energy use by 13%, bringing down power demand as well as energy costs.

Measuring land surface temperature can tell us how these interventions directly impact temperatures of surfaces, like roofs or roads. But measuring local air temperature also gives us important information about how reflective surfaces impact temperatures and people more broadly.

Shade offers instant relief 

Trees, as well as structures like awnings, bus shelters and covered walkways, intercept sunlight and prevent it from being absorbed by the ground — or by people. This dramatically improves how hot it feels outside, creating a respite even on scorching days.

While increasing shade may seem obvious, the benefits can be staggering. One study found that tree shade can reduce surface temperatures by up to 25 degrees C (45 degrees F) compared to sunny, asphalt-paved areas. Cities are taking note: A community-led project in Freetown, Sierra Leone, planted more than 1 million trees over the last five years to combat rising temperatures.

Thermal comfort indices can be used to measure how shade (or the lack of it) affects people, providing guidance for where to plant trees and build shelter.

Green spaces cool the air by releasing water

A grassy park often feels noticeably cooler than a sunlit sidewalk. This is in part thanks to “evapotranspiration” — the process by which water moves from the land to the atmosphere, both through plant leaves and direct evaporation of water. Much like how sweating cools the human body, evapotranspiration draws heat from the surrounding environment and uses it to transform liquid water into vapor, lowering local air temperatures in the process.

This property, combined with the shade they provide, makes urban trees an especially powerful tool for mitigating heat. Moist surfaces like soil and open water bodies have a similar effect, known as evaporative cooling.

Cities can tap into these benefits through strategies like green roofs, green spaces and permeable pavements (which have gaps that allow water to evaporate). Medellín, Colombia, has managed to decrease its local air temperature by 2 degrees Celsius (3.6 degrees F) by creating a network of “green corridors” — trees and vegetation strategically planted along roads, paths and bike lanes and connecting green spaces throughout the city.

The flip side is that evaporation also increases humidity, which can compound the health risks of extreme heat. When evaluating these solutions, it’s important to consider their impact on the heat index — a metric which combines air temperature and humidity — to more accurately assess how people feel outside.

‘Thermal delay’ shapes daytime and nighttime heat

Different materials absorb and emit heat at different rates, which shapes urban temperature cycles. For example, asphalt warms dramatically during the day and releases that heat well into the night, keeping cities sweltering after the sun goes down.

Water bodies, on the other hand, can absorb and store significant amounts of heat without much change in temperature. This is why water stays cooler than the air when it’s hot out — and why swimming is so refreshing on a summer day. During the day, air near the surface of a water body is cooled by the water and can be carried by breezes, lowering temperatures in nearby areas. At night, water releases only a small share of its stored heat very slowly, helping to avoid the high nighttime temperatures common in heavily built-up areas.

In 2005, Seoul, South Korea, demolished approximately 6 kilometers of elevated urban highway to uncover and restore the Cheonggyecheon Stream. After the project’s completion, researchers compared air temperatures near the stream to those in a developed area four blocks away and found it was nearly 6 degrees C (over 10 degrees F) cooler by the stream. Not only that, but the revitalized waterway has boosted tourism, created green space for residents to enjoy and lent a massive boost to biodiversity.

While thermal delay affects temperatures day-to-day, it also moderates temperatures over longer periods — reducing the occurrence of either extreme heat or cold because of the slow, constant release of stored heat. This is a primary reason why coastal land areas have milder climates than inland areas. The cooling and moderating effects of thermal delay are best measured using air temperature.

The rehabilitated Cheonggyecheon Stream in Seoul, South Korea has lowered nearby temperatures while creating new green space for residents to enjoy. Photo by Daniel Gauthier/iStock

Matching Solutions with Cities’ Cooling Goals

Each one of these infrastructure solutions offers value on its own. But to achieve broader or more complex goals — like reducing overall energy demand or keeping pedestrians and transit riders safe during heat waves — cities will often need a multi-pronged approach. By understanding the relationships between goals, heat metrics, cooling mechanisms and infrastructure solutions, cities can make more meaningful progress against extreme heat.

For example, a city might aim to lower overall temperatures while also keeping people cool in areas with high pedestrian traffic. To accomplish this, it could blend broad and localized strategies: A citywide effort to implement cool roofs and cool pavements would increase reflectivity across large surface areas, helping to reduce heat absorption and bring down air temperatures for the whole city. In neighborhoods with high pedestrian exposure — such as commercial corridors or transit hubs — incorporating more trees or shade structures could measurably reduce people’s heat exposure and health risks as they go about their days.

This kind of tiered strategy allows cities to deliver both widespread heat hazard reduction and immediate relief in the places people need it most.

Explore the grid below to see how different infrastructure solutions can be combined to meet various resilience and health goals.

Heat Goals, Explained

[Read more]

Cool people outdoors: Provide significant site- or corridor-specific reduction in thermal stress, as measured by thermal comfort indices like the universal thermal comfort index.

Lower area-wide temperatures: Reduce air temperatures across a large area throughout the day.

Flatten peak temperatures: Reduce air temperatures in the hottest period of the day across a neighborhood or city.

Reduce nighttime temperatures: Reduce nighttime minimum temperatures across a neighborhood or city. Particularly relevant for providing relief from heat stress during multi-day heatwaves.

Reduce surface temperatures: Reduce the absorption of heat by surfaces in the urban environment, as measured by land surface temperature.

Cool without adding humidity: Reduce air temperatures without contributing additional humidity to the urban environment, to manage the heat burden of an area as defined by heat index. Particularly relevant in areas with high baseline humidity during the warmest seasons.

Cool without water: Reduce the temperature of an area without requiring additional water resources. Particularly relevant in arid or water stressed regions.

Reduce indoor heat: Reduce the heat absorbed by buildings to reduce indoor air temperatures and thermal stress.

Reduce energy consumption: Reduce the heat absorbed by buildings in order to decrease energy demand for cooling the buildings.

A Toolkit to Protect Cities Against Extreme Heat

Heat is a universal challenge — but mitigating it offers cities an opportunity to respond with creativity and local insight. Each built environment is unique, and each piece of infrastructure can either exacerbate heat or help reduce it.

The solutions listed here offer a flexible toolkit. By understanding how sunlight interacts with the urban landscape and intentionally using infrastructure to influence these interactions, cities can achieve clear, measurable goals for urban heat resilience and protect their residents in a warming world.

To learn more, see WRI’s Urban Heat and Passive Cooling initiative.

The graphics in this article were produced by Sara Staedicke.

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