Urban Green Space and the Heat Island Effect: How Street Trees and Parks Cool Cities

Why Do Cities Get Hotter? The Mechanics of the Urban Heat Island Effect

Compare a weather station in the suburbs with one in a downtown area lined with high-rise buildings, and the downtown site runs hotter year-round. This is the "urban heat island" effect — named because a map of temperature contours shows the city center standing out like a hot island. Japan's Ministry of Land, Infrastructure, Transport and Tourism and Ministry of the Environment attribute this mainly to three factors: the expansion of surfaces covered in asphalt and concrete, the shrinking of vegetation and water bodies, and the growing volume of waste heat from buildings, vehicles, and air-conditioning equipment.

Asphalt and Concrete Store Heat

Bare soil and grass keep temperature rise in check because some of the sun's energy goes into evaporating moisture. Asphalt and concrete, by contrast, hold almost no moisture, so nearly all of the solar energy they absorb goes directly into raising surface temperature. It is not unusual for paved road surfaces to exceed 50°C in midsummer, and because of their specific heat and thermal conductivity, they continue releasing that stored heat slowly through the night. This is one reason nighttime temperatures downtown stay stubbornly high — the phenomenon known as "tropical nights." As green space and water surfaces are replaced by pavement with no capacity to hold moisture, this daily cycle of "storing heat by day, releasing it by night" tips further against the city.

A Third Factor: Waste Heat from Human Activity

Surface properties are not the only driver of rising temperatures. Waste heat from building air-conditioning units, vehicle exhaust, and the operation of factories and offices also pushes up downtown temperatures. The more concentrated the population, buildings, and traffic, the greater the waste heat per unit area. Ironically, the very air conditioners people run to escape the heat contribute to raising the outdoor temperature further — a vicious cycle that is one of the more troublesome aspects of the urban heat island effect.

Tokyo Rose 3.3°C in 100 Years — How That Differs from Global Warming

An analysis of Japan Meteorological Agency observation data from 1927 to 2021 found that average temperatures at sites with relatively little urbanization rose about 1.6°C over that roughly 100-year span, while Tokyo's average temperature rose about 3.3°C. Global average temperatures over the same period rose a little over 1°C, meaning that nearly half of Tokyo's temperature rise is thought to be an "extra layer" added by the heat island effect — that is, by urbanization itself. In other words, Tokyo's heat is the combined result of global-scale climate change and the heat the city itself has generated.

Osaka, Nagoya, and Other Cities Are Warming Too

The heat island effect is not unique to Tokyo. JMA statistics show that across ten major Japanese cities, the 100-year temperature rise averages about 2.7°C — more than 1°C higher than the average at sites with little urbanization. Osaka City's own published data show its citywide average temperature rising about 2.1°C over the past 100 years. In Nagoya, the number of "tropical nights" — nights when the low temperature never drops below 25°C — has increased by roughly 3.7 days per decade between 1931 and 2017. This appears to be a shared pattern across Japan's major metropolitan areas, wherever population and buildings are concentrated.

A Possible Link to Sudden Downpours

The heat island effect may not be limited to temperature alone. Cities covered in asphalt and concrete release more heat from the ground into the atmosphere, and added waste heat widens the temperature gap with the air above, a gap that some Ministry of the Environment documents and meteorological research groups suggest may contribute to the sudden, localized downpours often called "guerrilla rainstorms," which appear to be increasing in urban areas. That said, the causal relationship between urbanization and these intense localized rains has not been firmly established, and research on the topic continues.

How Green Space Creates a "Cool Island"

The phenomenon in which parks and green spaces alone stay cooler than their surroundings is called a "cool island" — the opposite of a heat island. As the name suggests, greenery creates small pockets of coolness within the city. This happens because plants don't just provide shade; they actively cool the air around them.

The Double Effect of Transpiration and Shade

Plants draw water up through their roots and release it as vapor through pores in their leaves, a process called transpiration, which draws heat away from the surrounding air as it occurs — the same principle by which sweat cools the human body as it evaporates, happening continuously across the surface of every leaf on every tree. On top of that, the shade cast by a tree's canopy blocks direct sunlight from reaching the ground, preventing the surface itself from heating up. A column published by the Japanese Society of Biometeorology explains that this combination of transpiration and shade tends to make green spaces cooler than the surrounding built-up areas. The same transpiration mechanism underlies the way riparian forests along rivers cool water temperatures, and the way headwater forests in the mountains capture rain and release it into rivers — it is a basic relationship that connects greenery and water more broadly.

Cool Air Seeping Out into the Surrounding Streets

Cool air generated inside a green space does not stay put — it flows outward into the surrounding city. This "seeping" effect has been observed as cool air spreading out fan-like from the edges of a green space, cooling not just the park itself but also nearby roads and residential streets. Because cool air is denser than warm air, it tends to spread slowly outward and downward from the edge of the green space, an effect most often observed on calm nights and in the early morning hours when wind is weak.

The Cooling Effect of Green Space, By the Numbers

The cooling effect of green space is not just a matter of perception — it has been confirmed numerically in multiple studies. Here we line up some of the representative findings for comparison.

Satellite Data Reveal a 1.5°C Drop

A study that combined satellite remote-sensing data on vegetation distribution with a mesoscale weather model found that vegetation produced a cooling effect of 1.5°C or more during summer daytime hours. What makes this study distinctive is that it evaluated the combined effect of green spaces scattered across an entire city, rather than a single park — evidence for the value of greenery as a citywide network rather than isolated points.

Osaka Castle Park: Up to 2.8°C, Reaching 250 Meters Away

Weather observations at Osaka Castle Park found that the temperature difference between inside and outside the park — the cool-island intensity — peaked between about 4 and 5 a.m., reaching 2.0°C or more, up to a maximum of 2.8°C. The cool air's influence was found to extend up to about 250 meters from the park's edge. During the day, the gap narrows due to sunlight and human activity, but as wind weakens overnight and into early morning, cool air accumulates inside the park and the difference reaches its peak — a time-of-day pattern that has also been observed directly.

What These Studies Have in Common

Although these studies differ in method and location, several common patterns emerge. First, the temperature differences mostly fall in a range from the high 1°C's to just under 3°C — not a dramatic swing to zero, but a realistic, noticeably-cooler effect that people can genuinely feel. Second, the effect tends to be larger at night and in the early morning than during the day, which is prompting a reassessment of green space's value specifically for easing tropical nights. And because the reach of the cooling effect is generally limited to a few hundred meters, cooling an entire city requires more than a single large park — it calls for distributing street trees and smaller green spaces so that their zones of influence overlap across the urban fabric.

Park Size and the "Seeping" Effect: How Far Does Cool Air Travel?

Not all "parks" are equal — their cooling effect on the surrounding area varies greatly with scale and shape. Research points to a rough threshold at which the effect becomes clearly noticeable.

Why 20 Hectares Is a Meaningful Threshold

Urban planning research indicates that large parks exceeding 20 hectares almost always show a clear cool-air "seeping" effect with temperature differences of 2°C or more. Twenty hectares is roughly the size of 4.3 Tokyo Domes — comparable to a major comprehensive park in a large city. Smaller parks and pocket parks do create localized coolness, but achieving a stable effect that cools the surrounding urban area as a whole requires a certain minimum mass of contiguous greenery.

Vegetation Density and Terrain Also Matter

On the other hand, some reports find that even a green space of around 8 hectares can produce a cool-air outflow comparable to much larger green spaces, provided it has a high vegetation coverage ratio and features like slopes. This shows that the effect depends not just on raw area but on how densely the greenery grows and how the terrain shapes the flow of cool air. For urban planners, this suggests that even where securing a vast new park site is difficult, increasing the density of existing green space or designing around slopes and water features may raise the cooling effect achievable within a limited footprint.

• Larger areas tend to produce a more stable cooling effect
• A green space with high vegetation coverage can rival a much larger one, even with a modest footprint
• Sloped terrain lets cool air flow downhill, extending its reach into the surroundings
• Water features (ponds, streams) add evapotranspiration and further boost the cooling effect
• A compact, clustered green space tends to build a more stable layer of cool air inside than a long, narrow strip

Putting Large Parks into Perspective

To get a sense of what "20 hectares" actually looks like, it helps to compare it with well-known urban parks. In Tokyo, Shinjuku Gyoen covers about 58 hectares, Yoyogi Park about 54 hectares, and Ueno Park about 53 hectares — all well above the 20-hectare threshold. Osaka Castle Park, mentioned earlier, spans about 105.6 hectares. All of these are large enough that a stable cool-air seeping effect can be expected.

Street Trees: The Everyday Coolness of Shade and Transpiration

Even without a large park nearby, street trees are the most familiar form of urban greening in daily life. Field studies of sidewalk thermal conditions have found that street trees reduce air temperature by up to about 2°C. Since most everyday walking — commuting, school runs, shopping — happens on streets rather than inside parks, more people may experience the benefit of street trees on a daily basis than they realize.

A 15°C Gap in Perceived Temperature Between Shade and Asphalt

The difference is even larger when measured as perceived temperature rather than air temperature alone. Measurements comparing surface temperature in tree shade versus asphalt exposed to direct sunlight have found gaps averaging about 15°C in some cases. The relief pedestrians actually feel from heat can be greater than what a thermometer alone would suggest. The cooling from shade — blocking sunlight directly — takes effect immediately, unlike the more gradual cooling from transpiration, which is why stepping into the shade of a tree feels instantly cooler.

Tree Species Choice Shapes the Effect

Not all street trees cool in the same way. Deciduous broadleaf trees such as ginkgo and zelkova provide strong shade and transpiration in summer when in full leaf, while letting sunlight through in winter once their leaves have fallen — making them easy to use flexibly across the seasons. Evergreen trees such as camphor, by contrast, maintain shade year-round, which is an advantage, but they also block winter sunlight, so their placement needs to account for how much daylight nearby buildings require. Street tree planning takes these species-specific traits into account, along with a road's orientation, width, and the uses of the buildings around it.

The Challenge of Maintaining Street Trees

Street trees serve many roles — shaping the streetscape, supporting disaster resilience, and reducing air pollution — but as they grow into large trees, maintenance costs (pruning, leaf litter management, dealing with roots that heave pavement) rise as well. A case collection from Japan's Climate Change Adaptation Platform (A-PLAT) positions the maintenance and use of street trees as one form of heat adaptation, one that depends on cooperation and mutual understanding between local government and residents. If heavy pruning shrinks a tree's canopy too much, both its shading and transpiration effects weaken, so maintaining an appropriate level of care is essential to preserving the cooling benefit.

Green Roofs, Green Walls, and Green Curtains: Greening the City Where Ground Space Runs Out

In dense urban centers where there isn't enough room at ground level for greenery, greening buildings themselves has become increasingly common. Green roofs, green walls, and green curtains all use transpiration cooling to reduce heat load on the building itself while also helping to ease the surrounding heat island effect.

A 10°C Gap in Measured Roof Surface Temperature

A measurement example cited by Japan's Ministry of the Environment found that an ungreened folded-plate metal roof reached a surface temperature of 45.5°C, while a greened roof stayed at just 35.1°C — a gap of about 10°C. A cooler roof surface means less of that heat transfers into the building's interior or the surrounding air, which also helps reduce the building's air-conditioning load.

Green Curtains Block About 80% of Solar Heat

According to the Ministry of the Environment's COOL CHOICE "Green Curtain Project," a green curtain with sufficiently dense foliage can block about 80% of incoming solar heat energy — a higher rate than bamboo blinds (50–60% blocking) or even high-performance heat-blocking window glass (about 50%). In one elementary school experiment, on a day when the outdoor temperature reached around 36°C, the gap in exterior wall temperature between a classroom with a green curtain and one without reached as much as about 7°C. Familiar plants like bitter melon (goya) or morning glory work on the same principle, as long as the greenery involves genuine transpiration. Unlike green roofs or green walls, which typically require professional installation, a green curtain needs only a planter and a net, making it something an apartment balcony or a house window can host at low cost — a form of greening individuals can start today.

Grass Schoolyards: Another Option

Greening the ground itself is also being pursued through the conversion of schoolyards to grass. Surveys by the Tokyo Metropolitan Government's Bureau of Environment have found that on clear summer days, grass schoolyards reach a peak surface temperature 8.3°C lower than dust-surfaced (bare soil) schoolyards, and air temperature 1.5 meters above the ground is 1.6°C lower as well. A schoolyard represents a substantial patch of green space that not only improves the thermal environment where children spend recess and physical education classes, but is also thought to produce a localized cool-island effect on the surrounding area. Japan's Ministry of Education, Culture, Sports, Science and Technology also cites the suppression of summer glare and temperature rise as an environmental benefit of grass schoolyards, and supports their continued development.

Nationwide Installation Data

According to an annual nationwide survey by the Ministry of Land, Infrastructure, Transport and Tourism, fiscal 2022 saw about 15.6 hectares of new green roof installations and about 4.4 hectares of new green wall installations across Japan. The survey, which has continued since fiscal 2004, reflects how green-roof and green-wall installation has become established as a heat island countermeasure alongside its role in urban decarbonization and creating a more pleasant streetscape.

Real-World Development: Otemachi Forest and Singapore's "Garden City"

Urban greening isn't only a matter of government policy — it is increasingly being incorporated into private redevelopment as well. A leading example is "Otemachi Forest" in Tokyo's Otemachi business district. We'll also look at Singapore, an overseas example of a nation that has placed greening at the center of its urban strategy.

A 3,600m² Forest Between Skyscrapers

Otemachi Forest is an urban woodland regenerated within a high-rise business district, covering about 3,600 square meters — roughly a third of the Otemachi Tower site. Tokyo Tatemono, the company that developed and manages the site, is also conducting joint research with the Forestry and Forest Products Research Institute to scientifically verify the effects of urban green space on the thermal environment and on ecological networks. The idea of creating a genuine forest in the heart of an office district was unusual when construction began, and the project is often cited as an example that changed how green space is positioned within urban development.

A 1.7°C Average Drop, Before and After Development

A comparison of on-site temperatures before and after development found an average drop of about 1.7°C. The site also temporarily stores rainwater to ease the sudden runoff during heavy rain, and is said to contribute to greater biodiversity — a striking example of multi-faceted environmental value in the middle of a downtown redevelopment. The way wildlife returns once greenery and water are reintroduced to a city echoes the case of kingfishers returning to urban rivers. Office workers also use the space as a place to rest, illustrating how the project achieves both heat relief and greater comfort at the same time.

Linking Isolated Green Spaces into an "Ecological Network"

Green spaces in city centers tend to be isolated, surrounded by buildings, which limits the variety of wildlife that can settle there. According to those involved in the project, Otemachi Forest contributes not only to improving the thermal environment but also to forming an "ecological network" that lets birds and insects move between it and existing green spaces nearby, such as the Imperial Palace grounds and Hibiya Park. Connecting these stepping-stone patches of urban greenery across distances that birds and insects can travel indirectly reinforces the citywide cooling effect as well.

An Overseas Example: Singapore's "Garden City"

Overseas, some countries have made the entire city a target of their greening strategy. Singapore launched its "Garden City" concept in 1967, setting up a dedicated division within its Public Works Department to drive intensive tree-planting. By the end of the 1970s, more than 55,000 trees had been planted, and by 2014 the country's tree count had grown to roughly 1.4 million. The stated aims were to ease the discomfort of a hot, humid climate year-round while also supporting foreign investment and tourism.

The Singapore government currently subsidizes roughly half the cost of green roof and green wall installations, encouraging building-level greening even in densely developed parts of the city center. This long-running effort to turn the entire nation into a "city in a garden" has drawn global attention from a heat island mitigation perspective as well.

Urban Planning That Connects Green Space and Water: Rainwater Storage, Sea-Breeze Corridors, and What We Can Do

Urban greenery doesn't only lower temperatures — it is also connected to water and the sea. Here we bring together everything covered so far, viewed through the lens of the water cycle and coastal cities.

A "Green Dam" That Absorbs Heavy Rain

Green space with soil and vegetation, unlike paved surfaces, can let rainwater soak into the ground and hold it temporarily. As more green spaces with rainwater storage capacity, like Otemachi Forest, are built, the volume of water rushing into sewers and rivers all at once during heavy rain can be reduced. Urban greening therefore also helps lower the risk of inland flooding and eases the sudden surges of water volume and pollution load that would otherwise reach river mouths and coastal areas.

Urban Planning That Keeps Sea-Breeze Corridors Open

In coastal cities such as those around Tokyo Bay or Osaka Bay, the daytime "sea breeze" that blows from the ocean toward land acts as a natural ventilation system, carrying heat away from the city center. But when high-rise buildings are packed too densely, they block this corridor and the sea breeze struggles to reach downtown. MLIT's urban design guidelines for mitigating the heat island effect emphasize the importance of arranging buildings and securing wide open spaces and green corridors so that breezes from the sea or mountains can be drawn into the interior of the city. Green space thus serves a dual role — as a source of cooling in its own right, and as part of the corridor through which wind travels.

Tokyo has translated this idea into a concrete standard. It defines a "wind corridor" as a sea breeze blowing at 4 meters per second or faster at a height of 50 meters, and aims to keep post-development wind speeds at 50% or more of pre-development levels so that development does not weaken the breeze too much. To meet this standard, waterfront redevelopment projects have limited building heights and widened the gaps between buildings. Green belts function as part of these "wind corridors" as well — serving simultaneously as standalone sources of cooling and as passages that carry cool sea air deep into the city center.

Pavement Technologies That Work Alongside Greenery

Heat island countermeasures are not limited to greenery. A pavement temperature comparison experiment by MLIT's Road Bureau confirmed that "water-retentive pavement," which stores rainwater inside the pavement and cools the surface through evaporation, and "heat-reflective pavement," which uses coatings that reflect infrared light, can both lower summer daytime road surface temperatures by around 10°C or more compared with ordinary asphalt. At intersections and plazas where street trees or grass cannot be planted, combining these pavement technologies with greenery elsewhere can raise the overall effectiveness of heat countermeasures across an area.

Policies to Raise Green Coverage, and the Future of Urban Greening

National and local governments are pursuing heat island countermeasures through a combination of policies — park and green space development plans, subsidies for green roofs and walls, and proper maintenance of street trees. Because creating and maintaining green space costs money, however, experts note the importance of weighing cost-effectiveness and prioritizing investment where the impact is greatest: new large-scale parks, green belts that serve as sea-breeze corridors, and the preservation of existing street trees. Urban greenery does more than cool how we feel — it also captures rainwater and helps keep the water cycle that eventually reaches the sea in a gentler balance. The next time you walk beneath a street tree or pause to rest in a park, it's worth remembering that the coolness you feel is supported by the real mechanisms of transpiration, shade, and the seeping flow of cool air.