The urban heat island effect describes an urban area that is warmer than nearby less-developed or rural surroundings. “Island” is an analogy for a pocket of higher temperature, not a literal island. The effect can involve hot surfaces, warmer air or both, and its strength changes by location, weather, season and time of day.
Cities have concentrated people, buildings and infrastructure, so urban development can reduce some direct exposure to nature while creating new environmental problems. Therefore, heat islands are one problem to assess. They do not affect every neighbourhood equally, and a citywide average can hide hotter streets or districts.
In this guide
What is an urban heat island?
For example, the U.S. Environmental Protection Agency’s heat-island overview explains that buildings, roads and other infrastructure can absorb and re-emit more solar heat than forests or water bodies. Consequently, temperatures can exceed those in outlying areas where built surfaces are concentrated and greenery is limited. EPA also distinguishes surface heat islands from atmospheric heat islands, so a satellite surface-temperature map and an air-temperature measurement do not answer exactly the same question.
Most cities can experience the effect to some degree. For example, the source article named New York, Tokyo, London and Hong Kong as dense international cities. However, population density is not the only driver. Vegetation, surface materials, building geometry, wind, cloud, moisture, waste heat and geography all influence the measured difference.
EPA reports that urban areas in the United States tend to have higher daytime and nighttime temperatures than outlying areas. Therefore, a local assessment must define the reference area, sensor type, height, season and time. EPA’s heat-island measurement guidance recommends choosing a method that matches the objective, such as public-health exposure, surface hot spots or energy demand.
However, the original page cited a 2017 paper in Geophysical Research Letters and said cities would experience twice as much heat stress as rural areas by the middle of the 21st century. The primary 2017 study by Wouters and colleagues was more specific: for a densely populated midlatitude maritime region, it projected that the increase in heat stress toward mid-century would be about twice as large in cities as in surrounding rural areas. The authors said other regions need additional assessment, so the finding is not a universal city forecast.
What causes the urban heat island effect?
Less vegetation and more heat-storing surfaces
For example, trees and other vegetation provide shade. In addition, evaporation from soil and transpiration from leaves move energy into latent heat, which can cool surfaces and air. When asphalt, concrete, stone, roofs and buildings replace natural cover, cities often lose some of those functions. Many conventional urban materials also absorb solar energy during the day and release stored heat later.
As a result, the effect can become especially noticeable after sunset, when sidewalks and other built surfaces continue releasing heat. However, heat islands are not only a summer or nighttime phenomenon. They can occur in any season and at any hour. Clear skies can increase incoming solar energy, while calm winds reduce heat removal; clouds and stronger winds can suppress the difference.
Urban form and human-generated heat
For example, building height, spacing and street orientation affect shading and wind flow. Dense “urban canyons” may store heat and restrict ventilation. Meanwhile, vehicles, factories, buildings and air-conditioning equipment release waste heat. The source also mentioned traffic and dense crowds. Their relative contribution depends on local activity, energy systems, climate and urban geometry.
What are the effects?
Heat exposure and health
Higher daytime temperatures and reduced nighttime cooling can increase heat exposure. Consequently, this can contribute to dehydration, heat exhaustion, heat stroke and death during extreme heat. Risk is not uniform: age, health, medication, outdoor work, housing, income, access to cooling and local nighttime temperature all matter.
The original article said heat had been the leading cause of weather-related deaths in many regions over the most recent 10- and 30- year periods. It did not identify a jurisdiction or dataset for that comparison. Therefore, retain it only as a historical statement, not a current universal statistic. Local public-health planning should use the relevant health agency’s current records and heat-warning thresholds.
Energy demand and air quality
Hotter conditions can increase the use of air conditioners and fans, raising electricity demand and utility costs. Consequently, the emissions effect depends on the electricity mix and time of demand. Higher cooling use does not translate into the same greenhouse-gas increase everywhere.
The legacy text said city heat mixes with vehicle exhaust to form ozone. More precisely, ground-level ozone forms through sunlight-driven reactions involving precursor pollutants such as nitrogen oxides and volatile organic compounds. Hot, sunny conditions can favour ozone formation when those precursors are present. Heat islands can therefore aggravate air-quality problems, but heat alone does not create ozone.
How can urban communities cool down?
Vegetation, shade and green roofs
For example, communities may add parks, green spaces, golf courses, tree-lined streets and urban farms, as the original page described. The useful strategy depends on who needs cooling, where exposure occurs, water availability, species, canopy growth, maintenance and competing land uses. Therefore, measurement should confirm whether a project cools a roof surface, pedestrian air, a neighbourhood or indoor space.
Meanwhile, green or ecological buildings can include vegetated roofs. EPA’s green-roof heat-island guidance explains that vegetation can shade roof surfaces and cool through evapotranspiration. However, performance varies with climate, plant cover, substrate, moisture, roof area and maintenance. A green roof must also meet structural, waterproofing, drainage, fire, access and local-code requirements. One project cannot guarantee a citywide temperature reduction.
Cool roofs and reflective pavements
Lighter or specially designed surfaces can reflect more solar energy. Meanwhile, the U.S. Department of Energy explains that cool-roof performance depends on solar reflectance and thermal emittance. Colour alone is not a complete specification, and glare, durability, climate, winter effects and maintenance also require review.
For example, the original page said white surfaces such as fresh snow reflect up to 90 percent of sunlight, while dark asphalt reflects about 5 percent. Those illustrative values depend on material condition and measurement. It also said New York, the largest city in the United States, added white-roof requirements to its building code as early as 2008. In addition, it described Los Angeles, the second-largest U.S. city, testing light-gray and white “cool pavement” pilots. These historical examples do not establish the result of a different city, material or project.
Where might HOENSOEY Cells fit?
Meanwhile, HOENSOEY Technology advocates “let the urban surface breathe freely” and calls its concept a “shallow water treatment system.” The original page described this as a creative way to restore a damaged environment, support harmonious building development, protect the environment and contribute to peak-emissions and carbon-neutrality targets. These are advocacy statements, not measured outcomes or product certifications.
The original proposed principle
The source proposed collecting rainwater from roofs, road surfaces, sports infrastructure and parking lots into HOENSOEY Cells. It said the cells would concentrate stored rainwater while providing stable support during stormwater retention, attenuation or infiltration. It then called the result a modern, innovative and environmentally friendly “shallow groundwater treatment application system.” That wording requires hydrogeological, water-quality, structural and regulatory review before use.
An underground storage cell does not directly prove a lower urban air or surface temperature. A plausible heat-related pathway would require a complete system that supplies vegetation or enables a cooler surface, followed by measured shade, evapotranspiration, surface temperature or air temperature. Performance would depend on water availability, planting, soil, climate, scale, pavement, drainage, overflow, structural design and maintenance.
The original page said the system could reduce the effect on people, create ecological landscapes, beautify the urban environment and let the surface breathe freely. It also listed three functions. They remain below as historical design claims with explicit evidence limits:
- Surface base: rapid drainage, regulation and storage, and infiltration in the shallow layer of the surface. Verify infiltration rate, water quality, groundwater separation, outlet and overflow.
- Power-free irrigation: a self-circulation irrigation system for green area. Verify hydraulic head, distribution, plant demand, drought conditions and maintenance; storage alone does not guarantee irrigation.
- Tree protection: protects the root system and prevents pavement damage from tree roots. Treat this as an objective, not a guarantee; species, root space, soil, pavement, utilities and arboricultural management remain decisive.
Evidence needed for a project
- Define whether the target is roof-surface, street-surface, pedestrian-air, nighttime-air or indoor temperature.
- Establish a baseline and comparison method, including sensors, locations, reference area, weather and monitoring period.
- Model and verify roof or pavement loads, cell properties, subgrade, safety factors, creep and installation.
- Design runoff collection, pretreatment, storage, underdrain, infiltration and safe overflow.
- Specify vegetation, soil, irrigation, establishment and long-term maintenance where evapotranspiration is part of the cooling pathway.
- Check utilities, groundwater, water quality, accessibility, structural and local planning requirements.
- Measure post-installation performance and report uncertainty instead of assuming the intended heat-island benefit occurred.
Bottom line
The urban heat island effect arises from changes in land cover, material properties, urban geometry, weather and human-generated heat. Communities can combine shade, vegetation, green roofs, cool surfaces, energy planning and heat-health measures. HOENSOEY Cells may support water storage or a planted system where project evidence supports their use. However, the cells alone should not be described as proven to reduce urban heat, protect every tree, prevent pavement damage or achieve emissions targets.