The urban heat island (UHI) effect is a weather and climate-related phenomenon where cities are much warmer than nearby rural areas. The temperature difference is often bigger at night than during the day and is most noticeable when there is little wind, during calm weather, and in summer and winter. The main reason for the UHI effect is changes to land surfaces, while heat from energy use is a smaller cause. Cities cover about 0.5% of Earth’s land but are home to more than half the world’s population. As cities grow, they usually expand and become warmer. The term "heat island" can describe any area that is hotter than its surroundings, but it most often refers to areas affected by human activity.

Monthly rainfall is higher downwind of cities, partly because of the UHI effect. Higher temperatures in cities can extend the growing season, worsen air quality by increasing pollution like ozone, and lower water quality as warmer water flows into streams, harming ecosystems.

Not all cities have a clear UHI effect, and the strength of the effect depends on the local climate. A city’s UHI impact can change based on its surroundings. Heat can be reduced by trees and green spaces, which provide shade and help cool through evaporation. Other solutions include green roofs, reflective surfaces that cool during the day, open areas that improve airflow, lighter-colored materials that reflect sunlight, and building materials that absorb less heat.

Climate change is not the cause of urban heat islands, but it is making heat waves more common and severe, which increases the UHI effect in cities. Compact and dense city planning may also worsen the UHI effect, leading to higher temperatures and greater risks for people living there.

Definition

An urban heat island is when a city is warmer than the nearby countryside. This happens because of how cities are built and used. Heat is trapped by the way land is used, such as having more roads and buildings. The way cities are planned, including how streets are arranged and how big buildings are, also contributes. Materials used in buildings often absorb more heat than natural materials. Cities have less airflow and fewer trees, plants, and water areas. Heat is also created by activities like cooking, heating homes, and industrial processes.

Description

During the day, especially when the sky is clear, surfaces in cities absorb heat from sunlight. These surfaces, such as roads and buildings, warm up faster than similar surfaces in rural areas. Because they can hold a lot of heat, urban surfaces act like storage containers for heat energy. For example, concrete can store about 2,000 times more heat than the same amount of air. A study found that urban surfaces like concrete absorb and store large amounts of heat during the day, which supports the idea of urban heat islands. This causes higher surface temperatures in urban areas, which can be seen using thermal imaging. During the day, this heating also creates winds that move air upward in the city. At night, the situation changes. Without sunlight, the air cools, and the movement of air slows down, making the air near the ground more stable. If the air becomes very stable, a layer of warm air forms close to the ground, trapping heat from city surfaces and keeping nighttime air temperatures higher in urban areas.

In general, the temperature difference between cities and nearby rural areas is more noticeable at night than during the day. For example, in the United States, urban areas are usually 1–7 °F (0.55–3.9 °C) warmer than rural areas during the day and 2–5 °F (1.1–2.8 °C) warmer at night. However, in dry climates like parts of southeastern China and Taiwan, the temperature difference is often larger during the day. Studies show that daily temperature changes depend on factors like local weather, seasons, humidity, plants, surfaces, and materials used in cities.

Seasonal changes in urban heat island temperature differences are less understood than daily changes. The relationship between rain, plants, sunlight, and surface materials in different climates is complex and affects how temperatures change throughout the year in urban areas.

Measurements and predictions

One way to measure the urban heat island (UHI) effect in cities is the UHI Index, developed by the Californian EPA in 2015. This method compares the temperature of an area being studied with temperatures from rural areas located upwind of the city, at a height of two meters above the ground. The temperature difference in degrees Celsius is recorded every hour. These differences are added together to create a total number called "degree-Celsius-hours," which represents the UHI Index for that area. This measure can be averaged over many days, and the result is expressed as "Celsius-hours per averaged day."

The UHI Index was created to help estimate how much air conditioning might be used in California and the related greenhouse gas emissions. However, it does not account for factors like wind speed, humidity, or the amount of sunlight, which can affect how hot people feel or how air conditioners operate.

If a city or town has a reliable system for collecting weather data, the UHI effect can be measured directly. Another option is to use complex computer models to simulate the area and calculate the UHI. These models can also help predict how rising temperatures from climate change might affect cities in the future.

In 1969, Leonard O. Myrup published the first detailed study to predict the effects of the urban heat island. He found that the heat island effect results from several physical processes working together. Usually, less evaporation in city centers and the thermal properties of buildings and pavement materials are the most important factors. Today, modern tools like ENVI-met simulate how buildings, ground surfaces, plants, and the surrounding air interact to study the UHI effect.

Causes

Urban heat islands (UHIs) occur because of how cities are designed. Dark surfaces, like roads and buildings, absorb more heat from the sun than lighter surfaces. This causes cities to be hotter than nearby rural areas during the day. Materials used in cities, such as concrete and asphalt, handle heat differently than natural materials found in rural areas. These differences affect how heat is stored and released, leading to higher temperatures in urban areas.

Transportation infrastructure, such as roads and parking lots, contributes to the urban heat island effect. For example, pavement in cities like Phoenix, United States, is a major source of heat during summer afternoons.

A lack of vegetation in cities also plays a role. Plants help cool the air through a process called evapotranspiration, where they release water into the atmosphere. A 2018 study by the U.S. Forest Service found that cities in the United States lose about 36 million trees each year. Fewer trees mean less shade and less cooling from evaporation.

The shape of buildings and streets can also increase heat. Tall buildings reflect and absorb sunlight, creating a "urban canyon effect" that traps heat. Buildings can also block wind, which reduces natural cooling. Heat from vehicles, air conditioning, and industries adds to the problem.

The type of land near a city affects its temperature. Areas near barren land tend to be hotter, while areas near vegetation are cooler.

How people design spaces, such as yards and parks, can reduce or increase urban heat. Using native plants in landscaping helps create cooler areas that survive with natural rainfall. These plants provide shade and moisture. Lawns with non-native grasses may not survive changes in weather. Large parking lots can become extremely hot and contribute to UHIs. Including trees, shade, and resilient plants in development can help prevent dangerous heat.

Air pollution in cities can worsen urban heat islands. Pollution changes how the atmosphere handles heat, which increases temperatures. Higher temperatures also lead to more ozone, a type of greenhouse gas that warms the planet.

Climate change does not cause urban heat islands but makes them worse. A 2022 report said that as global temperatures rise, heat stress in cities increases. This can lead to more frequent and severe heatwaves, which harm health and productivity.

Urbanization, or the growth of cities, increases heat risks. It replaces natural areas with surfaces that do not absorb water, like roads and buildings. Suburban areas usually have more greenery but can become hot if droughts reduce water for lawns. Suburbs that rely on cars often have large parking lots, which heat up quickly. Cities can be designed with parks and tall buildings to balance heat and climate.

Historic practices, like redlining in the United States, have led to unequal distribution of trees and green spaces. This can create areas with more heat, especially in neighborhoods with less access to healthcare, transportation, or cooling resources. These areas are more likely to experience extreme heat.

Housing conditions also affect heat. Living on upper floors, having dark roofs, or poor insulation can make homes hotter. Lower-income individuals may lack air conditioning or cannot afford higher energy costs. In dense cities, opening windows can bring in pollution. In Asia, high-density housing often has limited space for cooling improvements.

Space poverty, or having very small living areas, is a problem in places like Hong Kong. People in small, crowded homes often live in poorly ventilated buildings with little space for cooling. These residents may rely on public spaces like parks or libraries for relief from heat.

Impacts

Urban heat islands (UHIs) can change local weather patterns in ways beyond temperature. These changes include altering wind directions, forming clouds and fog, affecting humidity levels, and changing how much rain falls. The extra heat from UHIs causes warm air to rise, which can lead to more showers and thunderstorms. During the day, UHIs create a low-pressure area near cities where moist air from surrounding rural areas flows in, increasing the chance of cloud formation. Rainfall rates in areas downwind of cities are 48% to 116% higher than in upwind areas. Monthly rainfall is 28% greater between 20 and 40 miles (32 to 64 km) downwind of cities compared to upwind areas. Some cities show a total increase in precipitation of 51%.

One study found that cities can change the climate in areas two to four times larger than their own size. A 1999 study comparing urban and rural areas suggested that UHIs have little effect on global temperature trends. However, other studies say UHIs may influence global climate by affecting the jet stream.

UHIs can harm the health and well-being of city residents. Increased temperatures from UHIs can make heat waves longer and more intense. More people are exposed to extreme heat because of UHIs. At night, UHIs reduce the cooling effect found in rural areas, making heat waves more dangerous. Extreme heat can increase the risk of illnesses and deaths, especially among older adults.

Higher temperatures can cause heat-related illnesses, such as heat stroke, heat exhaustion, heat syncope, and heat cramps. Extreme heat is the deadliest weather event in the U.S. A study by Professor Terri Adams-Fuller found that heat waves kill more people in the U.S. than hurricanes, floods, and tornadoes combined. These illnesses are more common in medium-to-large cities due to UHIs. Heat illnesses can worsen when combined with air pollution, which is common in cities.

Increased temperatures can affect mental health by raising aggression, domestic violence, and substance abuse. Heat can also lower school performance. A study by Hyunkuk Cho of Yeungnam University found that more days of extreme heat each year are linked to lower student test scores.

High UHI intensity is connected to higher levels of air pollutants that build up at night, such as volatile organic compounds, carbon monoxide, nitrogen oxides, and particulate matter. These pollutants, combined with higher temperatures, can increase the formation of harmful ground-level ozone. While UHIs may increase polluted days, other factors like air pressure, cloud cover, and wind speed also affect pollution levels.

Studies in Hong Kong found that areas with poor outdoor ventilation have stronger UHIs and higher death rates compared to well-ventilated areas. A study in Babol, Iran, showed a significant increase in Surface Urban Heat Island Intensity (SUHII) from 1985 to 2017, influenced by geography and time. This research highlights the need for careful city planning to reduce health risks from UHIs. Surface UHIs are strongest during the day and are measured using land surface temperature and remote sensing.

Heat stress vulnerability refers to how likely a person is to be harmed by heat, often due to factors like age, income, or health. Groups at higher risk include the elderly, children, low-income households, and those with chronic illnesses. Elderly people are more vulnerable because they struggle to regulate body temperature and often have pre-existing health conditions. Children are also sensitive to heat, which can stress their developing bodies. High indoor heat can strain relationships and mental health.

UHIs can harm water quality. Hot surfaces like pavements and rooftops transfer heat to stormwater, which flows into streams, rivers, and lakes, raising water temperatures. Warmer water reduces biodiversity. For example, in 2001, rain in Cedar Rapids, Iowa, caused a 10.5°C (18.9°F) temperature rise in a nearby stream within an hour, killing about 188 fish. Similar events have been reported in other U.S. regions. Rapid temperature changes stress aquatic ecosystems.

Buildings can be 50°F (28°C) hotter than the air around them, causing rain to warm quickly and increasing water temperatures in nearby water bodies by 20 to 30°F (11 to 17°C). This rapid temperature change stresses fish and other aquatic life.

Permeable pavements may help by allowing water to soak into the ground, reducing heat transfer to water.

UHIs can worsen droughts and be worsened by them. Some species, like the grey-headed flying fox and common house gecko, thrive in urban areas due to warmer temperatures. In Melbourne, Australia, grey-headed flying foxes moved into the city as warmer winters made the climate similar to their natural habitat.

In temperate climates, UHIs can extend the growing season, changing how species breed. This is visible in changes to water temperatures.

UHIs have changed natural selection by reducing selective pressures like food availability and predation. In cities, insects are more common because they rely on environmental temperatures to regulate their body heat, and urban areas are warmer. A study in Raleigh, North Carolina, found that urban habitats support more insects than rural areas.

Options for reducing heat island effects

Strategies to improve urban resilience by reducing excessive heat in cities include: planting trees, using cool roofs (painted white or with reflective coating), using light-colored concrete, creating green infrastructure (such as green roofs), and using passive daytime radiative cooling.

Cities can be up to 5°C (9.0°F) warmer than nearby rural areas. About 40% of this temperature increase is caused by dark-colored roofs, while the rest comes from dark-colored pavement and less vegetation. Using white or reflective materials for buildings, roofs, pavements, and roads can increase a city’s overall reflectiveness, helping to reduce heat.

Expanding cities in circular patterns can worsen the urban heat island effect. Instead, cities should be planned in strips that follow natural water systems and include green spaces with different types of plants. Examples of this approach include cities like Kielce, Szczecin, Gdynia in Poland, Copenhagen in Denmark, and Hamburg, Berlin, and Kiel in Germany.

Planting trees around cities can increase reflectiveness and reduce the urban heat island effect. Deciduous trees are recommended because they provide shade in summer and allow sunlight in winter. Trees can lower air temperatures by up to 10°F (5.6°C) and surface temperatures by as much as 20–45°F (11–25°C). Trees also help fight global warming by absorbing carbon dioxide from the air.

Recent studies show that increasing the number of trees in cities can lower air temperatures near the ground. A 2024 study found that cities with more tree cover typically cool daytime temperatures by 1–2°C. Areas without tree cover within 10 meters were more likely to reach very high temperatures, while more tree cover led to cooler air. Tree diversity and coverage are linked to greater cooling effects in certain seasons.

Urban green infrastructure (UGI) includes networks of green spaces in both public and private areas. Examples include parks, tree-lined streets, private gardens, rooftop gardens, and other green areas. Proper use of UGI helps distribute heat more evenly, while poor use can worsen heat inequality. UGI reduces temperatures by providing shade and through evapotranspiration, which cools the air and land. Lack of UGI in marginalized communities can increase heat inequality by reducing the land’s ability to regulate temperature. UGI is seen as an effective, sustainable solution to heat inequality.

Painting rooftops white is a common strategy to reduce the urban heat island effect. Dark surfaces in cities absorb more heat, lowering a city’s reflectiveness. White rooftops reflect more sunlight, increasing reflectiveness. Green and cool roofs help cities manage extreme heat. Cool roofs are especially effective in warm regions, sometimes reducing cooling energy use for low-rise buildings. Green roofs may be more helpful in colder climates because they provide insulation and reduce heating needs in winter.

Using reflective coatings on rooftops is an effective way to reduce heat gain. A study in London found that white or reflective rooftops reduced outdoor temperatures more than solar panels, green roofs, or tree cover. During a 2018 heatwave, cool roofs lowered average outdoor temperatures by 1.2°C and up to 2°C in some areas. Tree cover reduced temperatures by 0.3°C, and solar panels by 0.5°C.

Replacing dark roofs with reflective materials requires less investment than other solutions and provides quick results. Cool roofs made of reflective materials reflect at least 75% of sunlight and emit at least 70% of absorbed solar radiation. In comparison, dark asphalt roofs reflect only 6% to 26% of sunlight.

Light-colored concrete reflects up to 50% more light than asphalt and reduces surrounding temperatures. Dark asphalt absorbs more heat, increasing surface temperatures. Replacing asphalt with light-colored concrete can lower average temperatures. However, if nearby buildings are not reflective, sunlight reflected from light-colored pavement can increase building temperatures, raising air conditioning needs.

Some paints are designed for daytime radiative cooling and can reflect up to 98.1% of sunlight.

Green roofs act as insulation during hot weather and cool the surrounding area. Plants absorb carbon dioxide and produce oxygen, improving air quality. Green roofs also help manage stormwater and reduce energy use. Cost can be a challenge for implementing green roofs. Factors like design, soil depth, location, and labor costs influence expenses. Comparing green roofs globally is difficult due to differences in design and the lack of a shared evaluation system. Green roofs also provide social, environmental, and public health benefits beyond cost.

Stormwater management is another way to reduce the urban heat island effect. Stormwater management controls water from storms to protect property and infrastructure. Urban surfaces like streets and parking lots prevent water from soaking into the ground, causing flooding. Using techniques like pervious pavement systems (PPS) allows water to pass through pavement, reducing heat through evaporation. PPS has been used in over 30 countries and helps manage stormwater and reduce urban heat.

Green parking lots use vegetation and materials other than asphalt to limit the urban heat island effect.

Green infrastructure or blue-green infrastructure includes networks that help solve urban problems by combining green spaces and water systems.

Society and culture

Luke Howard first studied and described the urban heat island effect in the 1810s. However, he did not give it a name. A report from that time said that the center of London was 2.1 °C (3.7 °F) warmer at night than the surrounding countryside.

Scientists continued to study the urban atmosphere throughout the 1800s. From the 1920s to the 1940s, researchers in Europe, Mexico, India, Japan, and the United States worked to better understand this effect. In 1929, Albert Peppler used the term städtische Wärmeinsel, which means "urban heat island" in German, in a publication. Between 1990 and 2000, about 30 studies were published each year. By 2010, this number had grown to 100, and by 2015, it reached more than 300.

In 1969, Leonard O. Myrup published the first detailed study that used numbers to predict how urban heat islands affect cities. His work reviewed existing theories and pointed out that they were too vague.

Urban heat inequity, also called thermal inequity, is when heat is not shared equally in cities. Some neighborhoods experience more heat than others, which can harm people living there. This unequal heat is often linked to differences in race, income, education, and age. In many cities, the most affected communities are those that have faced unfair treatment in the past. This issue is closely connected to the urban heat island effect, which is caused by increased urbanization.

Some studies show that the health effects of urban heat islands may not be the same for everyone. Factors like age, race, and income can influence how heat affects people. This raises concerns about whether these effects are a fairness issue. Research in the United States has found that communities of color are more likely to be affected by urban heat islands.

There is a link between neighborhood income and the number of trees. Poorer neighborhoods often have fewer trees than wealthier areas. Researchers think this happens because lower-income communities may not have the money to plant or care for trees. Wealthier areas can afford more trees on both public and private land. Another reason is that richer people often have more space to create green areas, while poorer housing is more likely to be rental properties where landowners try to build as many homes as possible.

Globally, the effects of urban heat islands vary by region. A study by Professor Kanging Huang and others found that the effects of heat have grown faster in the Global South in recent decades. In places near the equator, where it is already hot and humid, the impact of urban heat islands is especially strong. A World Bank study found that in Bandung, Indonesia, the hottest neighborhoods are 7.0° warmer than the coolest ones.

In the United States, there is a connection between race and exposure to urban heat islands. In most U.S. cities, people of color are more likely to live in areas with high heat intensity than white people in the same cities. A study by Angel Hsu and colleagues found that, in most of the 175 largest U.S. cities, the average person of color lives in a neighborhood with higher heat intensity than non-Hispanic white people. A 2023 report also found that historically redlined and low-income Black neighborhoods, which already face high heat, also have higher rates of violent crime, making these communities even more vulnerable.

Economic status also affects how urban heat islands impact people. Lower-income individuals are more likely to live in areas with high heat and less likely to afford air conditioning. Similar patterns are seen when comparing poor households to those with higher incomes.

Urban heat islands can have serious effects on African Americans with chronic illnesses. African Americans have higher rates of conditions like asthma and diabetes than the general population. Extreme heat can worsen these conditions, leading to problems like high blood pressure or strokes.

Researchers have found that neighborhoods with low socioeconomic status often have more impervious surfaces, like concrete, tar, and asphalt. These materials are linked to higher temperatures in cities.

Redlining was a housing policy created in the 1930s by the Home Owners' Loan Corporation (HOLC) under the Roosevelt Administration. This policy labeled neighborhoods as risky based on their population. Areas with more minorities and lower-income residents were marked as dangerous on maps, often in red. This limited access to loans and caused long-term neglect in these communities.

A study of 108 U.S. cities found that neighborhoods labeled "redlined" in the past are, on average, 2.6 °C (4.7 °F) hotter than non-redlined areas. This is partly because these areas have fewer trees and more heat-absorbing materials like asphalt. In cities like Durham, Fresno, and Pittsburgh, "D" graded areas have less tree cover than wealthier "A" graded areas, which are mostly white. In Phoenix, Arizona, formerly redlined areas have surface temperatures up to 10–15 °F (5.6–8.3 °C) higher than other neighborhoods.

These neighborhoods also have higher air pollution because they are near highways and industrial areas. In Los Angeles, freeways built in the 1950s passed through redlined areas like Boyle Heights and South Los Angeles. These areas now have higher levels of harmful emissions, such as diesel exhaust and fine particles, which increase health risks. Globally, studies show that air pollution can make urban heat islands worse. In China, a type of pollution called urban haze adds up to 0.7 °C of extra nighttime heat.

In response to these issues, some cities are starting programs to plant more trees and use reflective materials to reduce heat.

Examples

In 2020, Bill S.4280 was introduced to the U.S. Senate. This bill would allow the National Integrated Heat Health Information System Interagency Committee (NIHHIS) to work on extreme heat problems in the United States. If the bill passes, NIHHIS would receive funding for five years. It would also create a $100 million grant program to support projects that reduce urban heat, such as cool roofs, cool pavements, and improved heating and cooling systems. As of July 22, 2020, the bill has not advanced beyond its introduction in Congress.

New York City studied how different methods reduce heat. It found that street trees provide the most cooling per area, followed by living roofs, light-colored surfaces, and open space planting. From a cost perspective, light-colored surfaces, light-colored roofs, and planting trees along curbs are the most cost-effective ways to lower temperatures.

A proposed "cool communities" program in Los Angeles in 1997 estimated that planting 10 million trees, reroofing 5 million homes, and painting one-quarter of roads could reduce urban temperatures by about 3°C (5°F). The estimated cost for this plan was $1 billion. It was expected to save $170 million annually from lower air-conditioning costs and $360 million from health savings related to reduced smog.

A 1998 study of the Los Angeles Basin showed that even without strategic placement, trees can still help reduce pollution and energy use in urban heat islands. If widely implemented, Los Angeles could save $100 million annually, mostly from cool roofs, light-colored pavement, and tree planting. A citywide effort could also save at least $1 billion yearly by reducing smog levels.

Los Angeles TreePeople is an example of how tree planting can strengthen communities. This group helps people work together, build skills, and create pride through collaboration.

Los Angeles has created a Heat Action Plan to address extreme heat more specifically than the state of California’s solutions. The city uses the LA Equity Index to ensure that efforts to reduce heat impacts are fair and benefit all residents.

In 2021, the Climate Adaptation Planning Analysis (CAPA) received funding from the National Oceanic and Atmospheric Administration to map heat across the United States. Ten areas in Virginia—Abington, Arlington, Charlottesville, Farmville, Harrisonburg, Lynchburg, Petersburg, Richmond, Salem, Virginia Beach, and Winchester—participated in a heat watch campaign. This campaign involved 213 volunteers who collected 490,423 heat measurements across 70 routes. Data was sent to CAPA for analysis using computer programs that learn from data. After analysis, CAPA met with local organizers to discuss future plans for each town.

New York City started its "Cool Neighborhoods NYC" program in 2017 to reduce extreme urban heat. One goal was to increase funding for the city’s Low-Income Home Energy Assistance Program, which helps lower-income families afford cooling solutions.

Several cities in India, including Delhi, Bengaluru, Chennai, Jaipur, Ahmedabad, Mumbai, and Kolkata, experience significant urban heat island effects. These effects are caused by rapid urbanization, loss of green spaces, and heavy use of concrete. A report by The Hindu found that these cities are 1°C to 6°C warmer than nearby rural areas. Urban heat islands raise local temperatures, worsen heatwaves, increase cooling costs, and harm health, especially for vulnerable groups.

Mumbai, India’s financial center and one of the world’s most densely populated cities, is heavily affected by the urban heat island effect. Rapid growth, concrete use, and loss of green spaces have made Mumbai’s temperatures higher than surrounding areas. A report estimated that Mumbai may spend twice as much as New York City to manage heat caused by concrete use. This higher cost shows how serious the urban heat island effect is in Mumbai and its impact on the city’s infrastructure and residents.