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 greater at night than during the day and is most noticeable when winds are weak, especially during summer and winter. The main cause of the UHI effect is changes to the land surface, 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 people. As cities grow, they often expand and become warmer on average. The term "heat island" can describe any area that is hotter than its surroundings but is usually used to describe human-altered areas.

Monthly rainfall is often higher in areas downwind of cities, partly because of the UHI effect. Increased heat in cities can make growing seasons longer, reduce air quality by increasing pollutants like ozone, and lower water quality by warming water that flows into streams and harming ecosystems.

Not all cities have a clear urban heat island, and the strength of the effect depends on the climate of the area where the city is located. The impact of the UHI can change greatly based on local conditions. Heat can be reduced by adding tree cover and green spaces, which provide shade and help cool the air through evaporation. Other solutions include green roofs, materials that reflect sunlight, lighter-colored surfaces, and designs that improve airflow. These methods help reduce heat absorption.

Climate change is not the cause of urban heat islands, but it is making heat waves more frequent and severe. These heat waves increase the effects of urban heat islands in cities. Compact and dense city planning may also increase 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 warmth happens because of how cities are built. Factors include how land is used, the way cities are planned (such as street layouts and building sizes), materials in buildings that absorb heat, less air movement, fewer trees and water features, and heat from homes, factories, and other human activities.

Description

During the day, especially when the sky is clear, surfaces in cities absorb heat from the sun. These surfaces in urban areas often become warmer than those in nearby rural areas. Because they have high heat capacities, urban surfaces can store large amounts of heat energy. For example, concrete can hold about 2,000 times more heat than the same amount of air. A study shows that urban surfaces like concrete absorb and store significant heat during the day, which supports the idea of urban heat islands. This causes higher surface temperatures in urban heat islands (UHIs), which can be seen using satellite images that detect heat. During the day, this heating also creates winds that move air upward in the city's atmosphere. At night, the situation changes. Without sunlight, the air stops rising, and the city's atmosphere becomes more stable. If this stability is strong enough, a layer of warm air forms near the ground, trapping heat from urban surfaces and keeping nighttime air temperatures higher in UHIs.

In general, the temperature difference between cities and nearby rural areas is often greater at night than during the day. For example, in the United States, urban areas are typically 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 areas like parts of southeastern China and Taiwan, the temperature difference is usually larger during the day. Research shows that daily temperature changes depend on factors such as local weather, seasons, humidity, plants, surfaces, and materials in cities.

Seasonal changes in urban heat island temperature differences are less understood than daily changes. Complex interactions between rain, plants, sunlight, and surface materials in different climates affect how temperatures change throughout the year in urban heat islands.

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 a studied area with temperatures from rural locations upwind of the city, measured at two meters above the ground. The temperature difference in degrees Celsius is recorded every hour. These differences are added together over time, creating a value called "degree-Celsius-hours," which represents the UHI Index for that area. This value can be averaged over many days, but it is reported as Celsius-hours per day on average.

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

If a city has a reliable system for collecting weather data, the UHI can be measured directly. Another option is to use detailed computer models to simulate the area and calculate the UHI. These models can also help predict how rising temperatures due to 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. In general, less evaporation in city centers and the heat-absorbing properties of buildings and pavement are the most important factors. Today, advanced tools like ENVI-met simulate how buildings, ground surfaces, plants, and 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 sunlight than lighter surfaces, making urban areas hotter than suburban or rural areas during the day. Materials used in cities, such as concrete and asphalt, have different properties than those in rural areas. These materials hold heat longer and reflect less sunlight, which increases urban temperatures. This changes how energy is used in cities, often causing higher temperatures than nearby rural areas.

Transportation infrastructure, such as roads and parking lots, plays a major role in UHIs. For example, pavement in cities like Phoenix, United States, is a key reason cities become very hot during summer afternoons.

Another reason is the lack of evapotranspiration, which happens when plants release water vapor into the air. This process cools the environment. 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.

Geometric effects also contribute to UHIs. Tall buildings reflect and absorb sunlight, making cities heat up more efficiently. This is called the "urban canyon effect." Buildings can also block wind, which reduces cooling and traps pollutants. Heat from cars, air conditioning, and factories adds to the UHI.

Proximity to different types of land affects UHIs. Being near barren land makes cities hotter, while being near vegetation makes them cooler.

Landscape design, such as yards and city parks, can reduce UHIs. Using native plants that need less water and provide shade can help cool areas. Non-native grass lawns may not survive changes in climate or water availability. Large parking lots heat up more than surrounding areas. Including trees, shade structures, and resilient plants in designs can help prevent dangerous heat.

Air pollution in cities increases UHIs because pollution changes how the atmosphere handles sunlight. Higher temperatures from UHIs also increase ozone levels, which is a greenhouse gas that traps heat.

Climate change does not cause UHIs but makes them worse. A 2022 report by the IPCC stated that climate change increases heat risks in cities and makes UHIs stronger in Asian cities at higher warming levels. Rising temperatures worsen UHIs, leading to more heatwaves that could affect half of the world's urban population, harming health and productivity.

Heat and built infrastructure interact in ways that increase heat stress for city residents.

Urbanization increases heat risks by replacing green spaces with hard surfaces like pavement and concentrating people in smaller areas. Factors like greenery, water availability, and infrastructure design determine if an area has a UHI. Suburban areas often have more trees but can heat up quickly if droughts limit water for lawns. Suburbs that rely heavily on cars often have large parking lots that heat up quickly. Cities are more likely to have UHIs but can be designed with tall buildings surrounded by parks to balance heat and climate.

Historic practices, such as redlining in the United States, have led to unequal distribution of vegetation and resources. Areas with less access to healthcare, public transport, housing, and energy are more vulnerable to heat. These areas may experience micro UHIs, where certain neighborhoods are hotter due to lack of vegetation.

Housing conditions also affect heat inequity. Living on upper floors, having dark roofs, and poor insulation make homes hotter. Lower-income individuals may lack air conditioning or afford higher electricity costs. In dense cities, opening windows can bring in pollution. In Asia, high-density housing often limits updates to improve cooling.

Space poverty, or very small living spaces, is a problem in places like Hong Kong. Low-income residents live in overcrowded, poorly ventilated units with little space for cooling. These homes are often in old buildings with no room for improvements. Residents may rely on public spaces like libraries or parks for cooling.

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, and changing how much rain falls. The extra heat from UHIs causes air to rise more quickly, which can lead to more showers and thunderstorms. During the day, UHIs create a low-pressure area near cities, drawing in moist air from nearby rural areas. This can help clouds form more easily. Rainfall downwind of cities is often 48% to 116% higher than upwind. In some areas, monthly rainfall is 28% greater between 20 and 40 miles (32 to 64 km) downwind of cities compared to upwind. Some cities have seen a total increase in precipitation of 51%.

One study found that UHIs can affect the climate in areas two to four times larger than the city itself. A 1999 study compared urban and rural areas and suggested that UHIs have little effect on global temperature trends. However, other studies say that UHIs might influence global climate by affecting the jet stream.

UHIs can directly harm the health and well-being of people in cities. Because UHIs raise temperatures, they can make heat waves longer and more intense. More people are exposed to extreme heat due to the warming caused by UHIs. At night, UHIs can be especially harmful during heat waves because they reduce the cooling effect that rural areas provide. Prolonged exposure to extreme heat increases the risk of heat-related illnesses and deaths, especially for older adults.

Higher temperatures can lead to heat illnesses, such as heat stroke, heat exhaustion, heat syncope, and heat cramps. Extreme heat is the most deadly 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, largely because of UHIs. Heat illnesses can be worsened by air pollution, which is common in urban areas.

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

UHIs can increase the amount of air pollutants that build up at night, which affects air quality the next day. These pollutants include volatile organic compounds, carbon monoxide, nitrogen oxides, and particulate matter. Higher temperatures and these pollutants can speed up the creation of ozone, a harmful pollutant at ground level. Studies say that UHIs can increase polluted days, but other factors like air pressure, cloud cover, and wind speed also influence pollution.

Studies in Hong Kong found that areas with poor outdoor air ventilation had stronger UHIs and higher all-cause mortality compared to areas with better ventilation. Another study in Babol, Iran, showed that the intensity of surface urban heat islands (SUHII) increased from 1985 to 2017, influenced by geography and time. This research highlights the need for careful city planning to reduce the health risks of 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 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 may struggle to regulate body temperature and often have existing health issues. Children are also affected because their bodies are still developing. High indoor heat can strain mental and physical health and affect family relationships.

UHIs can harm water quality. Hot pavement and rooftops transfer heat to stormwater, which then flows into streams, rivers, and lakes, raising water temperatures. Warmer water can reduce biodiversity. For example, in August 2001, heavy rain in Cedar Rapids, Iowa, caused a 10.5°C (18.9°F) rise in a nearby stream within an hour, killing about 188 fish. The hot city pavement, not the cool rain, was the main cause of the fish deaths. Similar events have been reported in other parts of the U.S. Rapid temperature changes can stress aquatic ecosystems.

The temperature of nearby buildings can be up to 50°F (28°C) warmer than the air near the ground. Rainwater can quickly warm as it flows into streams, lakes, and rivers, creating excessive thermal pollution. This can raise water temperatures by 20 to 30°F (11 to 17°C), causing stress and shock to fish and other aquatic life.

Permeable pavements may help reduce these effects by allowing water to soak through into underground storage areas, where it can be absorbed or evaporated.

UHIs can worsen droughts and be worsened by them. Some species, like the grey-headed flying fox and the common house gecko, have adapted to urban heat and thrive in areas outside their usual range. Grey-headed flying foxes in Melbourne, Australia, have moved into cities because warmer winter temperatures make the climate more similar to their natural habitat in northern regions.

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

UHIs caused by cities have changed natural selection. Factors like food availability, predation, and water access are less variable in urban areas, leading to new selective pressures. For example, insects are more common in cities than in rural areas. Insects are ectotherms, meaning their body temperature depends on the environment. Warmer urban temperatures are ideal for their survival. A study in Raleigh, North Carolina, found that…

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 coatings), building with light-colored concrete, creating green infrastructure (such as green roofs), and using passive daytime radiative cooling.

The temperature difference between urban areas and nearby rural or suburban areas can be as much as 5°C (9.0°F). About 40% of this increase is caused by dark roofs, while the rest comes from dark-colored pavement and the loss of vegetation. Using white or reflective materials on buildings, roofs, pavements, and roads can help reduce the urban heat island effect by increasing the city’s overall albedo, which is the ability of a surface to reflect sunlight.

Expanding cities in a circular pattern worsens the urban heat island effect. Instead, cities should be planned in strips that follow natural water systems and include green spaces with diverse plant life. For example, cities like Kielce, Szczecin, Gdynia in Poland, Copenhagen in Denmark, and Hamburg, Berlin, and Kiel in Germany have built urban areas that stretch over large regions.

Planting trees around cities can increase albedo 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 10°F (5.6°C) and surface temperatures by up to 20–45°F (11–25°C). Trees also help fight global warming by absorbing carbon dioxide from the air.

Recent studies show that increasing street tree coverage cools air temperatures near the ground. A 2024 study of 110 cities found that tree cover typically lowers daytime temperatures by about 1–2°C. Another study found that areas without tree cover within 10 meters were up to five times more likely to exceed 32.2°C. Tree diversity and canopy 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, such as parks, tree-lined streets, private gardens, and rooftop gardens. Proper use of UGI helps distribute heat more evenly, while poor planning can worsen heat inequity. UGI reduces temperatures through shading and evapotranspiration, which is the process of water evaporating from plants. A lack of UGI in marginalized communities limits the land’s ability to regulate temperature, increasing heat inequity. UGI is widely seen as an effective, sustainable solution to heat inequity.

Painting rooftops white is a common strategy to reduce the urban heat island effect. Dark surfaces absorb more heat, lowering the city’s albedo. White rooftops reflect more sunlight and emit more heat, increasing albedo. 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 better in colder climates because they provide insulation and reduce winter heating needs.

Covering rooftops with reflective coatings has proven effective in reducing solar heat gain. A study by Oscar Brousse from University College London found that white or reflective rooftops in London reduced outdoor temperatures by up to 2°C during a 2018 heatwave. This was more effective than solar panels, green roofs, or tree cover.

Replacing dark roofs with reflective materials requires less investment than other solutions and provides quick results. Reflective materials like vinyl can reflect at least 75% of sunlight, while asphalt roofs reflect only 6% to 26%.

Using light-colored concrete instead of asphalt reflects up to 50% more light and lowers ambient temperatures. Black asphalt has a low albedo, absorbing more heat and raising surface temperatures. However, reflective pavements can increase building temperatures if nearby buildings are not also reflective, which may raise 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 warm weather and cool the surrounding area. Plants absorb carbon dioxide and release oxygen, improving air quality. They also help manage stormwater and reduce energy use. Cost is a challenge for green roofs, as expenses depend on design, soil depth, location, and labor costs. Each green roof’s context makes comparisons difficult, and focusing only on money may overlook social, environmental, and health benefits.

Stormwater management helps reduce the urban heat island effect by controlling water runoff from streets, sidewalks, and parking lots. A technique called pervious pavement systems (PPS) allows water to flow through pavement, reducing heat through evaporation. PPS has been used in over 30 countries and is effective for both stormwater management and heat reduction.

Green parking lots use vegetation and non-asphalt surfaces to limit the urban heat island effect.

Green infrastructure or blue-green infrastructure refers to a network that provides solutions for urban challenges.

Society and culture

The phenomenon was first studied and described by Luke Howard in the 1810s, though he did not name it. A report from that time noted that the center of London was 2.1 °C (3.7 °F) warmer at night than the surrounding countryside.

Studies of the urban atmosphere continued throughout the 1800s. From the 1920s to the 1940s, scientists in Europe, Mexico, India, Japan, and the United States worked to better understand the phenomenon.

In 1929, Albert Peppler used the term städtische Wärmeinsel (which means "urban heat island" in German) in a publication, believed to be the first use of the term. Between 1990 and 2000, about 30 studies were published each year; by 2010, that number grew to 100, and by 2015, it reached more than 300.

Leonard O. Myrup published the first detailed numerical study to predict the effects of the urban heat island (UHI) in 1969. His paper examined UHI and criticized earlier theories for being too vague.

Urban heat inequity, also called thermal inequity, refers to unequal heat distribution in cities or neighborhoods, which causes unequal effects on people living there. Unequal heat risks in cities are often linked to differences in demographics, such as race, income, education, and age. While the effects of urban heat inequity vary by city, negative impacts often affect historically marginalized communities. This issue is closely connected to the urban heat island effect, as increased urbanization is a major cause of urban heat inequity.

Some studies suggest that the health effects of UHIs may be uneven, as impacts can vary based on factors like age, ethnicity, and socioeconomic status. This raises the possibility that health impacts from UHIs could be an environmental justice issue. Research shows that communities of color in the United States are more likely to be affected by UHIs.

There is a link between neighborhood income and tree canopy cover. Low-income neighborhoods often have far fewer trees than wealthier areas. Researchers think that lower-income areas lack the money to plant and maintain trees. Wealthier neighborhoods can afford more trees on both public and private land. One reason for this is that wealthier people often have more land, which can be used for green spaces, while poorer areas often have more rental housing, where landowners prioritize maximizing housing density.

Globally, the effects of UHIs vary by region. While heat exposure is increasing worldwide, its impacts have grown faster in the Global South in recent decades, according to a study by Professor Kanging Huang and colleagues.

The unequal effects of UHIs on the Global South worsen existing environmental injustices. Many countries near the equator are naturally hot and humid, making them more vulnerable to UHIs. A World Bank study found a 7.0° temperature difference between the hottest and coolest neighborhoods in Bandung, Indonesia.

In the United States, there is a connection between ethnicity and exposure to UHIs. In most U.S. cities, people of color are more likely to live in areas with high Surface Urban Heat Island (SUHI) intensity than white people in the same cities. A study by climatologist Angel Hsu and colleagues found that, on average, people of color live in areas with higher SUHI intensity than non-Hispanic whites in all but six of the 175 largest urbanized areas in the U.S. A 2023 policy brief also noted that historically redlined and low-income Black neighborhoods, already affected by urban heat, also experience higher rates of violent crime, increasing risks in these communities.

Economic status influences the human effects of UHIs. People with lower incomes are more likely to live in UHIs and also less likely to afford air conditioning. Similar to the link between SUHI intensity and ethnicity, a similar pattern exists when comparing households below the poverty line to those with incomes more than double the poverty line.

UHIs can have strong effects on African Americans with chronic diseases. African Americans have higher rates of chronic illnesses, such as asthma and diabetes, than the general population. Research by Professor Pamela Jackson and colleagues shows that extreme heat can worsen these conditions, leading to health issues like hypertension or stroke.

Researchers have also found that the spread of impervious surfaces, such as concrete, tar, and asphalt, is linked to low-income neighborhoods in the U.S. These materials are predictors of temperature differences within 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 perceived investment value. Areas with more minority and low-income residents were marked as hazardous and outlined in red on HOLC maps. This limited access to housing loans and led to long-term neglect of these communities.

A study of 108 U.S. cities found that formerly redlined neighborhoods are, on average, 2.6 °C (4.7 °F) hotter than non-redlined areas. This was largely due to fewer trees and more heat-absorbing surfaces like asphalt. In cities like Durham, Fresno, and Pittsburgh, neighborhoods labeled "D" on HOLC maps have much less tree coverage than wealthier "A" graded neighborhoods, 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 face higher air pollution levels due to their proximity to highways and industrial zones. In Los Angeles, freeways built in the 1950s often passed through redlined areas, such as Boyle Heights and South Los Angeles. These areas now have higher levels of pollution, like diesel exhaust and fine particulate matter (PM2.5), which increase risks for respiratory and cardiovascular diseases. Globally, research shows that urban air pollution, such as vehicle emissions, can intensify the urban heat island effect. In China, a "haze effect" was found to contribute up to 0.7 °C of additional nighttime warming.

In response to these issues, some cities have started urban greening programs and reflective materials to reduce heat effects.

Examples

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

In New York City, studies found that street trees provide the most cooling benefit per area, followed by living roofs, light-colored surfaces, and open space planting. From a cost-effectiveness perspective, light-colored surfaces, light-colored roofs, and curbside planting offer the lowest cost for each degree of temperature reduction.

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

A 1998 study of the Los Angeles Basin showed that even when trees are not placed strategically in urban heat islands, they can still help reduce pollution and energy use. If implemented citywide, Los Angeles could save about $100 million annually, with most savings coming from cool roofs, lighter-colored pavement, and tree planting. Widespread use of these strategies could also save at least $1 billion annually by reducing smog levels.

Los Angeles TreePeople is an example of how tree planting can help communities grow stronger. TreePeople offers opportunities for people to work together, develop skills, build pride, and connect with others.

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

In 2021, the Climate Adaptation Planning Analysis (CAPA) program 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. After collecting data, volunteers sent equipment and information back to CAPA, where machine learning was used to analyze the results. CAPA then met with organizers in each area to discuss future plans for each town.

New York City started its "Cool Neighborhoods NYC" program in 2017 to reduce the effects of extreme urban heat. One goal of the program was to increase funding for the city’s Low-Income Home Energy Assistance Program. This program aimed to provide more support for cooling solutions for families with low incomes.

Several cities in India face significant urban heat island effects due to rapid urban growth, loss of green spaces, and heavy use of concrete. A report by The Hindu noted that cities like Delhi, Bengaluru, Chennai, Jaipur, Ahmedabad, Mumbai, and Kolkata have temperature differences of 1 °C to 6 °C compared to nearby rural areas. These heat islands raise local temperatures, worsen heatwaves, increase energy use for cooling, and harm the health of vulnerable people.

Mumbai, India’s financial center and one of the most densely populated cities in the world, is heavily affected by the urban heat island effect. Rapid urbanization, widespread use of concrete, and loss of green spaces have caused higher temperatures in the city than in surrounding areas. A report predicted that Mumbai will spend twice as much as New York City to manage heat caused by concrete use. This increased cost shows how serious the urban heat island effect is in Mumbai and how it affects the city’s buildings and residents.