The urban heat island (UHI) effect is a weather and climate-related occurrence where city areas 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 in summer and winter. The main reason for this effect is changes to land surfaces, while heat from energy use plays a smaller role. 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 on average. The term "heat island" can describe any area that is hotter than its surroundings, but it is most commonly used for areas affected by human activity.

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

Not all cities have a clear urban heat island, and the effect depends on the local climate where the city is located. A city's heat island impact can change based on its surroundings. Heat can be reduced by trees and green spaces, which provide shade and help cool the air through evaporation. Other methods include green roofs, reflective materials on buildings, ventilation corridors, and lighter-colored surfaces that absorb less heat.

Climate change does not cause urban heat islands, but it is making heat waves more frequent and severe, which increases the UHI effect in cities. Compact and dense city designs may also worsen the urban heat island 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 use land, the way streets and buildings are arranged, and the materials used in construction that absorb heat. It also happens because there is less air movement, fewer trees and plants, and fewer water areas. Additionally, heat from homes and factories contributes to the warmth.

Description

During the day, especially when the sky is clear, surfaces in cities absorb heat from the sun. These surfaces, such as roads and buildings, warm up faster than similar surfaces in rural areas. Because of their high heat capacity, urban surfaces store heat energy. For example, concrete can hold about 2,000 times more heat than the same amount of air. Studies show 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 high surface temperatures in urban areas, which can be seen using thermal remote sensing. During the day, this heating also creates convective winds in the air near the ground. At night, the situation changes. Without sunlight, the air cools, and the layer of air near the ground becomes more stable. If the air becomes stable enough, an inversion layer forms, trapping warm air near the surface. This keeps nighttime air temperatures in urban areas warmer than in rural areas.

In general, the temperature difference between urban and 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 nearby rural areas during the day and about 2–5 °F (1.1–2.8 °C) warmer at night. However, in dry climates like those in southeastern China and Taiwan, the temperature difference is larger during the day. Research shows that daily temperature changes depend on factors such as local climate, weather, seasons, humidity, vegetation, and the materials used in urban areas.

Seasonal temperature differences in urban heat islands 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 areas.

Measurements and predictions

One way to measure the UHI effect in cities is the UHI Index, developed by the California EPA in 2015. This method compares the temperature in the area being studied with temperatures in nearby rural areas that are not affected by the city. Measurements are taken at a height of two meters above ground level. The temperature difference in degrees Celsius is recorded every hour. These differences are added together over time to create a value called degree-Celsius-hours, which represents the UHI Index for the area. This value can be averaged over several days and is expressed as Celsius-hours per averaged day.

The index was created to help estimate how much air conditioning would be used and the greenhouse gas emissions that result in California. It does not consider factors like wind speed, humidity, or sunlight, which can affect how hot it feels or how air conditioners work.

If a city has a reliable system for weather observations, the UHI can be measured directly. Another option is to use a complex computer model to calculate the UHI, or to use a simplified method based on past data. These models help include the UHI effect in predictions about future temperature increases in cities due to climate change.

Leonard O. Myrup published the first detailed numerical study to predict the effects of the urban heat island (UHI) in 1969. The heat island effect results from several different physical processes working together. In general, less evaporation in city centers and the thermal properties of buildings and pavement materials are the main factors. Modern simulation tools like ENVI-met model how buildings, ground surfaces, plants, and the air around them interact.

Causes

Urban heat islands (UHIs) occur due to several common urban design features. For example, dark surfaces absorb more sunlight, causing areas with roads and buildings to become hotter than suburban and rural areas during the day. Materials like concrete and asphalt, often used for pavements and roofs in cities, have different thermal properties (such as how they store and transfer heat) and surface properties (like how much sunlight they reflect or emit) compared to rural areas. These differences change how energy is used in urban areas, often leading to higher temperatures than surrounding rural areas.

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

Another major reason is the lack of evapotranspiration, which is the process by which plants release water into the air. This often happens because urban areas have fewer trees and plants. In 2018, the U.S. Forest Service found that cities in the United States lose 36 million trees each year. With fewer trees, cities lose the cooling effects of shade and water evaporation from plants.

Other causes of UHIs are related to the shape of urban areas. Tall buildings in cities create many surfaces that reflect and absorb sunlight, increasing how quickly urban areas heat up. This is called the "urban canyon effect." Buildings also block wind, which reduces cooling through air movement and prevents pollutants from spreading. Heat from cars, air conditioning, industry, and other sources also adds to the UHI.

The proximity of urban areas to different types of land cover affects UHIs. For example, being near barren land makes urban areas hotter, while being near vegetation makes them cooler.

The design of personal landscapes, such as yards, and public spaces, like city parks, can greatly influence UHIs. Landscapes with native plants that survive on natural rainfall are more resilient to heat and can provide shade and moisture to surrounding areas. Lawns made of non-native grasses may not survive changes in climate, temperature, or water availability. Large parking lots can become much hotter than surrounding areas. Including tree cover, shade structures, and resilient plants in development plans can help prevent dangerous heat or UHIs.

High levels of air pollution in cities can worsen UHIs because pollution changes how sunlight and heat interact in the atmosphere. UHIs not only raise urban temperatures but also increase ozone levels, as ozone is a greenhouse gas that forms faster when temperatures rise.

Climate change is not a direct cause of UHIs but makes them worse. According to the IPCC Sixth Assessment Report from 2022, "Climate change increases heat stress risks in cities […] and amplifies the urban heat island across Asian cities at 1.5 °C and 2 °C warming levels, both substantially larger than under present climates […]."

The report also states: "In a warming world, increasing air temperature makes the urban heat island effect in cities worse. One key risk is heatwaves in cities that are likely to affect half of the future global urban population, with negative impacts on human health and economic productivity."

Interactions between heat and built infrastructure increase the risk of heat stress for city residents. Increased urbanization is a major cause of rising urban heat risks because it replaces natural areas with surfaces that do not absorb water, such as concrete and asphalt, and concentrates people into smaller spaces. Factors like the amount of greenery, drought conditions, and reliance on cars influence whether urban or suburban areas experience UHIs. Cities are more likely to have UHIs but can be designed with tall buildings surrounded by large parks to balance with the climate.

Historical urbanization processes, such as redlining in the United States, have long-term effects on land use and may create unequal access to vegetation in cities. Disparities in access to healthcare, public transportation, housing, and energy often leave some communities more vulnerable to heat. These differences can lead to localized areas with higher heat risks, called micro urban heat islands. Socially vulnerable groups often live in densely populated areas with little vegetation, which increases their risk of heat exposure and limits their ability to adapt.

Housing conditions also affect urban heat inequity. Living on the top floor, having a dark roof, or poor insulation can make homes hotter during heatwaves. Lower-income individuals may lack air conditioning or cannot afford higher electricity costs. In densely packed cities, opening windows for airflow can bring in pollution and odors. In Asia, high-density housing often has limited opportunities for building updates that improve cooling.

Space poverty, which refers to very small living spaces, is a problem in places like Hong Kong, where low-income individuals live in overcrowded, poorly ventilated units. These units make it hard to move around and often lack proper cooling. Residents in such conditions may rely on public spaces like libraries, parks, and cafés for cooling and space.

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 low-pressure areas where moist air from nearby rural areas flows in, helping clouds form. Rainfall downwind of cities can increase by 48% to 116%. Studies show that monthly rainfall is about 28% higher between 20 and 40 miles (32 to 64 km) downwind of cities compared to areas upwind. Some cities experience a total increase in precipitation of 51%.

Research suggests that UHIs can affect climate in areas two to four times larger than the city itself. One study from 1999 found that UHIs may not greatly influence global temperature trends. However, other studies suggest that UHIs might impact global climate by affecting wind patterns called jet streams.

UHIs can directly harm the health of people living in cities. Because UHIs raise temperatures, they can make heat waves last longer and feel stronger. More people are exposed to extreme heat due to this warming. At night, UHIs reduce the cooling effect that rural areas provide, making heat waves more dangerous. Extreme urban heat increases the risk of heat-related illnesses and deaths, especially for older people.

High temperatures can cause heat-related illnesses, such as heat stroke, heat exhaustion, heat syncope, and heat cramps. In the United States, extreme heat is the deadliest weather event. Studies show that heat waves kill more people than hurricanes, floods, or 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 urban areas.

Extreme heat can also affect mental health. Higher temperatures may lead to more aggression, domestic violence, and substance abuse. Heat can hurt school performance, as studies show that more days of extreme heat each year are linked to lower student test scores.

UHIs can increase air pollution. Pollutants like volatile organic compounds, carbon monoxide, nitrogen oxides, and particulate matter gather at night in cities. These pollutants, along with higher temperatures, can speed up the formation of harmful ozone. Studies note that UHIs may increase polluted days, but other factors like air pressure, cloud cover, and wind speed also affect pollution.

In Hong Kong, areas with poor air ventilation had stronger UHIs and higher death rates compared to areas with better ventilation. A study in Babol, Iran, found that the intensity of surface UHIs increased from 1985 to 2017, influenced by geography and time. This research highlights the need for careful urban 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 someone is to be harmed by heat, often due to age, health, or income. Groups like the elderly, children, low-income families, and people with chronic illnesses are more vulnerable. Elderly people struggle to regulate body temperature and often have health conditions that make them more at risk. Children are also sensitive to heat, which can stress their developing bodies. High indoor heat can harm mental health and strain 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 reduces biodiversity. For example, in 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 surfaces, not the cool rain, likely caused the fish deaths. Similar events have been reported in other U.S. regions.

The temperature of nearby buildings can be 50°F (28°C) higher than the air temperature, warming precipitation and runoff into water bodies. This can raise water temperatures by 20 to 30°F (11 to 17°C), stressing 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 the common house gecko, thrive in urban areas because UHIs create conditions similar to their natural habitats.

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

UHIs have altered natural selection. In cities, selective pressures like food availability and predation are less intense, leading to new survival challenges. For example, insects are more common in cities because they rely on environmental temperatures to regulate their body heat. A study in Raleigh, North Carolina, found that urban areas 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), and using light-colored concrete. Other methods include green infrastructure, such as green roofs, and 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-colored roofs, while the rest comes from dark-colored pavement and the loss of vegetation. Using white or reflective materials for buildings, roofs, pavements, and roads can help reduce the urban heat island effect by increasing the city’s overall reflectiveness.

Expanding cities in circular patterns is not helpful for reducing heat. Instead, cities should be planned in long strips that follow natural water systems and include green spaces with different types of plants. For example, cities like Kielce, Szczecin, Gdynia in Poland, Copenhagen in Denmark, and Hamburg, Berlin, and Kiel in Germany have been designed this way.

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 street-tree coverage cools air temperatures near the ground. A 2024 study of 110 cities found that tree cover typically lowers daytime temperatures by 1–2 °C. Another 2024 study found that areas without tree cover within 10 meters were up to five times more likely to reach 32.2 °C. Tree diversity and coverage are linked to greater temperature reductions in some seasons.

Urban green infrastructure (UGI) includes networks of green spaces in both public and private areas, such as parks, street trees, private gardens, and rooftop gardens. Proper use of UGI helps distribute heat more evenly, while poor use worsens heat inequality. UGI reduces temperatures through shading and evapotranspiration, which is the process of water released by plants. A lack of UGI in marginalized communities limits the ability to regulate temperatures, increasing heat inequality. UGI is a widely accepted solution to address heat inequality.

Painting rooftops white is a common strategy to reduce the urban heat island effect. Dark surfaces absorb more heat, lowering a city’s reflectiveness. White rooftops reflect more sunlight and emit more heat, increasing reflectiveness. Green and cool roofs help manage extreme heat. In warm areas, cool roofs can reduce cooling energy use for some buildings. Green roofs may be better in colder areas because they provide insulation and reduce heating needs.

Applying reflective coatings to rooftops is an effective way to reduce heat gain. A study by Oscar Brousse from University College London found that white or reflective rooftops in London reduced outdoor temperatures by 1.2 °C on average, up to 2 °C in some areas. This was more effective than solar panels, green roofs, or tree cover. 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. Reflective materials like vinyl can reflect at least 75% of sunlight and emit 70% of absorbed heat. In comparison, asphalt roofs reflect only 6% to 26% of sunlight.

Using light-colored concrete instead of asphalt can reflect up to 50% more light and lower temperatures. Black asphalt has a low reflectiveness, absorbing much of the sun’s heat and raising nearby temperatures. Replacing asphalt with light-colored concrete can lower average temperatures. However, if nearby buildings are not reflective, light-colored pavements may increase building temperatures, raising air conditioning needs.

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

Green roofs act as insulation during hot weather and cool the environment. 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, as factors like design, soil depth, location, and labor costs vary. Each green roof is unique, so comparing them based only on cost misses social, environmental, and health benefits. Global comparisons are difficult due to a lack of shared standards.

Stormwater management helps reduce the urban heat island effect. It involves controlling rainwater to protect property and infrastructure. Urban surfaces like streets and parking lots prevent water from soaking into the ground, causing flooding. Techniques like pervious pavement systems (PPS) allow water to pass through, reducing heat through evaporation. PPS has been used in over 30 countries and is effective for managing stormwater and heat.

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

Green infrastructure or blue-green infrastructure refers to a network of natural and built features that help solve urban challenges.

Society and culture

Luke Howard studied and described this effect in the 1810s, but he did not give it a name. A report from that time said the city center of London was 2.1 degrees Celsius (3.7 degrees Fahrenheit) warmer at night than the surrounding countryside.

Scientists continued to study how heat behaves in cities throughout the 1800s. Between the 1920s and 1940s, researchers in many countries, including Europe, Mexico, India, Japan, and the United States, worked to better understand this effect.

In 1929, Albert Peppler used the term "städtische Wärmeinsel" in a German publication, which means "urban heat island." Between 1990 and 2000, about 30 studies were published each year on this topic. By 2010, that number had grown to 100, and by 2015, it reached more than 300.

In 1969, Leonard O. Myrup created the first detailed model to predict how urban heat islands affect cities. His work reviewed existing ideas and pointed out that some earlier theories were too vague.

Urban heat inequity, also called thermal inequity, refers to how heat is unevenly spread across cities or within neighborhoods. This can lead to unfair effects on people living in certain areas. Heat stress often affects communities differently based on factors like race, income, education, and age. These effects are usually worse for groups that have been historically treated unfairly. This issue is closely connected to the urban heat island effect, as more urban development can worsen heat inequity.

Some studies suggest that the health effects of urban heat islands may not be the same for everyone. Factors like age, race, and income can influence how much heat affects people. This has led to concerns that urban heat islands may be an environmental justice issue. Research shows that communities of color in the United States are often more affected by urban heat islands.

There is a link between neighborhood income and the number of trees. Poorer neighborhoods usually 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 neighborhoods can afford more trees on both public and private land. One reason is that wealthier people often have more space for green areas, while poorer areas may have more rental housing, where landowners try to build as many homes as possible.

Globally, the effects of urban heat islands vary by region. While heat exposure is rising worldwide, it has increased faster in the Global South in recent years, according to a study by Professor Kanging Huang and others.

The uneven impact of urban heat islands on the Global South worsens existing environmental injustices. Many countries near the equator are naturally hot and humid, making them more vulnerable to heat. A World Bank study found that the hottest and coolest neighborhoods in Bandung, Indonesia, differ by 7.0 degrees Celsius.

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 surface urban heat island intensity than white people in the same cities. A study by climatologist Angel Hsu and others found that, in most of the 175 largest U.S. cities, people of color live in areas with higher heat intensity than non-Hispanic whites. A 2023 report also found that historically redlined and low-income Black neighborhoods, already affected by heat, also face higher levels of violent crime, increasing risks in these areas.

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 cooling tools like air conditioning. Similar patterns exist when comparing households below the poverty line to those with higher incomes.

Urban heat islands can have strong effects on African Americans with chronic health conditions. African Americans have higher rates of diseases like asthma and diabetes than the general population. These conditions can worsen in extreme heat, leading to health problems such as high blood pressure or stroke.

Researchers have also found that neighborhoods with lower economic status often have more impervious surfaces, like concrete, tar, and asphalt. These materials are linked to differences in temperature within cities.

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

A study of 108 U.S. cities found that neighborhoods once labeled as "D" grade (the lowest rating) are, on average, 2.6 degrees Celsius (4.7 degrees Fahrenheit) hotter than non-redlined areas. This was mainly due to fewer trees and more heat-absorbing surfaces like asphalt. In cities like Durham, Fresno, and Pittsburgh, "D" graded areas have much less tree cover than wealthier "A" graded neighborhoods, which are mostly white. In Phoenix, Arizona, formerly redlined areas have surface temperatures up to 10–15 degrees Fahrenheit (5.6–8.3 degrees Celsius) higher than other neighborhoods.

These neighborhoods also face higher air pollution because of their proximity to highways and industrial areas. In Los Angeles, freeways built in the 1950s were placed 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

Examples

Bill S.4280, introduced to the U.S. Senate in 2020, would allow the National Integrated Heat Health Information System Interagency Committee (NIHHIS) to address extreme heat in the United States. If the bill is passed, it would fund NIHHIS for five years and 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 has not advanced beyond the introduction stage in Congress.

The city of New York found that street trees had the highest cooling effect per area, followed by living roofs, light-colored surfaces, and open space planting. In terms of cost effectiveness, light-colored surfaces, light-colored roofs, and curbside planting provided the greatest temperature reduction for the lowest cost.

A hypothetical "cool communities" program in Los Angeles, predicted in 1997, estimated 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) at an estimated cost of $1 billion. This program projected annual benefits of $170 million from lower air-conditioning costs and $360 million in savings from reduced smog-related health issues.

A 1998 study of the Los Angeles Basin showed that even without strategic placement, trees can still help reduce pollutants and energy use. It is estimated that large-scale use of cool roofs, lighter-colored pavement, and tree planting could save Los Angeles $100 million annually, with additional savings of at least $1 billion per year from reduced smog levels.

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

Los Angeles has also started 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) 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 it was analyzed using machine learning algorithms. CAPA then met with campaign organizers in each area to discuss future plans for each town.

New York City launched 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, specifically to support cooling solutions for lower-income families.

Several cities in India face major urban heat island effects due to rapid urbanization, loss of green spaces, and widespread use of concrete. A report by The Hindu states that metropolitan areas like Delhi, Bengaluru, Chennai, Jaipur, Ahmedabad, Mumbai, and Kolkata have temperature differences ranging from 1 °C to 6 °C compared to their rural surroundings. These urban heat islands increase local temperatures, make heatwaves worse, raise energy use for cooling, and pose health risks to vulnerable groups.

Mumbai, India’s financial hub and one of the most densely populated cities globally, is heavily affected by the urban heat island effect. Rapid urbanization, widespread use of concrete, and loss of green spaces have caused temperatures in the city to rise compared to surrounding areas. A report estimates that Mumbai is expected to spend twice as much as New York City to manage heat caused by concrete use. This increased cost highlights the severity of the urban heat island effect in Mumbai and its impact on the city’s infrastructure and residents.