The urban heat island (UHI) effect is a weather and climate-related phenomenon where cities have much higher temperatures than nearby rural areas. The temperature difference is often greater at night than during the day and is most noticeable when winds are weak, the sky is clear, and during summer and winter. The main cause of the UHI effect is changes to the land's surface, 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 of the world’s population. As cities grow, they usually expand and become warmer. The term "heat island" can describe any area that is warmer than its surroundings but is most often used for areas affected by human activity.

Monthly rainfall is higher in areas 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 its characteristics depend on the climate of the region where the city is located. The impact of the UHI effect can change based on a city’s local environment. Heat can be reduced by trees and green spaces, which provide shade and cooling through evaporation. Other methods include green roofs, materials that reflect sunlight during the day, ventilation corridors, and using lighter-colored surfaces 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 development 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 rural areas. This warmth happens because of several factors: heat is trapped by how land is used, the way buildings and streets are arranged, materials that absorb heat, less air movement, fewer trees and water features, and heat produced by activities in homes, factories, and other human-made structures.

Description

During the day, especially on clear days, city surfaces become warm because they absorb sunlight. City surfaces often heat up faster than those in nearby rural areas. Because they have high heat capacities, city surfaces store heat energy. For example, concrete can hold about 2,000 times more heat than the same amount of air. Studies show that city surfaces, like concrete, absorb and store large amounts of heat during the day, which supports the idea of the high heat capacity effect in urban heat islands. This causes high surface temperatures in urban heat islands (UHIs) to be visible through thermal remote sensing. During the day, this heating also creates convective winds in the urban boundary layer. At night, the situation changes. Without sunlight, the air cools, and the urban boundary layer becomes more stable. If the air becomes very stable, an inversion layer forms. This layer traps warm air near the ground, keeping nighttime air temperatures in UHIs warmer than surrounding 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 rural areas during the day and 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 more noticeable during the day. Studies show that daily temperature changes depend on factors such as local climate, weather, seasons, humidity, vegetation, 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 climate zones influence how temperatures change over seasons 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 city area with that of rural areas located upwind, at a height of two meters above the ground. The temperature difference in degrees Celsius is recorded every hour. These differences are added together over time to create a number called "degree-Celsius-hours," which is the UHI Index for the area. This number can be averaged over many days and is expressed as "Celsius-hours per averaged day."

The index was created to estimate how much air conditioning would be used in California and the related greenhouse gas emissions. However, 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 good system for collecting weather data, the UHI can be measured directly. Another option is to use a detailed computer model of the area to calculate the UHI. A third method is to use an approximate approach based on past observations. These methods help include the UHI effect in predictions about how city temperatures might rise in the future due to climate change.

In 1969, Leonard O. Myrup wrote the first detailed study to predict the effects of the urban heat island. He found that the UHI is the result of several physical processes working together. Usually, less evaporation in city centers and the heat-absorbing properties of buildings and pavement are the most important factors. Today, tools like ENVI-met are used in simulations to model how buildings, ground surfaces, plants, and the air interact.

Causes

Urban heat islands (UHI) occur because of how cities are designed. Dark surfaces, like roads and buildings, absorb more sunlight than lighter surfaces. This makes urban areas much 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 change how heat is stored and released, often raising city temperatures compared to surrounding areas.

Pavements, parking lots, and other transportation infrastructure contribute to UHI. For example, pavement in Phoenix, United States, is a major cause of heat during summer afternoons. Another reason is the lack of evapotranspiration, which happens when plants release water vapor. The U.S. Forest Service reported in 2018 that cities in the United States lose 36 million trees each year. Fewer trees mean less shade and less cooling from evaporation.

Geometric effects also play a role. Tall buildings in cities reflect and absorb sunlight more efficiently, creating the "urban canyon effect." These buildings can also block wind, slowing cooling and trapping pollution. Heat from cars, air conditioning, and industry adds to the problem.

Proximity to different types of land affects UHI. Areas near barren land become hotter, while areas near vegetation stay cooler. Landscapes, such as yards and parks, can reduce UHI. Native plants that use natural rainfall are more resilient to heat than non-native grasses. Large parking lots heat up quickly and retain heat, but including trees, shade, and resilient plants can help prevent extreme heat.

Air pollution increases UHI by changing how the atmosphere handles heat. Higher temperatures also increase ozone levels, which trap more heat. Climate change worsens UHI, not by causing it, but by making it stronger. A 2022 report from the IPCC said that cities face greater heat risks as temperatures rise. Heatwaves could affect half of the global urban population, harming health and productivity.

Urbanization increases heat risks by replacing natural areas with hard surfaces like concrete. Suburban areas usually have more greenery but can become hot during droughts if lawns lack water. Sprawling suburbs with many cars and parking lots also contribute to heat. Cities can reduce UHI by designing tall buildings with nearby parks.

Historic practices, like redlining in the United States, have led to unequal distribution of vegetation in cities. This creates micro heat islands in certain neighborhoods. People in these areas often lack access to healthcare, public transport, and cooling options. Lower-income individuals may live in homes with dark roofs, poor insulation, or no air conditioning. In densely packed cities, opening windows can bring in pollution. In Asia, high-density housing often has limited space for cooling improvements.

Space poverty, or very small living spaces, is a problem in places like Hong Kong. Poorly ventilated, overcrowded homes make it hard to stay cool. Residents often rely on public places like parks or libraries for relief from heat.

Impacts

Urban heat islands (UHIs) can change local weather patterns beyond temperature. They affect wind patterns, cloud and fog formation, humidity, and rainfall amounts. 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 where moist air from nearby rural areas flows in, increasing the chance of clouds forming. Rainfall downwind of cities increases by 48% to 116%. In some areas, monthly rainfall is 28% higher between 20 and 40 miles (32 to 64 km) downwind of cities compared to upwind. Some cities experience 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 suggested that urban heat islands have little effect on global temperature trends. However, other research indicates that UHIs might influence global climate by affecting the jet stream.

Urban heat islands can harm the health and well-being of city residents. Because UHIs raise temperatures, they can make heat waves last longer and feel more intense. More people are exposed to extreme heat due to UHIs. At night, the lack of cooling from UHIs can be dangerous during heat waves, as urban areas stay warmer than rural ones. Extreme heat increases the risk of heat-related illnesses and deaths, especially among older people.

Higher temperatures can cause heat-related illnesses, such as heat stroke, heat exhaustion, heat syncope, and heat cramps. Extreme heat is the deadliest weather type 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 are also more likely when combined with air pollution, which is common in urban areas.

Increased temperatures can harm mental health. Higher temperatures may lead to more aggression, domestic violence, and substance abuse. They can also hurt school performance. A study by Hyunkuk Cho of Yeungnam University found that more days with 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. These pollutants include volatile organic compounds, carbon monoxide, nitrogen oxides, and particulate matter. The combination of these pollutants and higher temperatures can speed up the formation of ozone, a harmful pollutant. Studies show that UHIs can increase polluted days, but other factors like air pressure, cloud cover, and wind speed also affect pollution.

Research 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. 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 urban 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. It depends on factors like age, location, and daily routines. Groups at higher risk include the elderly, children, women, low-income households, and people 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 harm mental and physical health and strain relationships.

Urban heat islands also harm water quality. Hot surfaces like 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 August 2001, heavy rain in Cedar Rapids, Iowa, caused a 10.5°C (18.9°F) rise in stream temperature within an hour, killing about 188 fish. The heat from city surfaces, not the cooler rain, likely caused the fish deaths. Similar events have been reported in other U.S. regions. Sudden temperature changes stress aquatic ecosystems.

When nearby buildings are much hotter than the air, precipitation warms quickly, and runoff into water bodies can raise temperatures by 20 to 30°F (11 to 17°C). This rapid change stresses fish and other aquatic life. Permeable pavements may help by allowing water to soak into the ground, where it can be absorbed or evaporated, reducing heat.

Urban heat islands 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 due to warmer temperatures. Grey-headed flying foxes in Melbourne, Australia, adapted to city life after winter temperatures became warmer, resembling their natural habitat.

In temperate climates, UHIs extend the growing season, changing how species breed. This is evident in changes to water temperatures. UHIs also alter natural selection. In cities, selective pressures like food availability, predation, and water access are less intense, creating new challenges. For example, insects are more common in urban areas because they rely on environmental temperatures to regulate their body heat, and cities are warmer. A study in Raleigh, North Carolina, found that urban habitats support more insects than rural ones.

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 difference 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 reflectivity.

Expanding cities in circular patterns is not ideal for reducing 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. For example, cities like Kielce, Szczecin, Gdynia in Poland, Copenhagen in Denmark, and Hamburg, Berlin, and Kiel in Germany have used this approach to build large urban areas.

Planting trees around cities can increase reflectivity 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.

Studies show that increasing street tree cover can lower temperatures at pedestrian level, with results depending on climate, tree density, and city layout. A 2024 study of 110 cities found that tree cover typically cools 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 cooling effects in certain seasons.

Urban green infrastructure (UGI) includes networks of green spaces in both public and private areas, such as parks, street trees, private gardens, rooftop gardens, and other green areas. Proper use of UGI helps distribute heat more evenly, while poor use can worsen heat inequity. UGI provides shade and reduces temperatures through evapotranspiration. Lack of UGI in marginalized communities limits the land’s ability to regulate temperature, increasing heat inequity. UGI is widely seen as a practical, 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 reflectivity. White rooftops reflect more sunlight and emit more heat, increasing the city’s reflectivity. Green and cool roofs help cities manage extreme heat. Cool roofs are especially effective in warm areas, sometimes reducing cooling energy use completely 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 been shown to reduce solar heat gain. A study by Oscar Brousse from University College London found that white or reflective rooftops in London reduced outdoor temperatures the most during a 2018 heatwave, outperforming solar panels, green roofs, and tree cover. Cool roofs reduced average outdoor temperatures by 1.2°C, up to 2°C in some areas. Additional 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 like vinyl reflect at least 75% of sunlight and emit at least 70% of absorbed heat. In contrast, dark asphalt roofs reflect only 6% to 26% of sunlight.

Using light-colored concrete can reflect up to 50% more light than asphalt, lowering ambient temperatures. Dark asphalt has a low reflectivity, absorbing much heat and raising surface temperatures. Replacing asphalt with light-colored concrete can lower average temperatures. However, if nearby buildings are not fitted with reflective glass, sunlight reflected from light-colored pavement may increase building temperatures and air conditioning use.

Some paints are designed for daytime radiative cooling, reflecting up to 98.1% of sunlight.

Green roofs act as insulation during warm weather and cool the surrounding 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, making broad comparisons difficult. Focusing only on cost may ignore the social, environmental, and health benefits green roofs provide. Comparing green roofs globally is also difficult due to a lack of shared standards.

Stormwater management is another way to reduce the urban heat island effect. It involves controlling stormwater to protect property and infrastructure. Urban surfaces like streets and parking lots prevent water from soaking into the ground, causing flooding. Stormwater management techniques, such as pervious pavement systems (PPS), allow water to flow through pavement, reducing temperatures through evaporation. PPS has been used in over 30 countries and is effective for managing stormwater and reducing 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 networks that help solve urban problems by combining green spaces and water systems.

Society and culture

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

Researchers continued to study the urban atmosphere throughout the 1800s. Between the 1920s and 1940s, scientists in Europe, Mexico, India, Japan, and the United States worked to better understand the phenomenon using new methods.

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

In 1969, Leonard O. Myrup created the first detailed numerical model to predict the effects of the urban heat island (UHI). His work reviewed UHI and pointed out that earlier theories were too vague.

Urban heat inequity, also called thermal inequity, refers to the unequal spread of heat in cities or neighborhoods. This causes some communities to face greater risks from heat than others. Differences in heat stress are often linked to factors like race, income, education, and age. While the effects of urban heat inequity vary by city, it often harms communities that have been historically treated unfairly. 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 age, race, and income. This raises concerns that UHI-related health issues could be an environmental justice problem. Research has shown that communities of color in the United States are more likely to be affected by UHIs.

There is a connection between neighborhood income and the number of trees. Lower-income neighborhoods usually have fewer trees than wealthier areas. Researchers think that poorer communities may lack the money to plant and care for trees. Wealthier neighborhoods can afford more trees on both public and private land. A reason for this is that wealthier people often have more land, which they can use for green spaces, while poorer areas often have more rental housing, where landowners focus on maximizing housing density.

Globally, the effects of UHIs differ by region. While heat exposure is rising worldwide, its impact has increased more quickly in the Global South in recent decades, according to a study by Professor Kanging Huang and others.

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 that in Bandung, Indonesia, the hottest neighborhoods are 7.0° warmer than the coolest ones.

In the United States, there is a link between race 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, in all but six of the 175 largest U.S. cities, the average person of color lives in a neighborhood with higher SUHI intensity than non-Hispanic white people. A 2023 policy brief also noted that historically redlined and low-income Black neighborhoods, already affected by urban heat, also face higher rates of violent crime, increasing risks in these areas.

Economic status affects how people are impacted by UHIs. Lower-income individuals are more likely to live in areas with high UHI effects and less likely to afford air conditioning. Similar patterns are seen when comparing households below the poverty line to those with incomes more than double the poverty line.

UHIs can strongly affect African Americans with chronic illnesses. African Americans have higher rates of conditions like asthma and diabetes than the general population. These conditions can worsen during extreme heat, leading to health problems such as high blood pressure or stroke, according to Professor Pamela Jackson and colleagues.

Researchers have also found that the spread of impervious surfaces, such as concrete, tar, and asphalt, is linked to neighborhoods with lower incomes in the United States. These materials help predict differences in temperature 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 based on perceived investment risk. Areas with more minorities and low-income residents were often marked as risky and colored red on maps. This limited access to housing loans and caused long-term neglect in 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 mainly due to fewer trees and more heat-absorbing surfaces like asphalt. In cities like Durham, Fresno, and Pittsburgh, neighborhoods labeled "D" by HOLC 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 °F (5.6–8.3 °C) higher than other neighborhoods.

These neighborhoods also face higher air pollution levels because they are often 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 pollution from car emissions, such as diesel exhaust and fine particles (PM2.5), which increase risks of health issues like asthma, heart disease, and birth defects. Internationally, research has shown that urban air pollution, like vehicle emissions, can worsen the urban heat island effect. In China, a haze effect that traps heat contributes up to 0.7 °C of extra nighttime warming. This highlights the serious effects of urban heat islands, especially in communities affected by redlining and broader environmental factors.

In response to these issues, some cities have started 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 address extreme heat in the United States. If the bill passed, NIHHIS would receive funding for five years and would start 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 had not advanced beyond its introduction to Congress.

The city of New York found that street trees provide the most heat reduction per area, followed by living roofs, light-colored surfaces, and open space planting. From a cost-effective perspective, light-colored surfaces, light-colored roofs, and curbside planting are the least expensive ways to lower temperatures.

A hypothetical "cool communities" program in Los Angeles, planned in 1997, estimated that planting ten million trees, reroofing five million homes, and painting one-quarter of roads could reduce urban temperatures by about 3 °C (5 °F) at a cost of $1 billion. This program projected annual savings of $170 million from lower air-conditioning costs and $360 million from reduced health costs linked to smog.

In a 1998 study of the Los Angeles Basin, computer models showed that even without strategic placement, trees can help reduce pollution and energy use in urban heat islands. It was estimated that large-scale implementation of cool roofs, light-colored pavement, and tree planting could save Los Angeles $100 million annually. Additional savings from reduced smog levels could reach at least $1 billion per year.

Los Angeles TreePeople is an example of how tree planting can strengthen a community. TreePeople helps people work together, develop skills, build pride, and connect with others.

Los Angeles has also 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 levels 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 results to CAPA, where computer programs analyzed the information. CAPA then met with local organizers 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 to provide more cooling support for families with lower incomes.

Several cities in India face significant urban heat island effects due to fast city growth, loss of green spaces, and widespread use of concrete. A report by The Hindu states 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 cooling costs, and harm vulnerable people’s health.

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