Urban heat island

The urban heat island (UHI) effect is a natural event where urban areas become much warmer than nearby rural areas. This happens mostly at night when there is no wind, and it is strongest during the summer and winter. The main reason cities get hotter is because of changes made to the land, like buildings and roads. Another reason is the heat created by things like cars and air conditioners.

Cities cover only about half a percent of the Earth's land but are home to more than half of all people. As cities grow bigger, they also get hotter. This extra heat can change the weather nearby, making rainfall heavier downwind from the city. It can also make the growing season for plants longer, lower air quality, and harm water in streams and rivers.

Not every city has a strong urban heat island effect, and how strong it is depends on the surrounding environment. There are ways to make cities cooler, such as adding more tree cover and green space. These provide shade and help cool the air. Other helpful ideas include green roofs, using lighter-colored surfaces, and designing buildings to let in more air.

Climate change makes heat waves more common and stronger, which makes the urban heat island effect even worse in many places. Dense urban development can also add to this problem, leading to even higher temperatures for people living in cities.

Definition

An urban heat island is when cities feel warmer than the areas around them that are not built up. This happens because of how the land is used, the way buildings and streets are designed, and the materials used in buildings that soak up heat. Cities also have less greenery and water, and activities from people add more heat.

Description

Diurnal variability

During the day, especially when the sky is clear, city surfaces heat up quickly because they take in energy from the sun. Materials like concrete can store a lot of this heat—much more than the air around us. This makes city areas warmer than nearby countryside during the day.

At night, things change. Without the sun, the air above cities can become stable, trapping warm air close to the ground. This keeps city nights warmer than the surrounding areas. Usually, cities are a bit warmer during the day and even warmer at night compared to rural places.

Measurements and predictions

Urban Heat Island Index (UHII)

One way to measure how much warmer cities are compared to nearby countryside was created by the Californian EPA in 2015. It looks at temperature differences between an urban area and rural areas upwind from it, measured two meters above the ground. By adding up these temperature differences over time, we get a number in Celsius-hours, which is the UHI Index. This helps guess how much air conditioning might be used and how it could affect greenhouse gas emissions in California. It doesn’t include things like wind speed, humidity, or solar influx that can change how hot it actually feels.

Models and simulations

If a city or town keeps good weather records, we can measure the UHI directly. Otherwise, we can use computer programs to simulate it. These models help us understand how much hotter cities might get in the future because of climate change. The first detailed study of this was done by Leonard O. Myrup in 1969. He found that the main reasons for the UHI are less evaporation in city centers and the materials used in buildings and roads. Today, tools like ENVI-met can simulate how buildings, plants, and air all work together.

Causes

See also: Heatwave § Formation

Urban design

See also: Urban design

Urban areas can become warmer than nearby rural areas because of how they are built. Dark surfaces, like roads and roofs made from materials such as concrete and asphalt, absorb more heat from the sun. This makes cities hotter during the day.

Lack of plants and trees also makes cities hotter, as trees normally help cool areas through shade and water release. Tall buildings can trap heat and block wind, which normally helps cool things down. Heat from cars, air conditioners, and industries adds to this effect.

Poor landscaping

The way gardens and public spaces are designed can affect how hot an area gets. Landscapes with native plants need less water and can survive heat better. Non-native grasses often need more water and can’t handle extreme heat as well. Large areas of pavement, like parking lots, get very hot. Adding trees, shade, and plants that need less water can help keep areas cooler.

Air pollution

Dirty air in cities can make the urban heat island effect worse. Pollution changes how sunlight and heat behave in the air, which can raise temperatures even more.

Climate change as an amplifier

Further information: Climate change and cities and Climate change adaptation § Heat waves

Climate change makes the urban heat island effect stronger. As the world gets warmer, cities feel this heat more. This can lead to more frequent and stronger heat waves, which can be dangerous for people’s health.

Increased urbanization

More people moving to cities means more buildings and roads, which take the place of plants and open spaces. This makes cities hotter. Some areas may have less access to shade, cooling, and other resources, making heat problems worse for certain communities.

Impacts

UHIs can change the weather and climate around cities. They can affect wind patterns, create more clouds and fog, and change how much it rains. Cities can make the air more humid and lead to more showers and thunderstorms. The extra heat from cities can also create areas of low pressure, which can bring in moist air and make clouds form more easily.

UHIs can also affect people’s health. The extra heat can make heat waves worse and can harm people, especially older adults. Heat can cause illnesses like heat stroke and can make air pollution worse. Heat can also affect mental health, school performance, and can make some people more likely to get sick. Cities with poor air ventilation can have stronger UHIs and higher rates of health problems.

UHIs can also hurt water quality and animals. Hot surfaces in cities can heat up rainwater, which can harm fish and other water animals. UHIs can also help some animals thrive in places where they usually wouldn’t survive. For example, some animals in cities can live in areas that are normally too cold for them because of the extra heat.

UHIs can also increase the energy needed for cooling buildings. This can cost cities a lot of money and can also make the UHI effect worse at night because of the heat from air conditioners.

Options for reducing heat island effects

There are several ways to help cities stay cooler during hot weather. Planting trees, using white or light-colored roofs and pavements, and adding green spaces like gardens and parks can all make a big difference. These methods help reflect sunlight and keep the air and ground cooler.

Planting trees in cities

Main articles: Urban forest and Urban forestry

Adding trees to cities can help cool the air. Trees give shade in the summer and let sunlight warm the area in the winter, so they are very useful. Trees can lower the temperature of the air by about 10 °F (5.6 °C) and the ground by even more. They also help fight climate change by taking in carbon dioxide from the air.

Cool roofs and light-coloured concrete

Main article: Reflective surfaces (climate engineering)

Painting roofs white or using special reflective coatings can help keep cities cooler. Dark roofs and roads absorb a lot of heat from the sun, making the area warmer. White roofs reflect the sun’s heat away, which helps lower temperatures. Studies show that white roofs can reduce outdoor temperatures by up to 2 °C during hot weather.

Green infrastructure

Main articles: Green infrastructure and Green roof

Green roofs, which are roofs covered with plants, can cool the air and improve air quality. They also help manage water from rain and snow better. Using special surfaces that let water soak through can also cool the area by letting the water evaporate.

Passive daytime radiative cooling

Main article: Passive daytime radiative cooling

Special roofs that let heat escape into space can save even more energy than white roofs. These roofs work best in very hot, dry places and can lower temperatures around people walking on the street.

Society and culture

History of research

People have noticed that cities are warmer than the areas around them since the 1800s. A man named Luke Howard observed that London was warmer at night than the countryside nearby. Later, scientists studied this more closely. In 1929, a researcher named Albert Peppler used a term in German that means "urban heat island." Over time, many scientists have worked to understand this better.

Aspects of social inequality

Some parts of cities get hotter than others, and this can affect people differently. Often, communities that have less money or that have been treated unfairly in the past feel the heat more. These areas may have fewer trees and more hot surfaces like concrete and asphalt, making them warmer. This can lead to health problems, especially for older people or those with health conditions.

The US experience

In the United States, people from different backgrounds may experience the urban heat island effect differently. Studies show that some communities face more heat because of where they live. These areas also often have less access to things that cool the air, like trees or air conditioning. This can be especially hard for people with health issues.

Redlining, Urban Heat Exposure, and Health Inequity in U.S. cities

In the past, some neighborhoods were treated unfairly by rules that made it hard for people to get loans or improve their areas. These neighborhoods, often where minorities and lower-income families lived, are now often hotter because they have fewer trees and more heat-absorbing surfaces. This makes health problems worse for people living there.

Chief heat officers

Recently, some cities have started hiring special leaders called Chief Heat Officers. These leaders work to cool down cities by adding shade, creating places to stay cool, planting trees, and organizing efforts to fight the heat. Cities like Los Angeles and Miami-Dade County have started this work.

Examples

United States

In 2020, a bill was introduced in the U.S. Senate to help fight extreme heat. If passed, it would fund projects to cool cities, like using special roofs and pavements and improving heating and cooling systems.

New York City found that planting trees, using light-colored surfaces, and adding green spaces can help cool areas effectively and at lower costs. Los Angeles studied ways to reduce heat by planting many trees, changing roofs, and painting roads. These efforts could save money on energy and improve health.

India

Many cities in India, such as Delhi and Mumbai, feel much hotter than their rural areas because of rapid building and loss of green spaces. This heat can make health problems worse and uses more energy for cooling. Mumbai, in particular, spends a lot of money to deal with this extra heat.