Urban Heat Islands

What are urban heat islands?

The term “urban heat island” refers to the fact that cities tend to get much warmer than their surrounding rural landscapes, particularly during the summer. This temperature difference occurs when cities’ unshaded roads and buildings heat up during the day and radiate that heat into the surrounding air. As a result, highly developed urban areas can experience mid-afternoon temperatures that are 15°F to 20°F warmer than surrounding, vegetated areas.

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What causes Urban Heat Islands?

Cities have their own microclimates because they greatly alter the local landscape. The following are some of the largest factors that can cause cities to be hotter than their surroundings.

Low albedo, heat-storing materials

Dark surfaces (low albedo) absorb more energy from sunlight than lighter, more reflective surfaces. Cities are often full of dark surfaces like asphalt roads and concrete sidewalks, which is why your hand feels hot if you put it on a dark surface that has been in the sun. The amount of heat-absorbing materials in a city causes a temperature difference.

Lack of Trees and Other Vegetation

Trees and other vegetation, as well as soil, absorb and release moisture through a process called evapotranspiration. Evapotranspiration absorbs energy from the air to turn liquid water into vapor. This process cools the air because heat energy is used for evapotranspiration, similar to how sweating can help cool down the human body. Trees can also provide shade, leading to a cooling effect.

Urban Canyons & Urban Geometry

Tall buildings can create an urban canyon effect that blocks wind flow and prevents ventilation. Tall buildings also have solid walls that trap energy released to the atmosphere and away from streets. This causes air temperature to rise and makes the local environment hotter

Waste Heat

Densely populated urban areas have a lot of heat-emitting devices like cars and air conditioners. Heat-emitting devices from buildings also contribute heat released to the atmosphere, leading to higher temperatures in cities.

These images from the NIHHIS Urban Heat Island Mapping Campaign in Las Cruces, NM, show how the temperature can differ greatly (by 44.5 °F) between shaded grass and exposed pavement. Credit: David DuBois.

Why should we care about urban heat?

Over 80% of Americans live in urban areas according to the U.S. Census Bureau. Due to the urban heat island effect, those urban areas are likely hotter than rural areas, exposing people to higher temperatures. The Centers for Disease Control and Prevention (CDC) estimates that the annual rate of heat-related deaths per 100,000 population is 0.3 in large central metro areas—the highest mortality rate paralleled only by “noncore” rural areas, which also experience a mortality rate of 0.3 (CDC MMWR Report).

Higher temperatures compromise increase the risk of respiratory illnesses, heat exhaustion, heat stroke, and heat-related mortality. They also increase energy consumption needed to air condition homes and buildings, which leads to higher emissions of air pollutants and heat-trapping gases and exacerbates climate change.

Urban heat is not distributed evenly across a city. Neighborhoods in the same city at the same time can differ in temperature by 20°F, due largely to differences in the factors that cause the urban heat island effect.

This image from the DUMBO neighborhood of New York City shows tall buildings on either side of a street, forming an urban canyon. Credit: Hunter Jones

What can be done to address urban heat?

Communities and individuals can take many actions to reduce urban heat. Many of these actions have additional co-benefits. Examples include:

  • Plant trees along streets, especially over dark surfaces. Learn about the co-benefits of urban forestry at the Vibrant Cities Lab.
  • Add vegetation to urban spaces. Add cool surfaces on roofs, roads, and walls. Learn more about cool surfaces from the Global Cool Cities Alliance.
  • Improve access to public air-conditioned spaces.
  • Vary the height of new buildings to increase airflow and create shade canyons. Use more natural ventilation in buildings.
  • Take advantage of programs like LIHEAP and WAP to make energy more affordable and to waste less energy in maintaining a safe temperature at home.
  • Additional options to manage urban heat are available in the American Planning Association’s report on Planning for Urban Heat Resilience.

Where can I learn more about urban heat islands?