Understand 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 gain heat 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.

Why do we care about urban heat islands?

Elevated temperatures with urban heat islands affect a community’s quality of life and can adversely impact both natural and built environments. Their adverse impacts include:

• compromised human health and comfort with an increased risk of respiratory illnesses, heat exhaustion, heat stroke, and heat-related mortality; and
• increased energy consumption needed to air condition homes and buildings, which leads to increased emissions of air pollutants and heat-trapping gases.

What can we do to stay cool in our cities?

Deep shade along a tree-lined street in Brooklyn, New York. Planting trees to provide shade, especially next to paved surfaces, helps to reduce the urban heat island effect. Image by smallchan, used under a Creative Commons license.

Urban communities can do a number of things to help keep cool:

• plant trees along streets to make shade, especially over dark surfaces,
• add vegetation to urban spaces, including green roofs,
• whitewash roofs, roads, and parking lots (any large areas of black asphalt),
• provide more 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.

Where can I learn more about urban heat islands?

• NCA4 Chapter on Built Environment, Urban Systems, and Cities
• EPA’s Urban Heat Island Effect page
• NASA ClimateKids: What is an urban heat island?
• UCAR’s Urban Heat Island page