Urban Heat Islands

By Elaine Simmons

As Arlington County increases its density of buildings and residents, more people can access our many benefits. However, growth also increases the possibility of the phenomenon known as the urban heat island effect: cities experience higher air temperatures than non-urban areas.

On average, cities tend to be 1–7°F warmer than non-urban areas around them during the daytime and 

as much as 5°F warmer at night. On a summer day, the sun can heat buildings, roofs, and cars to temperatures 50 degrees higher than the surrounding air. Urban air quality also suffers since more pollutants are pumped into the air in densely populated areas, and stagnant air conditions during heat waves can trap pollutants near the ground. 

Natural surfaces like trees and plants have cooling effects from shade and “evaporative cooling,” whereby evaporating water from vegetation absorbs heat (much like sweat cools the human body). Artificial surfaces such as roads and buildings that replace vegetation typically lack those cooling effects and instead absorb and re-emit more heat, which makes their surroundings warmer too. 

As the planet warms, urban heat islands will only intensify higher temperatures. To cope with higher temperatures, cars and buildings consume more energy for cooling—frequently via fossil fuels—which worsens air pollution and contributes to climate change. 

An important way to fight the urban heat island effect is to reintroduce vegetation. This includes planting trees, expanding parkland, and installing “green roofs” designed to support plants. Building cool roofs and  pavements—which have bright coatings that reflect more sunlight and, therefore, absorb less heat—also   can reduce the urban heat island effect. A model by the MIT Concrete Sustainability Hub estimated that, if widely implemented, cool pavements could reduce the frequency of heatwaves by 41% across all US urban areas.

As residents, a key thing we can do is plant and properly care for tough native shade trees (like oaks, sycamores, hickories, tulip trees). Those of us with shady yards know that tree cover makes a HUGE difference in how hot it feels. A friend in McLean with lots of shade trees has measured a 10-degree difference in his yard compared to nearby exposed areas. Next time you venture out, notice how much better streets and sidewalks with tall tree overhang feel and look compared to those that are sun-beaten. To get a free tree, including installation, contact the Tree Canopy Fund established by Ecoaction Arlington. You may also register for a free tree on Arlington County’s Urban Forestry Dept web page. Or visit your local garden center.