The severe loss of trees during the May 16th tornado did more long-term damage to our region than some realize due to something called the “urban heat island effect.”  Our Environmental Quality and Climate Change committee, open to all members, reports that this term refers to microclimate differences that take place when cities’ unshaded roads and buildings gain heat during the day and radiate that heat into the surrounding air. Conditions are typically worse overnight when temperatures stay warm and when wind is lower.

According to a preliminary East-West Gateway Council of Governments study, parts of the St. Louis region can be 12 degrees warmer than other areas during the hot days of the summer. And those disadvantaged St. Louis neighborhoods, which were already without substantial tree cover and already hotter, were especially hard hit in the tornado. This can have major impacts on heat-related diseases, morbidity, reduced biodiversity and a significant increase in utility costs, to name a few negative repercussions.

Urban heat island is due to a combination of factors:

• Lack of trees and vegetation, which provide shade and also absorb and release moisture that cools the surrounding air.
• Dark surfaces, like asphalt roadways, absorb more energy from the sun and re-radiate that back to the environment creating warmer conditions vs. light, reflective surfaces.
• Urban geometry with tall buildings can create a canyon effect which blocks cooling winds.
• Waste heat from vehicles and mechanical equipment contributes to locally hotter conditions.

What can be done? There are many strategies to reduce urban heat island effect:

• Implement cool surfaces on roads, roofs and walkways.
• Build shade structures where people congregate and along walkways.
• Vary the height of buildings to reduce the canyon effect.
• Reduce hard surfaces and maximize vegetation in urban spaces, including green roofs.

When it comes to trees, planting trees along streets and in our parks is a major goal. The good news is that local organizations like Forest ReLeaf have been working to remedy the disparity in tree canopy in our neighborhoods for many years. They are now focused on developing a comprehensive plan to greatly increase tree cover with a long-term resilient solution based on community needs.

Everyone has a role to play in reducing the urban heat island effect. At the personal scale, replacing hard surfaces with vegetation and light, reflective surfaces can have a dramatic effect.  Support of broader efforts to replant our tree canopy throughout the region is extremely important, especially at this time.

National resources include the federal heat islands website and the Smart Surfaces Coalition.