The Role of Trees in Mitigating Nashville's Urban Heat Island Effect
Why We Should Conserve Trees to Manage Summer Heat
The urban heat island effect occurs when a city’s temperature is significantly higher than the suburban and rural areas surrounding it, which can have severe impacts on human comfort and health. However, urban heat can be diminished by planting more trees and maintaining green spaces throughout the built environment. Here, we’ll go over how heat islands work, what effects increased temperatures can have, and what kind of difference trees and other plants can make to mitigate the hazardous warming effect.
How Does the Urban Heat Island Effect Work?
Cities are hotspots of energy, where lots of people live and work in densely populated spaces. Many characteristics of urbanized locations contribute to creating a warmer area than its surrounding environments:
Impermeable surfaces – Hard surfaces like cement, pavement, asphalt, and rooftops, as well as building materials like metal and glass, absorb the sun’s heat during the day and radiate it throughout the night, preventing the location’s air from cooling down
Dense layout – In urban places, particularly in downtown city centers, tightly packed buildings don’t leave much space for surface-level air movement, allowing heat to collect as it bears down and radiates back up
Human activity – In densely populated areas, cars, factories, and other buildings generate heat (as well as pollution) from the energy they use
Less green cover – Urban areas with lots of impermeable surfaces but little to no green cover (such as grass, trees, and other plants) don’t have enough shade or moisture to cool the air and break up stagnant heat
How Does High Heat Affect City Conditions?
In addition to the heat island’s effect of high ambient temperatures, humid days can make the warmth of the hottest days more dangerous since air with lots of water vapor can hold more heat than dry air does. Moisture in the air can provide a cooling effect at first, but when it absorbs heat to its capacity, it can also result in the muggy humidity we Nashvillians experience on hot, moist summer days (this effect is measured as relative humidity).
Urban areas with concentrated air pollution are at high risk for smog development, where elevated temperatures “cook” air pollution, creating toxic chemicals that hang in the air in and over the city.
In extreme cases, excessive heat can cause damage to (or shutdown of) certain infrastructure services in urban areas. Electrical grids may be strained from high energy demands and high temperatures, which may result in a blackout. Heat radiating from asphalt roads can make driving dangerous due to softening or damage to the roads themselves, in addition to the increased heat. Similarly, airport traffic can be interrupted by hot tarmacs and air temperature conditions, while train services may be disrupted from the heat’s effect on the metal rails.
How Do Trees Reduce the Urban Heat Island Effect?
In urban areas with lots of buildings and hard surfaces, adding green spaces breaks up the built environment with sources of shade, heat absorption, and moisture release that can lower immediate area temperatures by multiple degrees. Trees reduce the urban heat island effect by:
Creating shade and reducing surface-level temperatures
Releasing moisture, pulling it vertically from the ground and releasing it into the air, allowing the moisture to absorb some of the heat and cool the air
Increasing the chance of weather events like rain and thunderstorms by releasing moisture, which can cool down hot summer days (although heat domes may result in precipitation downwind rather than above the city itself)
Removing particulate matter from the air, either by absorbing particles or by capturing them on leaves, reducing air pollution and, consequently, contributing to lower temperatures
Nashville's Urban Heat Challenges
In the summer of 2022, the metropolitan government worked with Nashville environmental nonprofits and local experts to measure temperatures and humidity in an effort to better understand where urban heat challenges are greatest and where mitigation efforts can have the most effect.
Residents can access the report here. The city also created a resource that uses information from the report to explain the results and implications so Nashvillians can better understand and take action to manage excessive heat.
Initiatives and Solutions to Mitigate and Manage the Urban Heat Island Effect
City planning departments across the US are increasingly taking measures to mitigate urban heat island effects, such as:
Planting more trees and gardens
Devoting more resources to maintaining existing trees and green spaces
Outfitting buildings and adapting open areas (such as using light-colored surfaces, using effective insulation materials, and planting green spaces)
Providing heat wave support to the most vulnerable among us, such as cooling centers and access to water
Residents can take simple actions to help themselves and their families manage the hottest of summer days:
Pay attention to weather reports for warnings of excessive heat
Recognize the symptoms of heat stress such as high body temperature, sweating, increased heart rate, unusual fatigue, nausea, headaches, dizziness, muscle pain, and fainting
Cover windows with blinds, shades, and/or curtains to reduce heat penetration into homes
Stay hydrated by drinking water, and take cool showers or baths
Consider the safety and comfort of small children and pets, who can’t easily communicate distress
Check in on loved ones to make sure they’re taking care of themselves
There are several ways communities can help support Nashville’s greenery to effectively help manage the city’s ecosystem, including the temperatures:
Plant and care for new trees
Maintain established trees in healthy conditions (make sure to water them during those hot dry spells!)
Get involved with local advocacy organizations to support tree planting, conservation, maintenance, and legislation that protects and encourages canopy cover
Educate yourself and others on the causes of heat islands and the importance of trees and other vegetation throughout the city
Trees and the Urban Heat Island Effect
While multiple characteristics of built environments contribute to the urban heat island effect, a lack of vegetation is a major factor of an urban area’s inability to moderate temperatures in the warmest months of the year. Luckily, trees are a low-cost, long-term option to manage heat in cities! Keeping our neighborhoods green and cared for makes an important impact on summer comfort and safety for communities at large.
Browse the Nashville Tree Conservation Corps’ tree sale to get some new trees in the ground, or volunteer with us to plant trees in open areas across the city. For regular updates on canopy news and care tips, subscribe to our newsletter!