Maybe during a July baseball game, you and the other spectators shifted your chairs to remain in the shade of a large nearby oak. Or while walking your dog, you crossed over to the side of the street that’s lined with tree canopy cover. While chatting with a neighbor in the driveway, maybe you both took a few extra steps to talk under the shade of a tree.
Trees are not a solution to extreme heat, by any means. But zooming in to neighborhood streets and even individual yards, trees offer significant cooling effects that can help keep communities safer during heat waves.
Shade
So just how much cooler does it feel in the shade?
The U.S. Department of Energy explains that “because cool air settles near the ground, air temperatures directly under trees can be as much as 25 degrees cooler than air temperatures above nearby blacktop.”
To see the amazing cooling power of trees for ourselves, the Cumberland River Compact conducted a simple experiment with kids and families at this summer’s Waterfest in Goodlettsville. Using handheld infrared laser thermometers to read surface temperatures, we gathered data to compare the temperatures of a sunny sidewalk versus under the shade of a tree. The results of this simple experiment showed a clear difference. The highest pavement temperature was recorded at 120 degrees with the coolest temperature of 65 in the shade.
With impressive numbers like these, you can see how shade provides not only relief on a hot day but can also positively impact your monthly electric bill. The shade of mature trees lessens the need for cranking up your air conditioner, especially when placed strategically on the hottest side of your house, usually the southwest. Amazingly, trees planted in the right places “can reduce air conditioning needs by 30%” (Arbor Day Foundation).
Evapotranspiration
It’s not just the shade of a tree that keeps us cool. The natural process of evapotranspiration also provides significant cooling on hot summer days. Evapotranspiration is the process by which the tree absorbs water from its roots, eventually dispersing it into the air when leaves react to sunlight. It’s almost like a tree sweating – and it has the same cooling action as when humans sweat. The water vapor released from the leaves cools the surrounding air and feels significantly cooler to us and “can help reduce peak summer temperatures by 2–9°F (1–5°C).” –United States Environmental Protection Agency
Heat Islands
These combined cooling effects of shade and evapotranspiration are especially important in cities because of the urban heat island effect. In a previous Root Nashville blog post, urban heat experts Dr. Alisa Hass and Dr. Adelle Monteblanco, Dr. Hass described this effect:
“The urban heat island effect is basically telling us that urban areas are warmer than the surrounding areas. This happens for a number of reasons. The big one is we have a lot of impervious surfaces — meaning concrete. So for all the rain that falls down, instead of actually incorporating into the ground and going into groundwater, it just washes off. And what rain really helps us with is cooling because any time we have evaporation, we have cooling. If we don’t have that water on the surface, we’re not going to have as much cooling. […] So urban areas can be a number of degrees warmer than rural areas (this is the “island”), which can make a big difference when you’re talking about heat waves.”
You can easily see this effect at work on the below Heat Exposure Index map (the “island” is downtown and nearby shown in the darkest red color, surrounded by cooler, more vegetated neighborhoods, in lighter shades):
This map displays recent data gathered through the “Understanding Nashville Heat” project led by the Mayor’s Office, with support from many partners, including the Cumberland River Compact. Learn more about the “Understanding Nashville Heat” project here and click through many interesting maps and layers of data on this easy-to-read StoryMap.
Take Action
There are many benefits of tree-planting beyond their cooling capabilities. Trees are the best filters for urban water runoff and to combat other forms of climate change such as habitat loss and increased flooding.
We encourage you to get involved with the Root Nashville campaign, and help members of your community plant trees and create cooler, more enjoyable neighborhoods for us all.
Two action steps you can take?
1. Become a Neighborhood Planting Captain to help your neighbors sign up for free trees
2. Help the campaign locate planting sites on private property
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