On a hot summer day in a city, one of the best places to cool off is in a forested park. Underneath the trees, temperatures can drop significantly, both from the shadow of the canopy and from the cool 'sweat' released by their leaves.
Even just a single tree could create a more comfortable microclimate, according to a new study. When researchers parked cars with temperature sensors underneath the shade of scattered trees around Washington, DC, they noticed a significant cooling effect.
Over the course of one hot day and night, the team gathered more than 70,000 air temperature readings in various different settings around the city. Compared to streets without any trees at all, those that had a smattering of canopies were cooler in the evenings.
Even when the sun set completely and the leaves stopped transpiring, the neighborhoods with trees experienced cooler temperatures throughout the night.
"There are plenty of good reasons to plant trees, but our study shows we shouldn't underestimate the role that individual trees can play in mitigating heat in urban areas," says environmental scientist Michael Alonzo from American University.
"City planners can take advantage of the small spaces that abound in urban areas to plant individual trees."
Trees are known to cool down city dwellers and city buildings, acting sort of like air conditioners.
Shade obviously plays a part, reducing the radiation that hits and heats the ground, but the transpiration of leaves on a hot day can also have an impact on local temperatures.
How to actually measure or model that impact is quite the challenge, especially when there are a bunch of other factors at play, including the extent of the canopy, the ground cover, the season, the health of the trees, and the time of day.
In the afternoon, for instance, some studies have found it takes at least 40 percent canopy coverage before the forest in a park is cooled down. Yet it's still unclear if those benefits last after sundown or extend to trees that are more spread out.
The study in Washington provides new information on the subject. In the afternoon, the authors found the canopy of a forested park cooled things down by 1.8 °C, which is higher than previous estimates.
Single trees had no such effect, but in the evening, those single trees made a difference. In the study, a single 15-meter-tall tree (49 feet) would cast a shadow a 14-meter shadow in the afternoon. By the evening, that tree's shadow increased to 56 meters. Practically, this meant that just a smattering of canopies could cover the same amount of ground as a dense forest by the end of the day.
Together, when the shadows of these individual canopies combined to cover 50 percent of an area, researchers measured significantly lower temperatures – up to 1.4 °C lower, to be exact.
Even after sundown, when the canopies of scattered trees only covered about 20 percent of the area, the team noticed a cooling effect.
"Evenings are not quite the respite from heat that we once had," says Alonzo.
"These distributed trees do help the city cool off in the evening and that's important for human health."
Planting trees is a way to provide these populations with ample shaded area, and even when there isn't space for a forest, the new findings suggest a single tree will do.
The study was published in Environmental Research Letters.