The shade from a single tree is a welcome relief from the hot summer sun. When that same tree is part of a small forest it creates a profound cooling effect.
According to a study published in the Proceedings of the National Academy of Sciences, trees play a big role in keeping our towns and cities cool.
According to the study, the right amount of tree cover can lower summer daytime temperatures by as much as 10 degrees Fahrenheit. The cooling effect is noticeable from neighborhood to neighborhood, even down to the scale of a single city block.
“We knew that cities are warmer than the surrounding countryside, but we found that temperatures vary just as much within cities. Keeping temperatures more comfortable on hot summer days can make a big difference for those of us who live and work there,” says Monica Turner, a University of Wisconsin-Madison professor in the Department of Integrative Biology and a co-author of the study.
Turner stated that impervious surfaces like roads, sidewalks, and buildings absorb heat from the sun during the day and slowly release that heat at night. Trees, on the other hand, not only shade those surfaces from the sun’s rays, but they also transpire or release water into the air through their leaves, a process that cools things down.
To get the maximum benefit of this cooling service, the study found that tree canopy cover must exceed 40 percent. In other words, an aerial picture of a single city block would need to be nearly half-way covered by a leafy green network of branches and leaves.
Traditionally, says Carly Ziter, lead author of the paper, studies like these have tended to focus on what’s known as the “urban heat island” effect. Those studies often use satellites to take ground surface temperature readings or measure air temperature within and outside the city. Studies have shown that developed, less vegetated cityscapes are much warmer than the rural lands around them. But this study, Ziter says, allowed researchers to look at temperatures on a much finer scale, down to the spaces “where we live our daily lives within the city.”
To get data at that local scale, Ziter and her collaborators had to get creative with their sampling methods.
Satellite measurements of ground surface temperature don’t really provide air temperature data, Ziter says, so they’re “not getting you quite as close to what people are actually feeling.”
But deploying enough air temperature sensors across town to get the fine-scale resolution they wanted was far too expensive. Research ongoing at the University of Wisconsin-Madison had temperature sensors strapped to 150 utility poles across the city and its surrounding countryside, but those sensors were often one mile or more apart — much too far to provide real-time data on temperatures in backyards and individual boulevards.
In the end, Ziter settled on a scaled-down solution to her sampling problem. All she needed was one sensor and two wheels.
In the summer of 2016, it wasn’t uncommon to see Ziter biking around the city of Madison with a small weather station strapped to the back of her bike. In all, she biked ten different transects of the city multiple times during different times of the day. The sensor on her bike marked her location and took an air temperature reading every single second as she rode, resulting in real-time data every five meters.
All told, she estimates that she biked 400 to 500 miles and was “in very good shape” by the end of the study. She also racked up a massive amount of data that showed just how instrumental trees are in moderating heat in cities.