If it feels hotter when you’re in a city than when you’re in a nearby rural area, it’s not your imagination. The urban heat island effect is caused by human activities and is a well-documented phenomenon.
Climate Central recently released a study ranking U.S. cities by the intensity of heat they produce. Three Midwest cities made the top 20: Chicago at number 7, Cleveland at 14, and Detroit at 15.
Many studies have shown the temperature differential between metropolitan areas and their rural neighbors. But thus far researchers have not quantified how urban temperatures are affected by adding shade. Ohio State University researchers say they are the first to use intricate 3D models to calculate how much of a cooling effect shade from trees and buildings has on cities’ surface temperatures.
“We know that when you walk in the shade, it’s more comfortable. This is intuitive, but it doesn’t measure the effect,” Jean-Michel Guldmann, study co-author and professor emeritus at Ohio State, told Centered. “The general idea of the research was to measure the influence of shade on local temperatures — for example, how much the shade reduced temperatures, what kinds of shade, what type of surfaces. … How could these measurements be helpful in terms of urban planning and design and greening of the city?”
The team had to gather tons of data from a 14-square-mile section of the city they studied, Columbus, which contained more than 25,000 buildings. They worked with some partners and used machine learning techniques to collect and analyze the data.
NASA satellite imagery that uses thermal infrared sensors helped to determine the land’s surface temperature. The Columbus auditor’s office provided building footprint data. The researchers integrated that with geographic information system (GIS) mapping to get each building’s exact size and orientation. The researchers used support from GIS software supplier Esri and open source data to collect information on elements such as roads and vegetation.
The team combined all this information with lidar data provided by the city of Columbus. Lidar, short for light detection and ranging, involves flying a plane over an area and sending laser pulses downward to determine the exact height of obstacles on the ground.
“The data were very dense. … There are millions of points — maybe billions — over a city. By processing this information, you can recreate approximately the height of each building. That information is also used to try to recreate the trees” for the model, Guldmann said. 3D city models typically do not include precise representations of vegetation like this.
“Essentially, we created a 3D city of both the built and natural environment in Columbus.”
Once they created the model, the researchers calculated the lines of sight, solar radiation, and projected shade in relation to the city’s structures. They analyzed which object threw the shade and where it landed — such as roads, grass, water, and other impervious surfaces. They put the information into a spatial regression model to determine variations in surface temperatures across the study area.
The researchers also determined how much the shade would change and affect temperatures over time. For example, they simulated young trees’ growth and the additional shade they would produce in the future, in addition to the effects from adding more trees to the area.
The research showed that although buildings do cause heat in urban areas, they also cast shade that lowers temperatures. Shaded rooftops especially offer cooling effects.
A 1% increase in a building’s area resulted in average surface temperature increases between 2.6% and 3%. Meanwhile, an increase of 1% in the area of a shaded rooftop caused average temperature decreases between 0.13% and 0.31%. Shading parking lots and roadways also decreased temperatures significantly, as did the presence of green spaces and water.
The simulation determined that young trees will provide more cooling when fully grown. On a 93 degree day, the fully grown trees would lower temperatures by nearly 3.5 degrees. The simulation also showed that adding 20 additional fully grown trees would lower temperatures by approximately an extra 1.4 degrees.
Guldmann says the analytic process used in this study could help cities develop greening and cooling strategies. The study describes in detail how city planners could replicate this process for their own analyses by using currently available resources. “There is no secret technology” they would need to adopt, Guldmann said.
“The building of the 3D city is complex, but you can use available software methods,” he said. “Most mid- to large-sized metropolitan areas have GIS departments … so they could do these things in-house.”
The Ohio State team intends to do further urban heat research that builds on their data and modeling. Specifically, they want to examine different socioeconomic groups’ access to shade.
“Because we have very precise data, we want to understand shade inequity, and also the effect on prices of housing,” Guldmann said.
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