Trees cool better than reflective roofs in vulnerable Houston neighborhoods

Researchers Kwun Yip Fung, Zong-Liang Yang, and Dev Niyogi at the UT Jackson School of Geosciences, along with colleagues from Spain and Canada, have created a new physics-based computer modeling framework that integrates indices of human comfort and social vulnerability with heat island mitigation strategies and a state-of-the-art urban climate modeling system.

The work was published in PNAS Nexus.

When the researchers applied the index to Houston, they discovered that trees, rather than roof treatments, provided the best relief from the heat in the most vulnerable areas. Vulnerability is assessed based on sensitivity factors such as socioeconomic status, household composition, and minority status as well as adaptive capacity factors such as housing type and access to transportation.

Heat islands occur in cities where structures such as buildings and roads absorb the sun's heat more than natural landscapes such as trees and grass. This higher heat leads to increased energy consumption from air conditioning, increased emissions from using more electricity, and compromises human health and comfort. This heat island effect can vary in different parts of the city, leading to differences in impact.

Most people are familiar with wind chill indexes used in the winter to describe how cold temperatures and wind interact to make people feel colder. Similarly, the heat index relies on both temperature and humidity to describe how conditions can make people feel hotter. Before this study, little research had been done to quantitively assess how the sun beating down on people makes them feel in an urban setting.

"If construction workers work under direct sunlight versus under the shade of tree cover, the comfort level will be very different," said Yang.

The universal thermal comfort index combines human comfort based on temperature, humidity, wind speed, and radiation. The researchers said it could be used in any community.

In their study, the researchers considered three different heat island mitigation strategies: painting roofs white to increase solar reflectance; planting vegetation on roofs to increase evaporation through the plants; and planting more trees, which increases evaporation and provides shade. In a generic city block, painting roofs white led to the biggest decrease in the index, especially during the day.

However, looking at different neighborhoods in Houston, the results became more nuanced.

The U.S. Centers for Disease Control and Prevention has developed a social vulnerability index as a measure of how sensitive neighborhoods are to socioeconomic factors and their capacity for adaptation. Classifying the neighborhoods in Houston according to this vulnerability index and then applying the human comfort index revealed that while painting roofs white was the best cooling option in places with low vulnerabilities, in places with higher vulnerabilities, planting trees was a better strategy.

"Now that we have developed the index of cooling and we have the vulnerability data, if we combine both of them, we can see which methods provide more cooling for those vulnerable neighborhoods," said lead author Fung, who conducted the research as part of his doctoral studies at the Jackson School.

The research revealed that places with high vulnerabilities also had more available space where trees could be planted, so the potential for adding trees was greater. They also had less roof area available for painting white or planting with vegetation.

"Now that we know the vulnerable neighborhoods have more space for planting trees, we should prioritize trees at those regions," said Fung. "And in those less vulnerable neighborhoods, we should prioritize other strategies like cool roofs and green roofs."

Applying the methodology to other cities may require other considerations. For example, in arid places like Arizona, trees would need to be selected for heat and drought tolerance. In northern cities, a lack of air conditioning plays a role in communities that are vulnerable to heat.

The new methodology could also be used to develop hybrid strategies, combining both rooftop treatments and tree planting, as well as other strategies such as reflective pavements.

"We see this as a baseline, but we are still exploring," said Fung. "Now that the index and the methodology have been developed, they can be applied to many other scenarios."