What are urban heat islands and how are they created?
An urban heat island is a pocket of heat within an urban center. Dense urban centers have a lot of buildings as well as roads, people and cars that generate heat, which affects the outdoor temperatures, thermal comfort outdoors and air pollutants. A century ago, only 20 percent of the population lived in a city. Now, 50 percent of people live in cities. The U.N. predicts that by 2050 about 70 percent of all population in the world will live in cities. In the U.S., we’re already at 80 percent. Another factor is a lack of vegetation. Because buildings cover entire regions of urban centers, there’s not much vegetation, which actually helps in cooling the outdoor air through evaporation. And lastly, tall buildings trap heat generated by cars, people and factories.
What effect do these urban heat islands have on our communities and our planet?
The main effect is literally the increase of outdoor temperatures, which affects outdoor thermal comfort. It also has an impact on air quality, air pollution and, ultimately, which is what matters, people’s health. The urban heat effect has also increased outdoor temperatures, which is estimated, depending on cities and regions, to be between one and eight degrees with an average of five degrees. This affects how much energy is used to air condition buildings. It also increases the heat stress of people who live in buildings without air conditioning, which leads to the increase in mortality rate.
What tools are being used today to detect urban heat islands that weren’t used in the past? How effective are they? And what are these new tools telling us?
We use many tools, including remote sensing of the temperature as well as humidity and air quality. The problem with these particular models is that they are all focused on a specific factor or a specific case study. What we need is a multiscale type of measurement to account globally within a city as well as specific to a building scale level, so we can improve the design and operation of our buildings while mitigating the urban island effect. That’s what we’re trying to work on in our group.
In a recent paper in the ASME Journal of Engineering for Sustainable Buildings and Cities, you discuss mitigation strategies. Can you walk us through those?
Increase the vegetation around your house. Studies showed that if we increase the vegetation from 15 to 30 percent you could reduce your temperature around your building or your home by 2 degrees. At the large scale of design, we need better urban architecture. Tall buildings and narrow streets trap heat. If we reduce the height of the buildings and increase the width of the streets we could reduce the impact of the urban heat island effect, or by using cool roofs or cool surfaces in general with a coating that is typically a white or light colored. This goes for pavement and roads—at least in an urban setting. Another way to mitigate – at least for buildings – is to have green roofs, which are roofs that can support vegetation. This reduces the amount of heat the roof absorbs. Ultimately, this improves the comfort as well as reduce the energy consumption due to air conditioning.