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October 22, 2014

Making sense of urban heat island: It's about the plants

Living in an urban heat island is a fact of life for over half of the global population, which lives in cities. In the summertime, the heat island effect is at its most intense, a known phenomenon whose causes were, until recently, poorly understood.

A new study by WSC scientists Jason Schatz (Ph.D. candidate, Environment and Resources) and Chris Kucharik (lead Principal Investigator) sheds light on why there are seasonal differences in the urban heat island effect, or the tendency of urban air temperature to be warmer than that of rural surroundings. Their study has implications for health, resource consumption and quality of life.

Schatz and Kucharik determined that the urban heat island effect is largely dictated by how much vegetation covers the land—the more vegetation, the cooler the air temperature.

In the summer, rural areas tend to be kept cooler by their abundant agricultural and natural vegetation, while the cement-and-asphalt jungles of urban areas retain more heat. This results in a larger summertime temperature difference between rural and urban areas than during times of the year when vegetation is scarcer, such as the fall and winter.

To illustrate, summertime temperatures in the study site, Madison, WI, averaged seven degrees Fahrenheit warmer at night and three degrees warmer in the daytime than those in the city’s surrounding rural areas. By comparison, the average wintertime temperature difference between urban and rural is about two degrees.

Vegetation plays a role in the difference between daytime and nighttime heat island effects, as well. Nighttime temperature differences are greater because abundant vegetation allows rural areas to cool down more quickly at sundown, while urban surfaces hold the heat acquired from the day longer into the night.

A bigger heat island effect in the summer matters because hotter temperatures are likely to be unwelcome in an already hot city, especially if climate change adds more hot days to the calendar, as scientists predict.

“This is all happening on top of climate change,” said Schatz, the study’s lead author.

Schatz says their study enhances the understanding of what climate change could mean locally in the Yahara Watershed, now and into the future. For example, more hot summer days could put more pressure on urban water supplies, especially if people are watering their lawns or gardens more.

“There are a lot of things that are affected by this much climate variation. It raises a lot of questions,” says Schatz.

Despite the questions raised, the knowledge gained about the mechanics of the urban heat island effect can aid decision making around mitigating it, as well as give scientists a clearer picture of the implications for human health, the water cycle, air quality, building energy demands and ecosystems.

“In a broad sense, our study helps us understand how urban climates work. The more we know, the better equipped we are to manage what is happening and what is coming,” said Schatz.

The study was published in the October 2014 issue of the Journal for Applied Meteorology and Climatology.

Read UW-Madison's coverage of the study.