A recent study by researchers at the University of Wisconsin in Madison offers one of the most detailed records of the variation in temperature between cities and the surrounding rural areas, known as the urban heat island effect.
Jason Schatz and Christopher Kucharik of the Nelson Institute wanted a data set that would accurately reflect how temperatures varied in Madison at the neighborhood level. They mounted 151 temperature sensors on telephone poles in areas with varying levels of building density. Each sensor took measurements every 15 minutes for 18 months between March 2012 and October 2013. The researchers then looked at what environmental factors could be influencing the variation in temperature from urban and rural areas.
Understanding the phenomenon is becoming more important as a growing number of people are moving to cities and climate change is driving overall temperatures higher, according to the study’s authors.
By 2030, about 87 percent of North America’s population is expected to live in urban areas, up from 80 percent of the current population, according to 2010 data from the World Health Organization. According to the study, a lot of that urban growth is likely to happen in small- to medium-sized cities like Madison, since half the people in urban areas live in cities that have populations between 100,000 and 500,000.
They found that the density of urban development and lack of vegetation had the greatest impact on temperature variation. Warming within Madison was more pronounced during the summer, and particularly at night, under calm, clear conditions.
Higher heat retention
Cities tend to retain heat more than rural areas because dark surfaces absorb heat in the day and concrete is much slower than vegetation to release heat at night. In areas with more vegetation, plant evapotranspiration helps to cool the air.
The researchers hypothesized that the greater amount of foliage in the summer could explain the seasonal differences of the urban heat island effect.
Other variable environmental factors like wind speed, cloud cover, humidity, soil moisture and snow cover also played a part in the warming effect. Land elevation and lake proximity also altered temperatures in specific areas.
City planners undervalue tree benefits
The study did not focus on ways to help cities become cooler, but plenty of other researchers are working on tackling the problem. One method that municipalities are trying is urban forestry, a tactic Madison would be well positioned to use, said Kathleen Wolf, a research social scientist at the University of Washington.
Although the field has been around for a couple of decades, broader acceptance of functional rather than decorative tree planting is fairly recent.
Increasing foliage cover is not the only way to help keep cities cooler. Cities can lower temperatures by incorporating reflective surfaces on roofs to deflect heat absorption. Another larger-scale approach could be to make cities more compact. According to a 2010 study from the Georgia Institute of Technology in Atlanta, urban sprawl, even in cities with relatively small populations, leads to a faster increase in the rate of extreme heat events than in compact cities.
A super-hot summer gets amplified
To Schatz, the summer of 2012 exemplified temperatures Madison and the rest of southern Wisconsin could see more frequently in July and August by the middle of the century.
That summer, Madison had 39 days above 90 degrees Fahrenheit. The annual average is usually 12 days over 90, according to National Oceanic and Atmospheric Administration data.
Schatz cited a 2011 report by the Wisconsin Initiative on Climate Change Impacts that predicted the southern part of the state will likely experience about 25 days above 90 F per year by 2050. Northern Wisconsin, which usually experiences about five days over 90 degrees, would likely exceed 90 F 12 times per year by midcentury.
— source scientificamerican.com