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Urban Islands

Project aims to show how cities' heat centers affect landscapes

Photo by David Hansen

Downtown temperatures are often higher than the temperatures in surrounding areas, a phenomenon that could have far-reaching effects on the environment and human health as the Earth gets warmer. “Islands in the Sun,” led by Department of Soil, Water and Climate assistant professors Peter Snyder and Tracy Twine, is aimed at understanding urban heat islands and mitigating the islands’ effects. The project is jointly funded by CFANS and the university’s Institute on the Environment. We asked for an update.

Heat islands involve many factors beyond just temperature; could you explain a bit about the tools you’re using and the data you’re collecting?

We are using a two-pronged approach to analyze the Twin Cities’ urban heat island. We have installed a network of more than 150 sensors located throughout the metro area from Minnetrista in the west to Woodbury in the east, and Hugo in the north to Apple Valley in the south. These sensors measure air temperature at 2 meters height and are mostly located on residential properties. Our goal is to have 225 sensors deployed by this winter. Our second set of sensors is installed on portable meteorological towers placed over different surfaces of the urban environment. They measure air temperature, wind speed and direction, skin temperature of materials, and we have net radiometers that observe how different surfaces influence the surface radiation budget.

In addition to the Twin Cities, you’re looking at heat islands in 100 cities around the world. How do the measurements you’re tracking here translate to other places—how might what you learn about the local heat island pertain to land use and planning in, say, Beijing?

The Twin Cities urban heat island study is unique because our city is located in a northern climate where we receive snowfall and can actually benefit from the urban heat island in winter. We are contrasting what we learn here with data we are analyzing in other cities including Rio de Janeiro, Riyadh, Paris and Tokyo. That analysis will allow us to determine the relative importance of differences in the landscape such as parks, proximity to water, building materials and land-use planning.
Some of the variations in temperature between a downtown and a nearby suburb might be just a few degrees. What difference do small variations make?

The variations in temperature are not always that small. During our July heat wave, the temperature difference between downtown Minneapolis and outlying areas reached 7-10 degrees F. These differences can have a significant effect depending on season. For example, in winter an urban core that is 10 degrees warmer can result in substantial savings in heating bills. In summer, a warming of this magnitude can contribute to increased cooling costs as well as health impacts such as an increased risk of hyperthermia.

The project has been running for about a year now. Any surprises so far?

Yes. This past winter’s lack of snow contributed to a muted signal, which is uncharacteristic. Typically the urban heat island is stronger in winter than in summer. This year, our very warm July created an urban heat island that was greater than what we experienced this past winter. This highlights the importance of studying not just air temperature but how the landscape is changing.

Another surprise is the moderating effect of the many lakes in the Twin Cities. We observed strong localized warming last fall when lakes released heat to the atmosphere that they stored over the summer. We observed the opposite effect this spring when lakes cooled their environment. This phenomenon is well known around larger lakes but we were surprised to see it near some of the relatively small lakes in the metro area.

Is there a point during the life of the project where you’ll say “OK, we have enough data”? How will you know when you get to that point?

No. Scientists never have enough data! Our hope is that we can continue the network beyond this particular project. We have recently been awarded a National Science Foundation grant that will further our research efforts. It is important to characterize the urban heat island in the context of climate variability, and this can take many years to evaluate. For example, this past summer, temperatures were some of the highest on record. This was an anomalous year and the more years of data we can collect, the better handle we will have on how our urban heat island relates to climate and the better options we will have to understand how we can reduce our urban heat island.

–Becky Beyers
Want to help collect data? Volunteer at


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