We were able to set up our two systems on the roof of the Minneapolis Convention Center November 21-28, 2011. We were fortunate to be able to see the roof between the snow melt from the snowfall on the 19th and before we have the snow cover through winter. We plan to go back in spring once the snow is gone. For now, here is a photo of our two systems. Both are measuring albedo (the reflectivity of the surface), absorbed sunlight, the temperature of the surface and air at four heights up to 2 m, and wind speed and direction. One system is set up over a black membrane covered with river rock (in the foreground), and the other is set up over white PVC (in the background).
Below are the first results from this experiment. One day (November 23, 2011) is shown as an example. This figure shows that the river rock surface is darker (has a lower albedo) and therefore absorbs more radiation. While this seems obvious, our ability to quantify these differences under a variety of environmental conditions (different seasons, cloudy vs. sunny days) will help us develop models that can simulate the effects of using these different surfaces to mitigate urban warming. It’s not enough to know that darker surfaces are usually warmer, we need numbers.
Net radiation is greater over the darker river rock surface than over the white PVC during daytime. Values are similar during nighttime. Noon albedo (reflectivity) is lower over the river rock than over the white PVC by 0.4.
Will Hertel has created an image that shows the features of the Twin Cities urban heat island.
The data are from September 15, 2011 at 4:00 am CDT. Urban heat islands are generally stronger in winter and during the nighttime. While we analyze data taken over the entire 24-hour day, the data at this early hour highlight the largest difference between the downtown and outlying areas on this particular day. Will interpolated among sites that include our volunteer network along with other standard temperature recording locations (such as airports).
This is one example of the data that we will be able to analyze with this project.
Thanks to those of you who have volunteered to help us with this project. We have received addresses from hundreds of interested people and are in the process of sorting through them. We’re grateful that so many of you are willing to help! With more sites being added, these preliminary data will be improved–more sites means less interpolation between existing sites. Stay tuned…
Our project was mentioned in Paul Huttner’s Updraft blog on MPR on Friday, November 11, 2011.
Our project was mentioned in a story about urban heat islands in the Star Tribune on 11/5/11.
Check it out!
We are setting up a dense network of temperature sensors around the Twin Cities Metro Area to measure the spatial and temporal variability of temperatures in a variety of urban and suburban environments. The goal of the project is to learn more about the behavior of the urban heat island effect in a large northern latitude metropolitan area. We are currently determining sites based on location and land characteristics.
If you live in the Twin Cities Metro Area and are interested in participating by housing a sensor on your property please follow the link provided below. The sensors are small (several inches long) and should not detract from your backyard aesthetics. We might contact you if we think your property might fall within our criteria. Please note that your interest does not guarantee that your site will be selected—it is possible that your location will not meet our selection criteria.
If selected, we will install a temperature sensor outside your house somewhere on your property and ask that it remain for 2-4 years. There is nothing required of you—the sensor is self-contained and automatically logs readings. We must have access to the sensor every 2-3 months to download data. We will be happy to provide the data for your site to you if you are interested.
If you are interested in housing a sensor on your property, please visit the Sensor Network page. If you have any questions please do not hesitate to contact us.
We use a variety of tools to study urban heat islands, including both measurements and modeling. Some observations we get from satellites as they look down on cities from space at different resolutions (just like digital cameras have different pixel sizes). In this post we’re going to talk about the measurements we are making on the ground.
We are making measurements of temperature, wind, and radiation over various surfaces such as asphalt parking lots, grass, and concrete. We measure air temperature at four heights near the ground (2m, 1m, 0.5m, and 0.25 m). We also estimate what is known as ‘skin temperature’ of these surfaces by measuring energy that radiates from the surface. This is the close to the actual temperature of the surface. In the case of an asphalt parking lot, if you walked barefoot on it in summer the skin temperature is what your foot would feel. We all know this can be much hotter than air just inches above the parking lot.
We are also measuring solar radiation as it comes through the atmosphere and warms the ground, as well as the part of this radiation that is reflected. A grassy park is cooler than an asphalt parking lot on a bright sunny day because of many differences—one of them is the fact that parking lots absorb more solar radiation than grass. The parking lot has a lower reflectivity than the grass. One way to mitigate the urban heat island effect is to increase the reflectivity of surfaces in urban areas.
Finally, we are in the process of deploying more than 100 temperature sensors to backyards around the Twin Cities Metro Area. By measuring temperature on a dense network from the downtown to outlying areas, we can better understand the make up of our urban heat island.
We just finished our new website for the project. At this site we can bring you the most up to date results of our research, information on our network of temperature sensors, and recent news that we will post on our blog.
Please look around the site and let us know what you think. It’s new and a work in progress so we appreciate any feedback. Many thanks to Michelle Beaman at the Insitute on the Environment who designed this website for us.
The Islands in the Sun project is sponsored by the University of Minnesota’s Institute on the Environment and College of Food, Agricultural, and Natural Resources and is led by Professors Peter Snyder and Tracy Twine. We are excited about the opportunity to highlight some of the things we are doing as part of the project on this blog. Let’s jump right in and talk about what is an urban heat island and what we are doing on this project.
Urban heat islands are regions of strong warming that are localized around the core downtown region of a city with progressively lower temperatures as one travels away from the central region of warming – hence the name “heat island”.
Urban heat islands exist because of large differences in land use, building materials, and vegetation between cities and their rural surroundings. This difference can be in excess of 1-10°F during the daytime and as much as 12-20°F at night. In addition, the localized warming has the potential to alter patterns of precipitation in metropolitan regions and the surrounding countryside and perhaps even influence the frequency and severity of severe weather.
The goal of this project is to better understand the mechanisms contributing to urban heat islands with a goal of finding ways to lessen their effects through landscape design. Given that more than half the global population lives in cities, there is urgent need to understand and mitigate urban heat island effects, especially during heat wave events when the risk of heat-related illness and mortality can increase dramatically.
The data that is collected in and around the Twin Cities Metro Area will be used to comprehensively map and understand the annual, seasonal, and daily behavior of our urban heat island. The data will also be used in conjunction with numerical models to explore how changes to the urban landscape could contribute to mitigating the deleterious effects of urban warming. Additionally, we will be evaluating the effectiveness of different engineering and building design practices that can reduce energy consumption and reduce urban warming.
Our ability to monitor air temperature across the Twin Cities Metro Area at over one hundred locations depends on volunteers like you. If you think you might be interested in helping us, stay tuned to a later blog post for information.
Please let us know what you think about our project and let us know if we can answer any of your questions on the project or on urban heat islands in general.