Just how do we measure the urban heat island?
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.