What is an “Urban Heat Island?”

Have you ever wondered why official temperatures for cities are actually taken at airports or other remote locations? If you have a thermometer in your car, perhaps on sunny days you’ve noticed that the temperature tends to increase the further you travel into a town, with the effects being greater in larger towns. Or, maybe you’ve noticed during snow and ice events that the roads get covered more quickly in rural or suburban areas than they do in town. I could go on giving examples, but these are all largely due to the urban heat island effect (UHI).

Cities tend to have large amounts of concrete, asphalt, and steel that absorb solar energy very readily. During the heat of a Summer day, moderate to large sized cities can be 5 degrees or more warmer than suburban and rural areas. That is why heat waves claim so many more victims in cities. It may be very hot outside of town, but it will usually be several degrees cooler than in town. Additionally, in rural areas any breeze will be able to help move that hot air, making it less stagnant and oppressive versus the wind obstructed cityscape. At night, suburban and rural areas will cool more readily because the surface has stored much less heat than the solid surfaces in town. This can lead to nighttime temperature differences of 2 to 5 degrees, even in the winter.

These heat island effects are significant enough that they have even been built into the GFS (Global Forecast System) forecast computer model. Note in the  https://weathermodels.com  image to the right how the Indianapolis, Cincinnati, and Louisville heat islands show up very well. These heat islands, along with irregular topography and highly obstructed wind flow, are why official temperature measurements are generally taken at locations like airports or other non-urban locations. These suburban locations allow for data collection that is more consistent with the majority of observation areas, rather than small urban heat sinks.

In the Winter, due to the lower sun angle, UHI effects are less intense but are still present and can have some impacts. I only live about ⅛ mile outside the far northern City Limits of Anderson, yet I frequently observe the temperatures at Broadway and Hartman Road (County Road 300 North) are 2 to 4 degrees warmer than they are in my subdivision. As such, during the onset of snow events, I also routinely observe that untreated roads in my subdivision will begin to cover with snow well before the roads in North Anderson do. During the course of running Madison County Weather Updates, I’ve also observed that, in uniform snow events, snowfall measurements inside the limits of the five larger towns in the county tend to be lower than those just outside the towns. This information, along with other factors, can have financial impacts in the decision making on when and how to treat roads, bridges, and parking areas. So, UHI is no small thing regardless of the season.

One last thought on the urban heat island effect: While UHI has been known for about 200 years and is firmly established, there are still a lot of questions and controversies regarding the effects (if any) on localized storm tracks and weather patterns. While there is no question that UHI *does* have perceptible impacts like the handful I mentioned above, and upon regional climate, there is less agreement about whether or not urban heat islands “create their own weather.” Studies like the ones mentioned in this article seem to show strong evidence that UHI at least enhances thunderstorm activity over larger cities, but there is still not strong agreement on the effects on “typical” local storm tracks. I believe there are observable effects, and in a later blog, I will address what I believe is happening to our local storm patterns as a result of the massive expansion of the Indianapolis metropolitan area to our southwest and west.

~Micah & Kristen

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