Post by Jason Schatz
What is the warmest place in Dane County? What’s more, how warm is it, and why is it warmer than everywhere else? To answer these three questions, we used our network of 150 temperature sensors, which have been collecting data across Dane County for the past 2.5 years.
The first question has an easy answer: downtown Madison is the warmest place in Dane County. More specifically, the Isthmus.
The second question—how warm is it?—has different answers depending on the time of day and the time of year. Compared to most of rural Dane County, downtown Madison averages 3°F warmer during the day and 7°F at night in the summer. The image below shows average temperature differences on summer nights; the Isthmus is the bright red strip between the lakes. Temperature differences are smaller during the winter, but the Isthmus is still warmest.
Why do these large temperature differences occur in such a small area? There are two main factors making downtown Madison warmer than the surrounding areas: (1) Heat storage in pavement and building materials, and (2) Urban canyons. More on urban canyons in a minute, but first, heat storage.
Pavement and many building materials can store a great deal of heat from the sun. In comparison, a blade of grass can store very little heat, which means that once the sun sets, the air temperature over a lawn drops much more quickly than the air temperature over a slab of pavement full of stored heat. So the more densely built an area is (and the less vegetation it has), the warmer it will be.
What about urban canyons? What are they, and why do they matter? Imagine a narrow street flanked on both sides by long, tall buildings. This is an urban canyon, which is similar to natural canyons, but composed of buildings and pavement rather than soil and stone. Tall, narrow urban canyons trap and prevent heat from escaping to the sky. The taller and closer the “canyon” walls, the more heat that bounces off surrounding buildings and stays trapped near the ground. Since the Isthmus has the highest concentrations of buildings in Madison, its air temperatures are consistently the warmest.
What about Madison’s lakes? Don’t they keep the Isthmus cool? Large bodies of water can affect nearby temperatures because of the difference between water and air temperatures. However, based on our data, Madison’s lakes rarely affect temperatures downtown, since they aren’t big enough. If the wind is particularly strong off the lakes, temperatures near the shore may change by a degree or two, but for the most part, lake effects on temperature are mostly restricted to within a quarter mile or so of the lakeshore.
To sum up, the more densely built parts of Madison are warmer, and the densest, the Isthmus, is the warmest. For more on local climatic differences in Dane County, see my previous post on the coldest place in Dane County.