Post by Jason Schatz
What is the coldest place in Dane County?
Oddly enough, summer is the best time of year to answer that question. Area temperature differences are strongest in the summer, so it’s easier to see hot and cold spots. But the patterns are the same throughout the year.
As part of our urban heat island study, we have measured the air temperature at 150 locations across Dane County for the past 2 1/2 years, including the bitterly cold winter of 2013/14. It was cold everywhere this past winter, but some places were colder than others, which made us wonder, what is the coldest place in Dane County?
It turns out there are a few places. This map shows average nighttime temperature differences in Dane County in August 2012.
As you can see on the map, rural areas were cooler than urban areas, but three rural locations (marked with arrows and white pixels on the map) were nearly always colder than the rest. They were colder at night, during the day, during the summer, and during the winter.
Cold spot #1 is a cornfield in a broad valley. Cold spots #2 and #3 are wetlands.
What makes these areas so much cooler than the rest of the landscape? Three factors are at work: abundant vegetation, openness, and low elevation.
(1) Abundant vegetation allows the surface to cool more quickly and store less heat compared to urban or unvegetated landscapes.
(2) Openness (i.e., a lack of surrounding trees and buildings) also allows the surface to cool quickly, because there is nothing to obstruct heat from escaping into the atmosphere. Openness does increase sun exposure during the day, but if there is also vegetation, air temperatures don’t get as warm as they would in the city, for example.
(3) Low elevation. Cold air is denser and heavier than warmer air and tends to settle into low lying areas, a process known as katabatic drainage (katabatic comes from a Greek word meaning “going downhill”). So valleys tend to collect colder air. This occurs mostly at night.
Have you ever seen frost in a low lying field on a chilly morning when everywhere else is frost-free? That’s vegetation, openness, and low elevation at work.
Let’s take a closer look at our three cold spots. Cold spot #1 (corn field in a valley) is heavily vegetated, very open (no trees or buildings), and in a particularly low lying valley. As you might expect, it is usually our coldest site.
Cold spots #2 and #3 are large wetlands. Like site #1, they are densely vegetated, very open, and lower in elevation than their surroundings.
Unlike site #1, they are not in deep valleys, but wetlands do tend to be slightly lower than their surroundings, which allows water and, in this case, cooler air to pool there.
We have a lot of temperature sensors (150 altogether), but we haven’t measured temperature everywhere. So these may not be the absolute coldest places in Dane County, but they’re definitely close. Wherever the absolute coldest spot is, you can bet it will look a lot like these three—rural, vegetated, open, and low in elevation.