The reason is simple: it never cools off in Atlanta. Sunset is a mere formality, with heat lingering through the night. Even excluding the effect of higher humidity, it is hotter in Atlanta because it does not cool off. On the other hand, Mazomanie, like the rest of Wisconsin, is always looking for an excuse to cool down.
Chart 1 shows in blue the hour-by-hour temperature in Mazonmanie (plus part of a forecast). The temperature tops out at 83 degrees. The hour-by-hour temperature for Atlanta is in red, and it tops out at 89 degrees. That's a 7% premium. An 89 degree day is pretty hot and an 83 degree day is tolerable, but that's not the half of it. Look at how the Atlanta nighttime temperatures stay in the mid-70s. That's real heat buildup.
To get an appreciation of how the buildup works, you can use the idea of a "degree day" that is used by the power industries to determine heating and cooling loads. The degree day is the difference between the temperature and a constant (60 degrees in the winter, and 65 degrees in the summer).
We apply the degree day idea to each of 24 hours in Chart 2. This chart shows how temperatures deviate from 65 degrees (a nice dog-like temperature) by the hour. These deviations can be thought of as "dog degrees," with a positive dog degree being part of a dog day.
Visually, you can see the distinction between Atlanta and Mazomanie over the course of the day. There are 263 degree-hours (number of hours x number of degrees in excess of 65) in the Mazomanie day but there are 352 degree-hours in the Atlanta day. This is a 34% difference.
This is why a high of 89 degrees in Mazomanie is cooler than a high of 89 degrees in Atlanta.
Click on charts to embiggen.
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