I guess that's the part I don't understand--why these fronts are rare compared with the lake breeze that occurs virtually every spring afternoon. I guess technically, the lake breeze line is a cold front, but the pneumonia front originates to the north and shoots rapidly down the lake in the late evening hours. They also seem to be associated with large, synoptic scale cold fronts, as all of the Midwest was much cooler today. However, while Mitchell had a 29 degree temperature drop in one hour, Truax in Madison only dropped 9 degrees, and this drop occured from 10 to 11 PM. La Crosse did not experience any rapid decline, but still dropped 30 degrees--but it was over a period of about 8 hours, like you would expect with a normal cold front. So, I think the pneumonia front originates from the normal cold front, which extended from about Green Bay to Minneapolis yesterday evening. The combination of the strong temperature difference across the front, the east-west orientation of the cold front, and the specific geography of Lake Michigan creates these rapid temperature drops which are only found along the west shore of the lake. I'm kind of curious about this now, maybe I will ask one of my professors.