What are the wettest and driest days of Madison’s warm season?

All things considered, we have really had a rather benign summer in southern Wisconsin thus far this year. Through Friday of last week, July was averaging just over 1 degree warmer than normal, with the majority of the contribution to this slightly warm month coming in the overnight lows, which have been 1.5 degrees above normal thus far. In addition, except for the 1.23 inches of rain we received when the heat broke July 23-24, we would be just about normal for July.

Somehow we got thinking about some characteristics of warm-season (May through October) precipitation in Madison in the midst of this spell of fine summer weather. Dr. Ed Hopkins at the State Climatologist’s Office was on the ready for our question, which was: What are the wettest and driest calendar days during the warm season in Madison? By “wettest” we mean the calendar day on which the most total accumulated precipitation has been recorded in Madison’s 153-year climatological record — and the least for the “driest” day.

It turns out that July 21 is the wettest day of the calendar year. This is consistent with the prevalence of thunderstorms near the height of the summer in most years. The driest day is Oct. 9, which is a little more difficult to explain. Certainly at that time of year we are not usually subjected to locally heavy rains from thunderstorms, but it is not clear what might distinguish early October or the weeks preceding or following it with regard to precipitation.

By that time of the year the Northern Hemisphere is just beginning to adjust to the absence of sunlight at high latitudes, but that is probably not a factor for weeks to come. It does correlate with our anecdotal sense that the weather remains quite nice in this region into mid-October in most years.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Seasons

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