If you think the first half of the summer has been unusually wet, it is not merely your impression — it is a measured fact.
Between May 1 and July 15, Madison received 21.85 inches of precipitation, punctuated by the 18th-wettest day in Madison history (3.3 inches on July 14) — which is 10.16 inches above normal.
In other words, since May 1 we have received nearly twice the normal rainfall in the wettest part of the year. In fact, this amount is the all-time record for May 1 to July 15 in Madison and one of only two such periods (the other was in 2000) during which 20 inches of rain or more has fallen. Perhaps unsurprisingly, in the 76 days of this interval, we have had only 30 completely dry days — well less than half. Consistent with this incredible total, only one time in this period have we had as many as five consecutive days during which not even a trace of precipitation fell — May 14-18.
Yet another interesting fact regarding this remarkable wet period is that over the same interval the daily average temperature has been 2.54 degrees Fahrenheit above normal. Historically, rainy late spring/early summers have been closer to average or even a bit cooler than normal as a result of the persistent cloud cover that usually accompanies such prolonged rainy periods. The four other rainiest May 1 through July 15 periods, for instance, averaged 0.55 degrees below average (2000), 0.44 degrees above average (2013), 1.76 degrees below average (2008) and 0.30 degrees below average (1993) — nothing at all like this year.
Since warmer air has a greater capacity for water vapor which is then available for conversion into precipitation, the combination of unusual precipitation amounts and higher than average temperatures is clearly suggestive of global warming.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
