High Dew Point Temperatures
July 23, 2005

Unusually high dew point temperatures pooled over southern Minnesota during the late afternoon and evening hours of Saturday, July 23. Both Pipestone and St. James reached 86 degrees at 5:00 pm, breaking the old record of 84 that was set at multiple locations on July 20, 2002. During the evening hours on that date, three west central Minnesota locations; Madison, Morris, and Olivia reported dew point temperatures of 84 degrees. Our examination identified one other episode of such extraordinary dew point values. A handful of southern Minnesota locations reported 84 degree dew point temperatures on July 29 and 30, 1999.

Note that there has only been about ten years of higher density hourly dew point temperature readings for Minnesota. The bulk of the automated weather stations were installed in the early to mid 1990's. The Twin Cities has hourly dew point records going back to 1945.

The dew point temperature at the Twin Cities International Airport on Saturday, July 23 topped out at 80 degrees at 9pm. 80 degree dew point temperatures are rare in the Twin Cities historical record. Since 1945, there have been only twenty-one hours of 80 degree dew point temperatures recorded. Ten of those twenty-one hours came in a ten hour period on July 12 and 13, 1995. The highest dew point temperature ever recorded in the Twin Cities was 81 degrees at 11:00 am on July 30, 1999.

Dew point temperatures in the low 80's can occasionally be seen across the Gulf Coast and the Upper Mississippi Valley. A handful of locations in the United States have seen dew points as high as 90 degrees F, especially in Florida and Louisiana. Some of the highest combinations of dew points and temperature on earth can be found in the costal regions of the Persian Gulf, the Gulf of Aden and the Red Sea where dew point temperatures as high as 93 degrees have been measured.

The heat index, a "feels like" measure which factors together temperature and dew point temperature, reached 125 degrees at Pipestone, MN at 5:00 pm on July 23. At that time, the air temperature was 93 degrees and the dew point temperature was 86. A quick scan of the historical database reveals only one other heat index value of 125 degrees. An air temperature of 97 degrees teamed with an 84 degree dew point temperature to create a heat index value of 125 degrees at the Red Wing airport at 3:00 pm on July 30, 1999.

[Addendum: July 28, 2005]
In a follow-up investigation, the State Climatology Office discovered that the Pipestone and St. James dew point temperature sensors were reading three to four degrees higher than neighboring stations during June and July of 2005. Therefore, dew point temperatures in Pipestone and St. James reported on July 23 may have been erroneously high. However, both stations are well-maintained, government-sponsored observation sites whose data are widely distributed. Thereofore, the 86 degree dew point temperatures will remain part of the historical database.


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URL: http://climate.umn.edu/doc/journal/dewpoint050723.htm
Last modified: July 28, 2005