The Iowa State Fair is underway in Des Moines. A few things have changed during the fair's history - the location and the time of year. Official weather records have been taken in two different locations in Des Moines (near the U.S. Courthouse before 1940 and then at the airport since then). The fair has been also been held in other parts of the state - some in eastern Iowa - before 1880.
It now spans for two weeks in mid August. Previously to 1900, the fair was held in September. As an open air event, it is quite vulnerable to weather. Over eight inches of rain fell on the 1977 State Fair, making it the wettest in history. Nearly half of that total fell on August 26th of that year. Last year's fair is the 10th wettest with 3.08" (2.48" on August 18th, 2015).
In 1983, seven days recorded top 10 warmest temperatures. Not surprisingly, state fairs held in September were some of the coolest on record. Most recently, 2004 landed in the 7th spot for coolest fair with an average temperature of 65.3°.
Temperatures have stayed pretty steady over the past century since the fair has been held in August. Official climatology reports from 1878 have all been from Des Moines. There were no fairs held in 1898 or during World War II.