Ball St 2005 was 91 degrees. ColoradoHawkeye provided this info below in a different thread a couple of days ago.
Nonetheless, I remember a few others being super hot, so I looked up all Aug/Sept games from 1979-2012...while there were many in the 70`degree and high 60s, most it seemed werehigh 70s/low80s. The highest in IC according to Farmer's Almanac was 92 degrees in 1999 vs Nebby (we lost 7-42). There were only 3 others that were above 90 degrees, 9/17/88 vs Colorado (we lost 21-24), 9/12/98 vs ISU (we lost 9-27), and 9/3/05 vs Ball State (we won 56-0).
I didn't look at temps of any away games other than ISU, and as it turns, the hottest 2 games we played in the state of Iowa were in Lames....9/10/83 (we won 51-10) it was 96 degrees high in lames....9/10/05 (we lost 3-23) in 94 degree heat.