I go by dewpoint (and the related heat index) more so than temperature - something I learned after moving from the arid American West to coastal Alabama and then the Upper Mississippi River Valley. 95F, even 100 was perfectly fine with a low dewpoint, but kills me when it's humid!

Generally:
- Dewpoints 70-75F: I will prehydrate, choose an easier route, and do any climbs very slowly.
- Dewpoints 75-80: Prehydration plus no climbing.
- Dewpoint over 80: Norecreational riding - transportation riding will, if possible, be delayed until it cools down.

Fortunately, western Wisconsin rarely has dewpoints above the mid 70s F. I did a few commutes in Alabama in that last category, and it was awful - much worse than my 112F commute during a Santa Ana event in California.