My house is in west Pennsylvania. I've been working on installing Bruce 3/4 solid hardwood floor on the first floor by myself. Underneath are walk out basement and garage. I have a dehumidifier in basement to keep the humidity at 60%. The installation is good so far. My wife has the habit of opening window in summer when the outside is cool. She set the AC to a high temperature actually AC will stop working. Then the cool air come in. I know in this situation the humidity can reach over 75% or even 80% late at night in damp weather. My 3-1/4" oak planks were acclimated in this environment. I just have nailed down 2/3 of the first room. This early morning my humidity meter showed 85%. Gush! I closed the window immediately and set the AC temperature. The humidity dropped below 70% very quickly. Normally we close all windows and set the AC temperature to normal before we leave the house to work.
It looks like my wife will not change her habit after I install the hardwood floor. But my deep concern is will opening windows at night cause my wood floor cupping? My guess is 50-50 of chance. I hope the excessive moisture woods get from night will evaporate after the temperature outside is rising and AC start running. If this can still cause cupping I'll persuade my wife to close window at least in damp weather. But I'm not clear. Can someone give help?

Are you sure your 85% reading is accurate? Is it always like this in Pennsylvania?

You should keep the humidity levels down in the house. Our house is always around 50% and I still see slight cupping only after 5 months on our 3/4 x 4" red oak floors. 3" floors may cup a little less but with such high humidity I would be very careful.

Which environment? High or low?

If you acclimated to high rh and installed at that it should not be cupping when exposed to that level. It should cup when the ac is on, dry on top, wetter on the bottom, no?