iMac screen fogging up

Consider the operating temperatures of the iMac and see if you’re not going beyond the limits. Electronics cannot (and shouldn’t) be operated beyond temperature limits. As much as heat is bad, extreme cold is not good either.

As already suggested, do not expose your iMac to “very cold air at night”.

I was having a similar issue when bringing my screen in from a colder temp indoors and turning it on immediately. This worked:

Turn off the display (or shut down the mac if the display is integrated), then leave it for 15-30 mins. The display will defog and get acclimated to the temperature. Then you can safely turn it back on again and it will not fog up.

I have seen this all the time in Minnesota when macs are cold much of the year. The blowers kick up air that condenses on the cold glass - especially at the bottom of the screen.

As long as it's not condensing and goes away within minutes - it should be fine. If you are worried, just turn off the Mac and let it adjust to the room temperature and humidity. 30 minutes should be enough. It is rare to see “fog” from a running / warm Mac so smell or observe for smoke or other particulate contaminants like flour or fine electric smoke and evaluate if you need to shut down the Mac and fix the environment first where the Mac draws cooling air or relocate the Mac.