The reason why this happens is condensation. When hot and cold mix, they produce moisture. Kind of like a cup at a resterant, after a while it starts forming dew.
Moisture
The AC runs to remove moisture from the air inside your vehicle. Moisture causes the fog inside of your windows. The ac runs to remove the moisture and the heater runs to heat the air.
High humidity levels, installing an HRV will fix that.
It has a bad seal or a crack allowing moisture to get in.
The AC takes the moisture out of the cabin air and that helps clear the inside of the windshield from moisture faster.
Check inside the car undre the dash , check the heater core.
It's condensation caused the variance of temperatures.
Try putting new weatherstripping and check if there is water in trunk or doors
when your in it, your breath. if not you, then you have moisture stuck in your car (seats, carpet, etc. that is retaining heat for a while and releasing slowly over time.
You're probably seeing moisture condensing. When your hair is wet, or if there is moisture in your car, and there is heat or just simply evaporation of water, you'll see some moisture collect on your window. Draw funny faces or write words on the window if you'd like.
== == The air temp outside is cooler than the air inside the car, so moisture forms on the inside of the glass surface. If this happens while you are driving, you may be adding to the problem by your breathing. Your breath contains a lot of moisture and it lands on the inside of the car windows. Solution............ run the defroster fan on hot and direct the warm air onto the windshield, and the side windows, PLUS open the windows at least an inch to let the dampness out of the car.
It could be that you have a leaking heater core, which resides inside your car. Another give-away for a leaking core would be a sweet smell inside the car and/or a sticky residue on the inside of your windshield.