Can't say exactly when it occurs except when it is too cold for a/c and too hot for heat. LOL Sometimes it fogs up on the outside sometimes inside, but ALWAYS in the very center of the windshield. Sometimes if I see it fogging up and I turn on the A/C it goes away even without turning on defrost. Sometimes it will get worse, and I roll down windows and it helps but doesn't go away. If it is on the outside the defroster will not take care of it so A/C and windows sometimes help. You just have to try different combinations. Embarrassing and aggravating when you have passengers. Have replaced the Heater Core, and had to replace the fan relay switch 4 times. But even though it has problems, it is comfortable to drive and I would buy another one in a hear beat. Makes me money and looks good. Love the fact it doesn't have electric doors. Manual doors last longer and take the abuse. Our company has vans with electric doors and they have all had to be fixed at some time or other, plus customers try to close them and sometimes damage them. I am so glad my van doesn't have many bells or whistles. Practical and cheaper to fix.