Reo on Yahoo says he has a plastic sheet to cover his windshield to prevent frosting (and morning scraping). But "I usually guess wrong," he says. "Either no frost has formed overnight or, more likely, I don't put on the cover and frost does form." He asks how to predict when frost may form. Watch the overnight forecast for clear skies, calm air and cool weather. Temperatures below freezing aren't needed. Radiational cooling chills glass even when the air is a few degrees above 32 degrees.