Erica's Answer:
If the atmospheric temperature is below freezing, and we cool down to the point of fog formation, the water droplets in the fog will only freeze when they touch an object that is below the freezing point. That is why, in freezing fog situations, you'll see a layer of ice crystals on your windshield, sidewalk, etc. The tiny water droplets in fog will remain in liquid form, in what's called a "supercooled" state, until they touch a surface below freezing.- Erica
