Watch out for frost gang. Noting our Winter coming on with a quick vengeance I scraped ice off my windows this morning. 

Frost may form on the ground although the reported temperatures are well above freezing. I experienced this a couple years ago resulting in a little high-side at 10MPH in front of Safeway = tank dent, bruises, broken bit$.   

Summed up:
Official temps are measured above the ground in a "Stevenson screen" (shelter). Ground temps may typically be much colder. Water vapor will turn directly into ice without first becoming dew.