The definitions of frost and freeze in the National Weather Service lexicon are confusing due to the facts that frost does not always form on surfaces that are freezing, and it is possible for frost to form when the air temperature is above freezing.
Air temperature is officially measured at 2 meters (about 6 feet) above the ground. On a clear and calm night, the ground temperature can be colder than the air at 2 meters. Other surfaces, such as roofs or car windows, may be even colder than the ground, depending on the radiative properties of the various surfaces.
Further complicating the matter is the fact that if the air is too dry, frost will not form even at temperatures well below freezing. Moisture saturation must occur in the air in order for water vapor to freeze onto surfaces.
The National Weather Service issues a Frost Advisory when temperatures of 32 degrees are expected to occur in scattered areas and a Freeze Warning when most areas will go well below 32 degrees.