Frost is defined as a layer of ice that forms on surfaces that are at or below 32°F (0°C) . The ground or any surface must be at or below 32°F for frost to form. However, sometimes frost can occur on your lawn overnight, even though your thermometer may never have dropped to the freezing mark. This is because cold air on clear, calm nights sinks to ground level. Temperatures at the ground can be lower than the temperature only a few feet higher where your thermometer may be located. The National Weather Service (NWS) says that a frost can form when the temperature falls below 36°F (2°C), with it being more than likely in rural areas. Frost becomes more widespread when the temperature falls below 32°F (0°C) . A frost can occur anytime temperatures drop to 37°F (3°C) or lower, and most often, frosts are associated with clear skies and calm winds overnight and during the early morning hours on a cold day. On the other hand, a freeze only occurs when surface temperatures drop below 32°F (0°C), or the point at which liquid water freezes.

In summary, frost occurs at or below 32°F (0°C), but it can also occur at temperatures slightly above freezing due to local conditions.