Frost is defined as a layer of ice that forms on surfaces that are at or below 32°F. However, frost can occur on surfaces even if the official temperature is above freezing, 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 the thermometer may be located. Therefore, the ground or any surface must be at or below 32°F for frost to form. However, if the thermometer is just a few feet above the ground, it may not give an accurate reading for frost. The temperature where the frost actually forms, near the ground, is what is important.

It is also important to note that frost will only form if the ground temperature matches the dew point. The dew point is the temperature at which the air is saturated with respect to water vapor over an ice surface. It is more difficult for water molecules to escape a frozen surface.

In summary, frost forms when the ground or any surface is at or below 32°F, and the ground temperature matches the dew point. However, frost can occur on surfaces even if the official temperature is above freezing, because the temperature near the ground can be lower than the temperature a few feet higher where the thermometer may be located.