Frost

Expect frost to form on an aircraft when the following conditions exist:

The aircraft skin temperature is below freezing. Note that the aircraft skin temperature can be colder than ambient due to radiation cooling or cold soak fuel. Ice can form even when the outside air temperature (OAT) is well above 0°C (32°F). An aircraft equipped with wing fuel tanks may have fuel that is at a sufficiently low temperature such that it lowers the wing skin temperature to below the freezing point of water. If an aircraft has been at a high altitude, where cold temperature prevails, for a period of time, the aircrafts’ major structural components such as the wing, tail and fuselage will assume the lower temperature, which will often be below the freezing point. This phenomenon is known as cold soaking. While on the ground, the cold soaked aircraft will cause ice to form when liquid water, either as condensation from the atmosphere or as rain, comes in contact with critical surfaces.
The air temperature is close to the dew point temperature (within 3°C)
The dew point is below freezing

Hoar frost appears as a thin, uniform deposit of ice with a fine, white, crystalline texture. It will need to be removed prior to flight, but will not reform.

Manufacturers often publish guidelines with respect to an acceptable amount of frost for takeoff when it is confined to the lower wing surface within the fuel tank area. However such guidance is typically reserved for larger transport category aircraft. Always consult the aircraft flight manual or pilot operating handbook regarding an aircraft’s ability to operate within icing conditions.