(a) Photoelectric detection of the dew point has been used and is based
upon achieving an equilibrium condition of the mirror surface during which the
amount of dew or frost remains constant. This method agrees with visual methods,
within .1 C., down to -35 C.
(b) When the dew point is at or below the freezing point, the formation
of frost is not always positive due to the lack of a crystal nucleus for the frost
crystals to form on. In this case, supersaturation of the