Embodiments of the present invention provide a frost point hygrometer apparatus utilizing dry ice and ethanol, or liquid nitrogen, as cryogenic coolant. FPH apparatus in accordance with embodiments of the present invention includes a copper cold finger with a sink immersed in a liquid cryogen to provide cooling power throughout the profile, a polished mirror disk residing at the opposite end of the cold finger with ambient air passing over it, a nichrome heater wrapped around the narrow shaft of the continuous cold finger and mirror piece to provide heat to the mirror, an optical source and detector, including an infrared light-emitting diode and a photodiode, to monitor the mirror's reflectivity as condensate accumulates in the form of dew or frost, a biconvex lens to focus the light reflected from the mirror into the photodiode, and a calibrated thermistor embedded in the mirror to measure the frost point temperature.