Frost point hygrometer

From Glossary of Meteorology

frost point hygrometer

An instrument similar to the dewpoint hygrometer, but especially suited for low frost points.

The strong cooling necessary for these measurements is provided either by multistage Peltier devices or by evaporating low-boiling-point (cryogenic) fluids like liquid nitrogen. Using this technique, frost points down to -100°C can be measured. Electrical resistance wires are used to provide heating.