The Behavior of the Snow White Chilled-Mirror Hygrometer in Extremely Dry Conditions
Oltmans, S. J.
Barnes, J. E.
- Publisher: American Meteorological Society
Journal of Atmospheric and Oceanic Technology,
The Snow White hygrometer, made by Meteolabor AG, Switzerland, is a new chilled-mirror instrument using
a thermoelectric Peltier cooler to measure atmospheric water vapor. Its performance under dry conditions is
evaluated in simultaneous measurements using the NOAA/CMDL frost-point hygrometer at Boulder, Colorado;
San Cristo´bal, Gala´pagos Islands, Ecuador; Watukosek, Indonesia; and Mauna Loa Observatory, Hawaii.
The Snow White exhibits a lower detection limit of about 3%–6% relative humidity, depending on the sensor
configuration. This detection limit is determined by the temperature depression attainable by the thermoelectric
cooler. In some cases, loss of frost-point control within layers with relative humidity below this detection limit
caused inaccurate measurements above these dry layers, where the relative humidity was within the detection
range of the instrument.
The sensor does not operate in the stratosphere because of the large frost-point depression and the large
potential for outgassing of water from the instrument box and the sensor housing. The instrument has some
capabilities in the tropical tropopause region; however, the results are somewhat mixed.