"Snow White" is a chilled-mirror hygrometer for radiosonde
application, and is a product of a Swiss company
See also their
Snow White page.
Snow White may be used as the reference hygrometer for
radiosonde humidity sensors, and
as the hygrometer for upper tropospheric and lower stratospheric
The chilled-mirror method is directly based on the thermodynamics,
and thus is regarded as the standard method for measuring
water vapor concentrations.
A small mirror (3 mm x 3 mm for the Snow White) exposed
to the ambient air is cooled continuously so that
it's temperature equals to the dew or frost point temperature.
A lamp and phototransistor are used
to monitor the thickness of the dew or frost layer on the mirror.
The advantage of the Snow White hygrometer is as follows:
The meteolabor AG has nearly 20-year experience of developing
chilled-mirror sensors for aircraft, laboratory, and surface
measurements. The development of a sensor for radiosonde application
began in 1996.
- Snow White uses a Peltier cooler, which is a thermoelectric
device producing a temperature difference between its two sides.
The hot side of the Peltier device is cooled by the ambient air,
and the mirror attached on the cold side is cooled electrically.
Snow White works with a 9 V dry cell battery
(for the control circuit and lamp) and a 1.5 V dry cell battery
(for the Peltier cooler and sensor-housing heater).
Compared with cryogenic frost point hygrometers,
Snow White may have difficulty in extremely dry regions of the
atmosphere such as the middle stratosphere
(less than 3-6% relative humidity, for example)
due to the limitation of the Peltier cooler.
However, Snow White should be capable of measuring water vapor profiles
from the surface up to the lower stratosphere.
- Thermocouple thermometry is used for the temperature
measurements of Snow White, and the mirror itself is
one of the two contact points of the thermocouple.
In other words, the mirror is the thermometer at the same time.
Therefore, the error and delay of the temperature measurements
are generally small.
- Snow White has the characteristics of low cost (similar to
that of ozonesondes), small size, light weight, and easy operation
(as a plug-in instrument; without any calibration needed on-site).
Everyone who is interested in measuring water vapor profiles
can use Snow White quite easily.
- As a common nature of chilled-mirror sensors, it is not easy
to distinguish between the existence of cloud droplets/particles
and the supersaturation. However, with a heater on the sensor housing,
Snow White may be able to provide the information
on the total water content in clouds.
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