Special Model "SOWER-Snow White"

SOWER is collaborating with Meteolabor to improve the performance of the Snow White hygrometer by making world-wide measurements and by proposing several modifications. Here are some details of the special model, SOWER-Snow White.

(1) House-Keeping Data Outputs

The special model provides us the following three house-keeping data outputs during the sounding (available since August 2000).

  1. Phototransistor output voltage (spec(ular)/mirror voltage)
    This parameter represents the intensity of the light reflected on the mirror, that is, the thickness of the dew or frost layer on the mirror. Thus, this voltage data should be constant while the humidity content is within the limit of the Peltier cooler. In practice, we need the following empirical temperature compensation to the measured voltage.
    ptv2 = ptv * ( 1.46 + tref * ( -0.0133 + tref * 81.7 * 10^-6)) 
    ptv2: compensated phototransistor voltage 
    ptv : measured phototransistor voltage
    tref: Snow White reference temperature 

    When the mirror temperature is at the dew or frost point temperature, the compensated voltage should be around 1.5 V. If the mirror becomes completely clear up due to insufficient cooling ability, the voltage will become around 2 V, which is the dry mirror voltage.

  2. Peltier current
    An indicator of the strength of the Peltier cooling. The larger the current is, the stronger the cooling is. The maximum value for this model is about 1.4 A.

  3. Peltier hot-side temperature
    An indicator of the performance of the aluminum radiator. The Peltier cooling could be minimized if this temperature equals to the ambient temperature. In reality, this temperature is 5-10 K greater than the ambient temperature.

(2) Temperature-dependent Phototransistor Output Gain

In previous models, this gain was constant throughout a sounding. This means that the lower the water vapor pressure becomes, the slower the time response becomes. Thus, layering structures in water vapor in and above the upper troposphere might have been smoothed due to slower time responses. In March 2001, this gain became dependent on the ambient temperature. In the near future, this gain will become dependent on the dew or frost point temperature.

(3) Day-type versus Night-type

The "day-type" Snow White has a styrofoam cover on the sensor housing and the hot-side radiator to prevent them from solar radiation. We propose to equip a special air inlet tube made of black-colored stainless steel sheet to avoid possible contamination problems. See the next section, "Operating Systems."
The "night-type" Snow White exposes the sensor housing and the radiator completely and should be used at night. A sophisticated night model has been available since October 2000.

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