The Hot Plate sensor provides a measure of the density of the snowflakes by forcing the top and bottom of the plates to be the same (hot) temperature. As snow (or rain) falls on the plate, they cool it.
The sensor reacts by applying voltage to maintain the equal temperatures. The voltage is then converted to Watts (a measure of the power taken to evaporate the precipitation). This power is divided by the heat needed to evaporate one gram of water, thus providing the total grams of water falling on the hot plate. Photo taken 7 February 2012.
Credits
NASA / Gail Skofronick-Jackson
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