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~ optical snow rate sensor in detail. 7 2. 9. 1 Optical Snow Rate Sensor 2. 9. 1. 1 Operating Principle The optical snow-rate sensor is designed to measure precipitation rates with high time resolution during snowfalls. The optical sensor operating principle is based on the instantaneous blocking of ;'lTght beim.byOi.ndivid.q~l SIlowflakes. The resuffing elecTrical signalis a time function of the num- be~~rid;ize of crystals. This signal is processed to provide a recorder output as a linear function of precipitation rate. A one-meterlight beam is generated bya 6-volt incandescent bulb and para- DOfic'reflector. The s~~sor-is a 20 mID diameter CDS-photo cell,-sllielded by a 28-ITl"~diameter tube to limit extraneous light ancrproteclfroms11,o\v'- buliCf-':'ups':--' --,-- . ----.--- ,. Small snowflakes generate a signal of approximately 100 U v peak at the out- put of the sensor. This signal is amplified by a transistor 3-stage common emitter amplifier. The third stage is biased slightly "on" and it is driven to saturation by the majority of the snowfalke signals. The typical output signal from the amplifier is thus a square-topped pulse with an average width of 2 ms. This pulse drives an integrator with a time constant of approximately 5 seconds. The output of the integrator is connected through an emitter follower to a 0-1 ma Rustrak recorder. ~ Power for the lamp and electronics is supplied by a 6-volt automotive type lead-acid battery. The Rustrak motor is driven by 1.15 VAC 60 Hz when available. A 6-volt DC motor is used at remote locations and is also driven from the 6-volt battery. The timing accuracy of the DC opera.ted motors is not adequate, and a battery-driven esca.pement movement timer is used to provide accurate 5-minute marks on the chart paper. 2. 9. 1. 2 Performance There have been cases where windblown snow has either completely or partially filled the sensor shield. Of course, the recorder output falls to zero if the shield is fully blocked. Occasionally, a partial blocking and wind vibration of the blocking particles creates a spurious signal, constant at approximately one half scale. This condition can be recognized and that portion of the record discounted. Another condition that can be easily re- cognized are wind spik'es, caused by blowing snow, that occur during gusty wind conditions. These spikes appear as rapid changes in the recorded output that are easily distinguished from the nor~al, slower rate, fluctu- ations created by falling snow. As with any gauge, careful siting eliminates ~ 29