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~ many of the exposure-related problems. There have been a few cases where the sensor head assembly has been com- pletely rimed over. This has not occurred often enough in the areas where the majority of the optical sensors are operating to become a problem. It can, of course, be a serious problem in areas where riming conditions are frequent. Electrical heaters or 1. R. lamps are a possible solution where sufficient power is available. ~ The input signal threshold is somewhat temperature dependent. It increases at a: rate of 2% per oC as the temperature decreases. This has not caused an appreciable error because of the limited temperature range over which significant snow precipitation occurs in the Upper Colorado River Basin. Condensation has not been a problem, either in the sensor head assembly or in the electronics and recorder housing. 2. 9. 1. 3 Calibration Two methods of calibration are utilized: a spinning reticle, and comparison with the snow collected on plastic sheets. The sensors are calibrated on a routine basis by a spinning rod (Figure 14). A rod 3 mm in diameter by 20 cm long is spun from its center by a small constant speed AC motor. The rod is placed so that it intercepts the optical path twice per revolution. The resulting signal produces a recorder deflection of O. 7 full scale when the recorder is calibrated to read O. 3 inches of water equivalent at full scale. A screwdriver adjustment on the front panel of the instrument is used to adjust the recorder to exactly O. 7 full scale. ~ ~ The instrument has been calibrated during storm conditions in the Park Range area at Rabbit Ears Pass and Buffalo Pass. The calibration has been accomplished by comparing the weight of snow collected on a polyethylene plastic sheet during a time interval with the integral of the precipitation rate signal indicated by the instrument over the same time interval. Vari- ous size sheets and various time intervals have been tried; but the bulk of the calibration data were derived from snow collected on three-by-three foot sheets during five and fifteen minute periods. These short period cal- ibrations are consistent with the purpose of the instrument, which is to measure the nearly instantaneous precipitation rate. A box with approxi- mately 811 high sides located on or slightly above the snow surface is used to hold a stack of plastic sheets. A sheet is exposed to the falling snow for the desired time period; then, the sheet is gathered by the corners and re- moved, exposing a fresh sheet. The snow is transferred to a lightweight polyethylene film bag and weighed. ~ The most serious source of error in this procedure is that due to wind effects. 30