SNOTEL Precipitation Sensor Issues


SNOTEL SWE and precipitation sensors are intended for water resource management and not for following hour by hour changes to the snowpack. With some discretion however, these sensors can provide surprisingly accurate near term information.


The snowpack model can use either SWE or precip as input, however SWE has several disadvantages. If it is raining at the SNOTEL site, it may be snowing above, and the sensor will miss some or all of this precip as the water drains out of the snow. Also, if rain soaked snow is later frozen hard by cold weather, the pressure from new snow on top may not be properly transmitted to the pillow below.  The SWE pillow may also be affected by snow creep, which can exert extra pressure not reflective of the snowpack. Because of the location of the Sasse SNOTEL, this may not be much of an issue unless there is a huge amount of new snow. Also, because the snowpack is relatively firm, the pressure from new snow may take awhile to be transmitted down to the pillow. That is, SWE data may lag precip data. From the recent December Sasse data, it appears that this lag may be in the range of 4 hours.


The snowpack model uses precip sensor data for these reasons unless it is obviously wrong. To understand the limitation of the precip gauge, consider how it works. As shown in the figure, it’s a vertical cylinder about a foot in diameter, open at the top, and high enough to capture a season's worth of precipitation in any form. A wind shield surrounds the opening. The gauge is plumbed to a pressure transducer that sends its data to a data logger for later transmission. The tube is primed with a non-freezing solution, and a small amount of light oil is added to reduce evaporation.

The following is taken from these most helpful SNOTEL sensor related web pages by Prof. Mark Williams and by Randall Julander, NRCS Water Supply Specialist:


(These pages also discuss SWE pillows in detail.)


For the most part, I'll skip causes of malfunction of the sensor itself, such as a major mechanical or electrical failure, etc. Unless there is reason to do otherwise the safest bet (as in sailing) is to trust your instruments and try to understand what they are telling you – which may be one of the problems below:

Precip gauge

Precip Value Too Low

Precip Value Too High


snow capped gauge

snow cap on gauge, warmed by sun; cap falls in, sensor reports significant increase in precip during period without precip.

air bubbles in gauge plumbing; the common cause of instability

wind screen related

ice problem (e.g. a frozen plumbing line and resultant increase in pressure at the sensor.)

diurnal fluctuations caused by daily heating and cooling

birds, rodents and detritus falling into gauge and causing plugs in the plumbing line


sensor/transducer instability, etc.

leaking gauge, a failing

transducer, lack of oil to prevent evaporation, etc.


given all these error potentials, sensor flutter of plus/minus

0.2 inch is acceptable [and 0.1 inch is optimistic.]