Snow Depth  Data Frequently Asked Questions

Q1. How does a snow depth sensor work?

Q2. What does ultrasonic mean?

Ultrasonic simply means sound waves with frequencies that are above the range of human hearing. The depth sensor’s ultrasonic ranging system operates at a frequency of 50 kilohertz (50,000 cycles per second). You can 'hear' the clicking of the sensor if you are near it when it takes a reading.

Q3. Does the depth sensor have limitations?

Anything that interrupts the "clicks" from making a return trip from the sensor to the snow and back again will cause erroneous measurements. The following are the most likely causes:
• The sensor is not perpendicular to the snow surface
• The snow surface is rough or uneven and doesn't reflect well, perhaps this is due to uneven melting or drifting.
• The snow surface absorbs rather than reflects sound because there is low density (<5%) new snow on the surface.
• The transducer is obstructed by ice or debris
• Strong winds are blowing the echo out from under the sensor
• While it is snowing, large or numerous snowflakes will cause a bad reading (most often the case for bad readings during a storm event).

Q4. How do I know if a particular snow depth value in a report is valid?

Once you begin looking at the depth values you'll realize that occasionally there are erroneous measurements that show up. All depths are processed through automatic quality check routine that attempts to weed out or flag values that are outside the realm of possible. If you see a -99.9 value the original value was weeded out. In other reports (particularly ones of the TK format) after the quality checking is done a flag is applied to all the values. The suspect flag "SS" means that the data are probably invalid, a valid "VV" flag means that the data passed the quality check and an "EE" flag indicates that the data have been edited and should be considered valid. The first reading of the day (generally the midnight reading) is usually the only data edited.

Q5. Why does the depth decrease after a storm?

Q6. Can the snow depth sensor tell me how much new snow has fallen?

Yes. Use the hourly data and subtract the most recent reading from one before the storm began, the difference is the new snow since the storm.

Q7. How can I learn more about the Judd Communications Depth Sensor?

Q8. What if I have another question that is not answered here?