Skip

Idaho Snow Depth FAQ

Snow Depth Data Frequently Asked Questions

Q1. How does a snow depth sensor work?

Q2. What does ultrasonic mean?

Q3. Does the depth sensor have limitations?

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

Q5. The depth sensor from my favorite site is not giving reliable data (-99.9), when do you plan to fix it?

Q6. Why does the depth decrease after a storm?

Q7. Can the snow depth sensor tell me how much new snow has fallen?

Q8. If a site doesn’t have a depth sensor, how can I estimate how much snow is on the ground? Or has fallen in a recent storm?

Q9. How can I learn more about the Judd Communications Depth Sensor?

Q10. What if I have another question that is not answered here?

Q1. How does a snow depth sensor work?

The NRCS uses Judd Communications Ultrasonic Depth Sensors to measure snow depth. The key component of the system is the ultrasonic transducer. The transducer is first used as a speaker to transmit an ultrasonic series of "clicks" down at the snow surface. The transducer then is used as a microphone to listen for clicks reflected back from the snow. By measuring the amount of time that it takes the clicks to travel from the transducer to the snow surface and back again, the distance to the snow can be calculated based on the speed of sound. Since the speed of sound is faster in warm air and slower in cold air, the depth sensor also measures temperature with a thermocouple and adjusts the depth reading based on the speed of sound and current temperature to calculate the distance to the snow. By subtracting the distance to the snow from the height of the sensor above the ground the depth of snow is determined.

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:

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. The depth sensor from my favorite site is not giving reliable data (-99.9), when do you plan to fix it?

Unfortunately, our snow survey office in Boise does not make special trips to repair depth sensors because they are not critical to our water supply forecasting operations. The depth sensors are considered a “bonus" sensor at SNOTEL sites. Our primary sensors are our precipitation gage and snow pillow that measures snow water content. If one of these sensors also starts having difficulties this triggers a special site visit and while we are there we will also generally fix any other problems, such as a broken depth sensor. Depth sensors that are still not repaired by our normal summer field maintenance trips, will be fixed at that time.

Q6. Why does the depth decrease after a storm?

New powder snow can be very light and fluffy but as time passes it settles and compacts becoming denser. As the density increases the snow depth decreases.

Q7. Can the snow depth sensor tell me how much new snow has fallen?

Yes. If the site has a depth sensor 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. Alternatively you can look at the "Rate of Change Reports" under the Daily Data section of the Current Water Year page. These reports show the change in the swe, precipitation and depth readings for the past 4 days.

Q8. If a site doesn’t have a depth sensor, how can I estimate how much snow is on the ground? Or has fallen in a recent storm?

There are some "tricks" to getting a ball park estimate to these two questions.

For a good estimate of the amount of snow on the ground, divide the snow water content by density of the snow. Snow density varies site to site and with time during the winter, but a rule of thumb for most sites is– Dec 1 = 0.15, Jan 1 = 0.20, Feb 1 = 0.25, Mar 1 = 0.30, Apr 1 = 0.35. So as an example, in February a site that has 15 inches of snow water (swe), probably has about 15 inches / 0.25 = 60 inches of snow depth. For the most accurate density value for an area and the elevation you are interested use these:

To determine how much new snow fell in a recent storm, look at the "Rate of Change Reports" under the Daily Data section of the Current Water Year page. If the site has a depth sensor the increase in the snow depth is shown for the past 4 days. If the site doesn't have a depth sensor, note the amount of snow water (swe) that the site gained, then divide that number by 0.10 or 10%, since 10 inches of new snow generally contains 1 inch of water equivalent. For example if a site picks up 1.5 inches of snow water, then according to this rule 1.5 inches / 0.10 = 15 inches of new snow. Take a powder day – we’re sure your boss will understand.

Q9. How can I learn more about the Judd Communications Depth Sensor?

Follow this link to the manufacturer's manual.

The following document requires Adobe Acrobat.
Adobe Acrobat Document Judd Communications Depth Sensor Manual (89KB)

Q10. What if I have another question that is not answered here?

Please contact someone on the snow survey staff.