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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?
Q2. What does ultrasonic mean?
Q3. Does the depth sensor have limitations?
- 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?
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?
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.
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- Idaho Basin Snow Density vs. Elevation Graphs
January 1 February 1 March 1 April 1
- Idaho Basin Snow Density vs. Elevation Graphs
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.
Judd Communications Depth Sensor Manual (89KB)