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Hilberg's Tips --  The Difference between the Rain Gauge Amount and Snow Water Equivalent (SWE)
Before you know it wintery
precipitation may be visiting several of you over the next month.  Here is another tip to keep in mind when measuring snow.

If you measure snow, there are five (yes, five) potential separate measurements you can make:

  • 1) The melted precipitation in your rain gauge (daily precipitation).
  • 2) The depth of the new snow that has accumulated in the past 24 hours (since your last daily observation) to the nearest tenth of an inch.
  • 3) The liquid water equivalent of the new snow, obtained from taking a snow core then melting and measuring the water. THIS IS NOT THE AMOUNT FROM YOUR RAIN GAUGE!
  • 4) The total depth of snow and ice on the ground, old and new, reported to the nearest half inch.
  • 5) The liquid water equivalent of the total snow on the ground, obtained from taking a snow core then melting and measuring the water. In the instance where new snow is the only snow on the ground, then 2 and 4, and 3 and 5 could be the same.

We see too many observers copying their precipitation amount into the new SWE field. SWE is a separate measurement! If there has been rain followed by snow, or snow changing to rain, the amount in your rain gauge does not represent the liquid water equivalent of the snow. In windy situations where gauge catch may be poor, SWE from a snow core will likely be a more accurate measurement of the precipitation. In such a case, you can enter the SWE as your precipitation with a note in the comments about the situation and what was in your rain gauge. Here is a recent example. I had an inquiry from an observer who found her liquid precipitation in the gauge was 1.03" and that from a snow core from her from her snow board was 1.16". It was windy during the storm and she wondered if perhaps the wind had prevented all of the snow from making it into her gauge. Yes! The 4-inch gauge is not a great collector of snow in windy situations, so a correctly done snow core is more accurate. In this case, she should enter her SWE as the daily precipitation amount and indicate, in the comments, her gauge amount and that it was likely affected by the winds.

MAIN POINT: If you don't take a snow core and melt and measure the water, then leave "Melted value from the core to the nearest hundredth" as NA.

You can view a short video on how measure SWE on YouTube