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More on Measuring Snow

Nolan Doesken,

I'm up here in the Stove Prairie Area (#47) and I've got a question regarding snowfall measurements. After a heavy snowfall this morning, I measured the new snow depth on the snow board as 1.7 inches. I then swept away an portion of the snow board and measured this again several hours later and came up with 3.2 inches additional snowfall. But when I measured the area that hadn't been swept, I got 4.5 inches (as compared to a total of 4.9 inches). I did this because I knew the snow was wet and would compact. Which is the correct way to measure and report new snow depth (for CoCoRAHS and an official weather station)?

I delight in the fact that you picked up immediately on an observing problem that has long haunted official National Weather Service snowfall observations. Depending on when and how often you measure, you can come up with distinctly different measurements for the very same snow. We are actually writing a research paper on this topic for publication in the Bulletin of the American Meteorological Society later this year.

That being said, what I believe makes the most sense is to report the greatest accumulation on the snowboard noted prior to clearing it completely at 7 AM or thereabouts. For example, if you checked but did not clear the board at 10 PM and found 3.7". Then the snow stopped and
when you came out at 7 AM it was only 3.2". The correct report for the 24-hour snowfall for this situation would be 3.7" -- the greatest accumulation prior to melting or settling.

But for your example above, the best reading is 4.5" -- since there never really was 4.9" on the board during the 24-hour period. From a National Weather Service perspective, either answer could be correct. Their guidelines state that you can measure and clear your snowboard up to a maximum of 4 times per day at 6-hour intervals. Anything more than that inflates the readings too much. We have found that the difference between summing 4 6-hour totals versus one 24-hour measurement can be 15-20% for many storms, so we still favor reporting the greatest accumulation on the board prior to melting, settling or blowing. Only problem is that you would technically need to be there measuring continuously to see when that occurs. But in practice, the snow settles and melts very little at night, so early morning readings often are very
close to the max accumulation for the day.

There. You got more than you asked for, but I hope that helps.

Nolan

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