Snow: How deep?

During and after storms the National Weather Service in Mount Holly frequently posts snow totals. Chances are that scanning the amounts, you might say, "Wait a minute. We didn't get that much!"

The observations come from weather-service snow spotters throughout the office's coverage area, which extends from the Delmarva to northwest Jersey.

Snow measurement is more art than science, and in this particular storm the totals are likely to be all over the place, probably more so than usual. Here are the issues:

Wind --It's hard to measure snow when winds gust to 50 m.p.h., one spot can be almost bare, and another have a neatly sculpted foot-deep drift. A place where 6 inches was measured three hours ago may have 3 inches now, even though it has been snowing heavily.

Surface -- In late February, snow is going to have a hard time sticking to any dark surface that has absorbed the sun's growing energy in preceding days. Plus, even behind the clouds the sun has an effect. So a ruler stuck into a driveway is going to come with a different reading from one poked into the snow atop a car roof.

Deep concept -- A factor that might trump them all is the distinction between "snowfall" and "snow depth." The latter is the amount on the ground at a given time.

Snowfall is almost an existential concept. It is freshly fallen snow now matter how ephemeral. For example, if it snows 3 inches, and a change to rain makes it disappear, obviously the snow depth is 0, but the snowfall still goes into the books as 3 inches.

Yesterday, from 2 to 4 inches was measured in parts of the region, and some of that compacted or melted away even though it snowed most of the day as temperatures stayed above freezing, and the February sun did its unseen work.

So even if all that accumulationg had skipped along merrily to the next phase of the water cycle before the snow picked up last night and this morning, it would count toward the "snowfall" total.

Once the snow slacks off today, the snow depth should rather quickly go down as the flakes compact and melt. But if the showers and flurries persist into tomorrow and add an additional inch, that would be tacked on to the "snowfall" total.

Thus those "snowfall" totals can be double what you see on the ground after the snow ends.

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