Snow Density so Far This Season

Last year (Winter 2007-2008) we had a lot of snow, amazingly so. More snow than many have experienced around here (Moscow, Idaho), and more than one would expect be soon repeated. But this year we have had a lot of snow also. As an engineer who does a lot of `structures' work, and as someone who is just plain curious about snow, and weather in general, I have been taking some measurements. And not only have the measurements assuaged some curiosity, they have also been of interest to building officials and other engineers.

Around here we might go through an entire winter with hardly any snow at all ... a few inches falling in a day, and melting off later that day or within a day or two. Or some years we may get snow, maybe half a foot deep, and it may last for weeks or so. And some years we may have snow on the ground for as much as 100 consecutive days. But I would say rarely do we have a foot of snow, and both this year and last we have had over two feet. (And this year isn't over.)

But back to th snow density. This year I found that the snow fell with a density (Specific Weight) of about 7 lb / cu. ft., or with a Specific Gravity of 0.11. (Specific Gravity is the ratio of some thing's density with respect to the density of water.) Or, stated differently, the snow fell and accumulated at the rate of 9 inches of snow per 1 inch of equivalent standing water. Wow! ... that's cool - I always heard it said `10 inches of snow equals one inch of water'.

As it kept snowing I kept measuring, ... to feed my curiosity of what happens as it gets deeper. Does it pack (itself)? And what happens when it starts raining, or melting? And now I have a better idea. Here, so far, it fell and accumulated at an overall Specific Gravity of about 0.11, regardless of depth. Well, that is, until it started melting, or we got rain. Then it got denser.

Here are the numbers.

They are presented by Date, Depth, Ground Snow load, and Specific Gravity.

12/23/2008 ... 13 in. ... 8 psf ... 0.11

12/27/2008 ... 20 in. ... 12 psf ... 0.11

12/28/2008 ... 15 in. ... 16 psf ... 0.20

01/02/2009 ... 14 in. ... 15 psf ... 0.21

01/05/2009 ... 27 in. ... 26 psf ... 0.185

01/06/2009 ... 24 in. ... 30 psf ... 0.24

01/07/2009 ... 13 in. ... 16 psf ... 0.23

01/11/2009 ... 9 in. ... 13 psf ... 0.28

The first two measurements show that as we took on snow it accumulated at a Specific Gravity of 0.11.

Then we had some warm weather and melt. The depth dropped and the density increased.

Then we got some more serious snow. The density shown is the average through the profile, so it includes the heavy stuff already on the ground. I suspect that if I was able to somehow measure the density of just the new stuff, it would be significantly less than the average.

And then we went into another period of rain and melt. The last measurement shows the density is almost three times that of how it (first) fell. (And we could further say that only (1 over 0.28 =) 3.6 inches of snow equals one inch of water.) Interestingly, this snow was still definitely snow (in feel and appearance). It was not ice, or slush. I suspect that ice and slush (which were around but I didn't measure it) would have even greater Specific Gravities. Obviously???!!!

Note that the last value of each line (the Specific Gravity) is either italic or bold. I also compared the numbers to the values using `best fit' curve information provided in a study by the University of Idaho for Ground and Roof Snow loads. That report gave average Ground Snow load per depth information, which I then manipulated into Specific Gravity information. And so the values in italic reflect my measurements that were less than the average; and those in bold were greater. And so what this means (to me, anyway), is that density can and does vary even in the same place, and not so much by depth, but definitely in accordance with melt and rain. And, in fact, that the Univ. of Idaho average information should be considered just that ... AVERAGE ... and not used to specifically predict any particular density (or snow load).

All these measurements were taken basically in one place in Moscow, where it was fairly open. I was curious, also, as what the densities might be like under (leafless but big) deciduous trees, which would carry the snow initially, and then let it melt and/or fall. Is suspect that the densities were greater in such places, but the standing water equivalents less. But, I had other things to do, and couldn't measure everything everywhere.

And I still haven't addressed greater geographic variation. I suspect at higher (colder) elevations the snow falls with even less density. And I suspect that closer to the ocean, where clouds are really packed with moisture, snow might fall denser. And I think that drifting snow is denser (but I'm not sure). I am fairly confident that when I shovel snow the pile I make is denser than the stuff around it and made it from.

Maybe winter is over, and maybe not. Last year this time it was hardly over. If we get some more serious snow, maybe I'll get some more measurements. And if I happen to be traveling, maybe I'll get some measurements in some other geographic regions (besides Moscow).

So much to think about, and measure; so little time.

References

Varying Snow Density, Jeff Filler, Associated Content.

Ground and Roof Snow Loads for Idaho, R.L. Sack, A. Sheikh-Taheri, University of Idaho, Department of Civil Engineering, Moscow, Idaho 83844.