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July 2007

How Much Does Snow Weigh?

If you've ever shoveled any snow, you know that there is no one correct answer to the question above. In some instances you can have 10" of very light and fluffy snow, while in other cases you can have 3" of wet and heavy snow. The actual weight of the snow doesn't depend on the depth of it, but rather the amount of water that's in the snow. The amount of water in the snow varies because of the difference in snow crystal structure. Snow which forms and falls in colder temperatures is typically fluffier because the snow crystals grow lots of branches. These branches allow for lots of air to be trapped in the snow pack. Snow which falls at warmer temperatures gathers a lot of cloud droplets on the crystals, which cause them to collapse the branches, reducing the amount of air trapped in the snow pack.The first image below shows an example of snow crystals which form in cold temperatures and create fluffy snow. The second image shows flakes which are "rimed" with cloud droplets, forming heavier snow. The images are courtesy of Don Moore, National Weather Service in Billings, MT.

 

Dendrite Snowflakes

 

Rimed snowflakes

 

So, just how much does our current snow pack weigh? You can't guess or even estimate the weight based on the depth of the snow on the ground or your roof. As of 10 am Monday, 5 January 2009, Spokane has officially received 77.6" of snow so far this season. Since 1.5" of that fell and melted in November, we've had roughly 76" of snowfall that is still on the ground. But obviously the depth of the snow on the ground isn't that deep. Officially, there was 25" of snow on the ground at the Spokane Airport. So where did the other 51" go? It's all still there, just compressed. The fluffy snowfalls that we had in December had lots of trapped air. Over time, the snow settles as the fine snow crystal branches break. The weight of the new snowfall also acts to compress the old snow beneath it. You no doubt have experienced this if you try to walk across your lawn or some other location where the snow is undisturbed. The snow we have on the ground is very compressible.

So the only real way to determine how much the snow weighs is to determine how much water is in the snow pack. At the National Weather Service Office as well as at the Spokane Airport, we do this once each day. A core sample of the snow is taken and melted down, which tells us how much water is in the snow. But this is a rather laborious process and most people don't have the equipment to do this. So how can you tell how much your pack weighs?

In most winters, there are periods of melting, refreezing, snow, then rain, etc. This allows some of the moisture to melt into the ground. But this current winter, residents north of about Interstate-90 haven't had much rain or melting. As a result, pretty much what has fallen since the 12th of December is still on the ground in your snow pack.

As an example, below is a comparison of the snow pack measurements at Spokane Airport.

Total Precipitation 12 Dec 2008 - 5 Jan 2009 Water in Snowpack on 5 Jan 2009
4.41"
4.2"

Thus, for most locations this year, it's a fairly good approxmiation that the total precipitation that has fallen since 12 December is the amount of moisture that's in the snow pack. However, once a location gets enough rain and warmth to melt some of that snow pack, this approximation will no longer be valid.

Once you know how much water is in your snow pack, it's an easy calculation to determine the weight. A cubic foot of water (a box 12" on all sides) weighs 62.42796 pounds. Thus, one square foot of water one inch deep would be 1/12th of this amount, or 5.2 lbs. Multiplying this number by the amount of liquid in a snow pack gives you the weight (per square foot) of the snow.

Below is a map which shows the weight of the 5 January snow pack (total precipitation multiplied by 5.2 pounds per square foot) for various official NWS Observations. As you can see, the numbers vary considerably.

 

Weight of Snow

 

Historically, what is the heaviest snow pack observed in the Inland Northwest? Unfotunately, the actual snow core measurements are only taken at the official Spokane location. So while heavier snow packs were likely observed in various locations (e.g. Cascade valleys, Kellogg, Sandpoint, etc), all we can know for certain is from the observations at Spokane.

The heaviest snow pack at Spokane was measured on 12 February, 1969 at 6.95" of liquid, or 36.1 pounds per square foot. This was 11 days after the record snow depth of 42" was achieved. By the 12th the pack had compressed to 19" with additional rain and wet snow.

As a matter of reference, last winter on February 19th, the snow depth at the NWS Spokane office was 15", but there was 5.1" of liquid in that snowpack. The difference between that situation and the current one is that last winter we had several rain events that washed much of the snow off of the roofs. So the snowpack on the ground was not indicative of what was on the roofs at that time.

The forecast for the next few days is for warmer temperatures and snow changing to rain. Initially, the snowpack will act as a sponge and just absorb any rainfall. In otherwords, this will just add weight to the snowpack, even as the snow depth decreases. Eventually, the snowpack will become so warm and wet that it won't be able to hold any more water and it will begin to release it into the ground. Once that happens, the approximations used in this article will no longer be valid.


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