High Density Snow
4th February 2023
Another slightly damp day in Lochaber today.
The Cairn Slope down from Nid Ridge.
A fair bit of grass on the surface of the snowpack.
The experimental method. I measured snow density using a small commercially available kit. A cutter (not shown) cuts a block of snow of known volume (not sure the actual volume, but would say around 10cm3). This block of snow is then placed in a bag which is weighted using a spring balance which is scaled in terms of density rather than weight.
As the snowpack was, to put it diplomatically, not the most technically interesting I decided to do a bit of an investigation into the density of the snow. Although snow density is not something I measure day to day as a forecaster, a general knowledge of it can be useful for context. I found the slight softer surface layer to have a density of around 380 kg/m3, and the lower icier layers to be around 450 to 500 kg/m3 (there was much more scatter in these results). The density of pure ice (discussed in more detail below) is around 917 kg/m3, this means that even the denser lower layer, the snowpack is roughly half as dense as the material that makes it up, or to put another way, that half this snow is actually free space.
The table below shows the literature values for various types of snow and ice.
| Typical densities of snow and ice (kg/m³) | |
|---|---|
| New snow (immediately after falling in calm) | 50-70 |
| Damp new snow | 100-200 |
| Settled snow | 200-300 |
| Depth hoar | 100-300 |
| Wind packed snow | 350-400 |
| Firn | 400-830 |
| Very wet snow and firn | 700-800 |
| Glacier ice | 830-917 |
Source: Paterson, W.S.B. 1994. The Physics of Glaciers.
Unsurprisingly the Lochaber snowpack is currently quite dense (5 to 10 times that of cold new snow in calm condition). Looking at the transition from dense snow to ice there are various stages (assuming that the snow does not melt first, which in in this case it is likely to do).
Névé is a granular type of snow which has been partially melted, refrozen and compacted, but is generally not yet multiyear. I am not sure why Névé was missed of the Paterson table above but tends to have a density greater than about 500 kg/m3.
Firn (from Swiss German firn “last year’s”, cognate with before) can be loosely defined as dense, multiyear snow ice. It tends to be denser than névé and is an intermediate stage between snow and glacial ice. The upper density of firn is well defined as 830 kg/m3 as and is associated with closure of the pore space.
Glacial Ice Pure ice has a density of about 917kg/m at 0°C and at atmospheric pressure. Ice behaves like most solids, with increasing density as temperature drops, with the density of pure ice being 920 kg/m3 at -23°C and values reach 922 kg/m3 for the coldest ice to be found, in the Antarctic ice sheet. Ice density also increases slightly with pressure having a density of 921 kg/m3 (at 0°C) under the load of 4 km of ice, which is typical of the East Antarctic plateau. However, glacial ice will tend to contain air pockets which will reduce the density to within the range quoted in the table above.
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Sam
4th February 2023 5:41 pm
Great blog BF. As you said – not the most interesting of snowpacks at the moment – so I really enjoyed your technical input. Thanks
lochaberadmin
4th February 2023 6:30 pm
Glad you enjoyed it. Lets hope the snowpack get more interesting soon!