...Snowpack Strength...

The air in the pore spaces has water vapor in it. In fact, like the atmosphere in general, this air will remain saturated with respect to the ice around it. Things get really interesting because the vapor pressure over different places on the ice crystals is not the same, so water molecules migrate around and redeposit due to the pressure differences, very much like water vapor in atmospheric weather systems. This vapor movement changes the shape and structure of the individual ice crystals as well as the strength of layers and the entire snowpack. It is a constant process and cannot stop, although very cold temperatures tend to slow things down because the air spaces cannot hold as much vapor.

Since the snow is constantly changing, it can either be gaining or losing strength. Internal changes in the strength of snowpack layers can also be caused by the mechanical action of packing the snow underfoot, but they are usually related to temperature.

How Snow Gains Strength

When the temperature at one point in the snowpack is not much different than the temperature of the snow above and below it, then we say we have an equitemperature--or isothermal--snowpack, and a shallow or non-existent temperature gradient. Gradient is just another word for change over a distance.

The ground is warm. If you could harness the energy rising from just a 2 meter patch of ground you could run a T.V. indefinitely. Since the ground stays right around freezing, warmer air temperatures and deeper, denser snow will encourage an equitemperature snowpack.

In a relatively warm and deep snowpack, water vapor migrates from pointy areas of high vapor concentration (like the tips of snow crystal arms) to low, hollow areas of low vapor concentration (like the point where two round crystals touch each other). Over time, elaborate snow crystals will become more rounded and individual crystals will become "glued" together. This is the strengthening or rounding process.

Snow can also gain strength through the melt-freeze process. In the spring, free water is released into the upper layers of the snow pack by the heat of the day. When this water refreezes at night, it tends to do so on and between larger snow grains, so smaller grains eventually disappear and "corn" snow develops. This snow is unstable during the afternoon when water lubricates a thick slab of individual snow crystals. After a hard night freeze, however, the snow grains are cemented together and very strong. This is why spring avalanche hazard is very predictable.

How Snow Loses Strength

If the temperature is markedly different as we pass downward through the snowpack layers, then we have a steep temperature gradient. A temperature gradient can exist through the entire snowpack or merely between two layers of the snowpack. The critical difference is 1 degree C in 10 cms. Since the ground is warm everywhere, shallower snowpacks in colder climates are far more prone to temperature gradients.

If a sufficient temperature gradient exists in the winter snowpack, the free water molecules in the pore spaces tend to move upward in response to lesser vapor pressure above. Encountering colder air, these molecules deposit on the under-side of the snow crystals above them. Over time, the crystals will develop a faceted, square appearance and will lose their ability to interlace with each other. This is the weakening or faceting process that creates uncohesive, sugary snow--called depth hoar--seen in Continental snowpacks, usually near the warm ground.

Once formed, depth hoar crystals tend to persist for very long periods of time. Rounds and facets can and do change into each other if the temperatures allow it. The problem with large facets is that there is not as much surface area relative to mass for bonding. It's kind of like pouring glue over a barrel full of basketballs--they just won't be as strong as a barrel full of glued BBs.