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Snow Metamorphism – Ever Changing Snowpack

September 6th, 2006

I wanted to post this document as a great primer into Avalanche Forecasting

 

 Thanks,

 Jeff Pierce

Avalanche Advisor

 

 

Snow Metamorphism:

The Force Behind Our Ever-Changing Snow pack

 

The mountain snow pack is in a constant state of change. From its creation in the Fall to its demise by Summer, avalanche forecasters study its mood swings and monitor influences that cause it to transform throughout the Winter. You know that many rocks go through geologic changes, or metamorphose, from the influence of tremendous heat and pressure. The term metamorphism, borrowed from geology, is also used to describe changes that take place within the snow pack. Like the earth’s crust, the snow pack is comprised of different layers, each having its own unique characteristics in hardness and density. Some layers are formed by diverse snow crystals falling from the sky; some develop from drifting. Sometimes the snow melts on the surface and then re-freezes to form an ice crust that later becomes buried. Each layer, regardless of origin, is ultimately influenced by metamorphism. But unlike rock, snow exists very close to its melting point. Thus, it takes only subtle differences in pressure and temperature to bring about change.

Soon after a snow crystal lands from the sky, it begins to change. It continues to change, or metamorphose, along with its neighbors until it finally melts in the Spring. There are three types of snow metamorphism- equilibrium, kinetic, and melt-freeze- that take place in the snow pack. Equilibrium metamorphism simplifies the original crystal making it more round. Thus, we refer to the resulting snow grains as “rounds,” and the process as “rounding”. Kinetic metamorphism turns the snow grains (new or old) into angular shapes with sharp corners and flat faces, or facets like on a diamond. We typically refer to these as “squares,” or “faceted grains,” or simply “facets.” In the Spring melt-freeze metamorphism builds large, round grains on the snow surface called “corn snow”.

Before we examine the three types of metamorphism in more detail, here’s some background information that will help you understand how these processes work in snow.

  1. Snow pack properties commonly found in our continental climate zone:
  • Snow depth varies greatly, even over short distances.
  • Snow density varies from layer to layer – fresh powder is about 70kg/m3 (7% water, 93% air); a hard layer created by drifting is about 400kg/m3 (40% water, 60% air).
  • Snow grains from different layers vary in size and shape.
  • Air in the pore space between grains is saturated (100% relative humidity)
  • Warmer pore spaces hold more water vapor than colder pore spaces.
  • Snow temperature is generally warmer close to the ground, near 0 degrees Celsius, because porous snow is a good insulator (only 1/10,000 as efficient as copper for heat conduction).
  • Snow is colder nearer the surface because of cold air temperatures and from longwave radiation heat loss to the atmosphere.
  • The snow pack temperature gradient is usually non-linear as it varies from the warmer ground to the colder top surface.
  1. The driving force behind the type of metamorphism that will take place, equilibrium or kinetic, is the temperature gradient in the snow pack. A small gradient of 10c/10 cm) leads to kinetic metamorphism (faceted grains). Influences that control the temperature gradient include:
  • Snow depth – highly variable
  • Terrain – aspect, elevation, or geothermal areas
  • Weather- warm or cold, clear or cloudy, dry or snowy periods all affect the snow differently.
  1. Another key player is vapor pressure. This is the pressure of confined vapor, such as found in the air spaces of the snow pack. Some important concepts to remember:
  • Vapor pressure is lower over a colder ice or snow grain than a warmer ice or snow grain.
  • If an ice grain warms, water molecules sublimate into pore space.
  • If the ice grain cools, water molecules redeposit onto the ice grain.
  • If the pore space becomes supersaturated (>100% relative humidity), water molecules are attached to the colder grains with a lower vapor pressure where the deposit onto the ice.
  • Vapor pressure is greater over a convex ice shape (points) than over a concave ice shape (cups)
  • Vapor flows more freely when the layer density is lower.
  1. Snow temperature of a layer helps to determine the rate of metamorphism. If the snow is warm (e.g., -10C to -50 C), the process occurs faster than if the snow is cold (e.g. -100C to -150C). Metamorphism comes to a virtual standstill at -400C. But Colorado’s snow pack rarely dips below -200C, and then only near the surface. Now let’s venture out into the field, dig some holes in the snow and gather data. We’ll apply the concepts above to scenarios that can be found in Colorado’s snow pack.

Snowpit No. 1

This snowpit is dug on flat ground on a mild, -50C day. The measurements taken are:

  • Snow depth = 100cm
  • Snow temperature near the ground = 00C
  • Snow temperature near the surface= -50C

What can we determine about this snow pack? The temperature gradient is 50C/m (0.50C/cm). Since the gradient is weak, rounding will dominate. The snow is relatively “warm” so there is sufficient water vapor for transport. There will be a transfer of mass (water molecules) from areas of high vapor pressure (convexities) to areas of low vapor pressure (concavities), through sublimation (the necks of the grains will grow and the connection point between grains will strengthen). This process strengthens the snow pack. Rounded grains with strong bonds between grains form strong snow layers.

Snowpit No. 2

We’ve dug this snowpit several days after cold weather has set in. Here are the measurements:

  • Snow depth = 100cm
  • Snow temperature near the ground = 00C
  • Snow temperature near the surface = -100C

What do we know about this snow pack? The temperature gradient is 100C/m (10C/cm), which is twice the gradient we found in Snowpit No.1. The snow pack is still relatively warm so any metamorphism that takes place will progress at a “normal” rate. And with the strong temperature gradient, kinetic metamorphism has taken over. Therefore, we can expect squares “facets” to grow, and this will weaken that snow layer over time. And the lower the snow density, the faster the growth of facets. In this case the water vapor doesn’t slowly migrate and deposit in the concave areas of lower vapor pressure. The strong gradient forces the molecules to leave the warmer ice grains and reattach directly onto the colder grain nearby. This occurs progressively up through the snow pack as long as a sufficient temperature gradient is sustained. If this process were to continue for a few weeks, the resulting snow grains would look similar to loose granular snow or even sugar. These large, angular grains called depth hoar, which is a result of advanced kinetic metamorphism. Note the weak bonds between the large grains can be easily broken when stress is added, such as the weight of a person or snowmobile. This is the bane of avalanche forecasters throughout the world. This type of snow, whether in a thick or thin layer, cannot support much weight. Since these layers are subject to collapsing and causing an avalanche, they are monitored closely by avalanche forecasters.

Snowpit No. 3

Now let’s dig a snowpit on a typical day in the springtime. The measurements taken are:

  • Snow depth = 100cm
  • Snow temperature near the ground = 00C
  • Snow temperature mid-pack = -10C
  • Snow temperature near the surface = 00C
  • Average snow density = 300kg/m3 (30% water, 70% air)

What do we seen in this snow pack? It is much warmer at the surface. There is only a negligible temperature gradient in the mid layers as the snow approaches isothermal conditions (near 00C throughout). And the surface snow is starting to melt. This is the end stage of the snowpack’s life. Density has increased because the snow has settled over time and the pore space has decreased. Layers that developed early or mid-winter are losing their identity because of prolonged equilibrium metamorphism. All of the grains are rounding and the snow pack is gaining strength. Thus, spring snow conditions are less risky for avalanches. When the snow surface melts during the day and refreezes at night (regardless of the time of year), melt-freeze metamorphism takes over. During the melt stage the smaller grains melt first, providing free water in the snow, and the bonds are destroyed between the grains. Wet snow avalanches become likely on steep slopes, especially around rocky areas that soak up heat on sunny aspects. When the snow pack refreezes, free water freezes onto the remaining ice grains, making them even larger than before. This is how “corn snow” develops for good spring skiing. The snow is very strong in the frozen stage and very weak in the melt stage. Avalanche forecasts often call for different danger ratings from morning to afternoon.

Summary

These simplified snowpits are good examples of how the three types of snow metamorphism work. But combinations of the contributory factors explored here are almost endless, making the snow pack a complex structure that develops and metamorphoses throughout the Winter. Its many layers, and the constantly changing forces acting on them, pose a formidable challenge to the avalanche forecaster.

Avalanche Team

August 17th, 2006

I wanted to write a blog on the website to talk a little bit about the Avalanche Team.  In the transition of ownership and the re-newed interest in Mt. Waterman, I wanted to find out what patrollers are still out there looking to patrol at Mt. Waterm and find out which of those patrollers might be interested in working with the Avalanche Team. 

 

This group will be responsible for all of the Avalanche Training on the mountain and will take part in Avalanche mitigation, and some forecasting.  Mt. Waterman is one of the only mountains in S. California with ligitimate avalanche concerns, and every member of the team with have to go through training to address the seriousness of the mountain.  We have had avalanches at Mt. Waterman and will have them in the future.  Please respond to this blog with your intentions and thoughts regarding the Avalanche Team at Mt. Waterman.

 

 

Thanks,

 

Jeff Pierce

Mt. Waterman Avalanche Advisor

Sourthern California Avalanche Advisor

Patrol Party Rescheduled

July 28th, 2006

The patrol party will take place in conjunction with the cliimbing day event at Pt. Dume, and not on 7/29 as originally scheduled.

How to contribute to this blog

July 8th, 2006

To contribute, simply login using your original Mt. Waterman Ski Patrol username and password. Your account is already set up and ready for use. Then click on the “admin” link on the lower left, and follow the menus to the “write” option. Then simply enter your posting. Alternatively, you can comment on an existing entry.

Lastest news

July 3rd, 2006

Check out today’s article in the Pasadena Star News!

http://www.pasadenastarnews.com/news/ci_4006686

Restarted!

June 29th, 2006

The mtwatermanpatrol.org website is back up and is (almost) fully functional. Patrol members will automatically be signed up for this blog with their patrol website usernames and passwords. If you don’t remember it, e-mail me at webmaster@mtwatermanpatrol.org. It will take me a few days to get everyone registered for the blog, so please be patient. If you’re not registered in a couple of days, e-mail me. Also, the password protection function of the regular website may not be functioning for a few days. If you notice any problems, have any suggestions, or want to volunteer to help with the website, please contact me! Also, let’s try to make use of the blog. Thanks!

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