Observing Snow

Snow On the Ground Changes Through Time To understand the changes in the snow pack through time, we must first consider the mechanical breakdown of the intricate, delicate, crystals of falling snow into relatively round mature ice grains. The delicate arms of a snow crystal are easily broken. Constant motion and vibration of the water molecules breaks off the delicate exterior features. Ultimately, these pieces lodge in the spaces between the crystal's arms. Through this process of destructive metamorphism, the original crystal eventually becomes a rounded ice ball. Most of us have observed a decrease in volume as snow settles through time. Destructive metamorphism dominates when there is NOT a significant temperature difference from the top of the snow pack to the bottom. Typical conditions could be located in the top layers of the snow pack, or during warm spring days when the outside air temperature is similar to the ground temperature. The literature often refers to this process as equi-temperature metamorphism, but classroom experience proves that simpler terms communicate the concepts more clearly. In nature, crystals lose their points due to molecular motion, wind, and direct pressure. Physically breaking the snow crystals, for instance stomping on them or disturbing them with a shovel, will produce the same effect. The crystal arms are broken and then rounded grains fuse by freezing into larger crystals in a process called sintering. Snow crystals resulting from destructive metamorphism make good snowballs...they compact easily and this part of the snow pack can become very hard and dense. The destructive metamorphism of a stellar snow crystal. From a "Field Guide to Snow Crystals" by Edward LaChapelle

Chapter 3 English / Language Arts Content Standards B - 1, 2, 3

Science Content Standard A -14 (a)

Late in the spring, daytime air temperatures will rise well above freezing causing the ice crystals in the top layers to melt. The water percolates downward through the snow pack and eventually refreezes. We refer to this process as Melt - Freeze Metamorphism. During the day, when the snow is warm, it forms a wet granular layer of slush often nicknamed "corn snow." When the sun sets, freezing temperatures harden this layer into an icy sheet. A deep melt-freeze episode results in a hard, crusty layer that frustrates animal movement, and often covers the available food supply in a frozen sheet of ice. Ice crystals also become bonded together due to pressure, and often from the weight of the snow pack above. Pressure Metamorphism causes individual ice grains to change into larger ice crystals. These forces make the snow pack very dense and very strong. Firnification occurs if enough time passes so that both meltfreeze processes and pressure create very large and dense ice crystals. If the firn is compressed further into even denser and larger ice crystals with very little air pockets; the result is glacier ice. We will study more about the movement of glacier ice in Chapter 5.

 

The transition from snow crystals to firn crystals to glacier ice
From "Glaciers" by Michael Hambrey & Jurg Alean

Constructive Metamorphism As the winter season progresses and snow accumulates on the ground, the snow itself becomes an important insulating layer. The blanket of snow helps retain the latent heat of the ground. Temperatures at the ground's surface tend to hover within several degrees of freezing. Conditions can be significantly warmer on the bottom of the snow pack than at the snow's surface, where it is exposed to the chilly arctic winter air. If the temperature gradient is at least 18° F per meter, then the process of constructive metamorphism, where new crystals are actually formed, dominates. The water vapor present near the warmer ground surface is under higher pressure than the cool water vapor at the top of the snow pack. Natural dispersion of energy causes water vapor to move from warmer to cooler, and from higher vapor pressure to lower. The greater the temperature gradient from bottom to top, the more quickly the vapor transfers. As the water vapor contacts cooler temperatures it crystallizes directly from vapor to frozen solid ice crystals in a process called sublimation. Crystals can change from solid to vapor, or vapor to solid, without ever passing through the liquid state. In a snow pack with a significant temperature gradient, large six-sided, cup shaped depth hoar crystals form a loosely packed layer at the bottom. Many small non-hibernating mammals depend upon these loose snow crystals for easy construction of tunnels throughout the subnivean environment. This "sugar snow" can often be the weak and unstable layer that causes avalanche hazards. depth hoar crystal

Science Content Standard A - 4, 8 (c), 9

 

Air/Snow Temperature in degrees Centigrade

The nighttime temperature profile over snow covered ground.
Air temperature is lowest on a clear night right at the snow surface.
The ground temperature is relatively stable hovering around zero degrees Centigrade.

From "Life in the Cold" by Peter J. Marchand

 

Science Content Standards A- 4,15 B-1,2,3,4,5,6

A student is recording temperature every ten centimeters in the snow pit The objective of this practical exercise is to use, illustrate, and confirm the concepts presented thus far in the curriculum. The class digs a snow pit, records data and then analyzes it back in the classroom. The class documents temperatures throughout the snow pack and observes differences in snow crystal structure and metamorphism in the snow pack.   A density box is a wonderful professional tool. It is included in the Denali Foundation Snow Kit, or can be ordered (see the Resource Appendix.) In a pinch here's an alternative. This method is very functional, but depends on knowing that density of water is 1 kg/liter. In a classroom it does not illustrate the concept of density as clearly or easily as having a box of known volume. A Homemade Snow Sampling Density Box Use two #10 cans. Using a marker and ruler, calibrate one in equal increments from 1 - 100. This # 10 can should have a bottom, but no lid. Remove both the top and the bottom from the second #10 can. This will be your snow sampling tool. Using the sampler tool, carefully insert it horizontally into the snow pack, trying to fill it without disturbing the crystals. When you think you have a good sample, brush away the surrounding snow and carefully slide it out. Slice off any extra snow that is hanging over the edges of the can. Dump your sample into a plastic bag in order to transfer it to the other can. Put your snow sample in the calibrated #10 can in a warm place and allow it to melt. Record the measurement of the water line after total melting. Your measurement will correlate directly as a percentage. If you measure 18; the density will be 18% or 0.18 kg/liter. This works easily because the density of water is the standard, 1 kg/liter.

The Snow Observation Journal contains detailed snow pit directions as well a data collection sheets. In order to classify crystals, student will use the charts provided in the Snow Observation Journal. It often works well to appoint a recorder for each snow pit group. The recorder will need a clipboard and pencil to note the observations, while the other students can participate in the various steps of collecting data from the pit. Groups of 3-5 students work nicely. Orient the length of your pit perpendicular to the sun so that the exposed wall will be in the shade. Dig a pit, exposing a wall of snow approximately 3 feet wide and at least 6 feet long. Be careful not to disturb the wall where we are going to take measurements! Insert measuring sticks on either side of the snow wall with the zero end sitting on the ground. Slowly slice the snow pack using a wooden tongue depressor like a knife. You can feel the changes in the snow pack as you move it through the snow. When you feel a change, place your tongue depressor sideways across the layer to mark it. Observe and tell the recorder at what depth these changes occur. These are the layer boundaries. Repeat this process until you reach the ground. You may also be able to see the different layers. Record and draw all observations. Take temperature measurements of the ground, air, and snow pack at 10 cm intervals. Classify snow crystal types every 10 centimeters. Carefully scrape the crystal sample onto the black felt. Using a hand lens and the classification charts provided in the Snow Observation Journal, record the types of crystals found at each depth and on the surface. Remember, once snow lies on the ground it begins to change, and the crystals you will observe will probably fall into one of the metamorphic categories. Using the density box, take a snow sample from the top and bottom of the snow pack. Weigh each sample and record density calculation on the Data Collection Sheet found in the Snow Observation Journal. If skis are available, a shear test can be preformed to determine which layer is weakest and may imply risk of avalanche. The skier should stand parallel to the snow pit wall about 1 foot away. If nothing happen the skier should start to gently bounce and work up to small jumps. Observe the layer that the snow slides on, and how easily the slide occurred. Snow Pit Sketch   When drawing your Snow Pit Sketch include: meter sticks, layers, temperature, density data collection points, and any other visual observations

Snow Journal pg. 14   Technology Content Standards A - 2 C - 1, 2, 3   Math Content Standards A -1,2,3,4, 5,6 B-1,2,3,6,7,8   Problem solving Performance Standard age 11-14 2   Statistics / Probability Performance Standard age 11-14 1   Math Communication Performance Standards age 11- 14 2,3

Snow Journal pg. 19   Math Content Standard B - 1 D - 1,2,3,4,5   Problem Solving Performance Standard Age 11-14 1   Science Content Standard C-2, 7   Reasoning Performance Standard age 11-14 1

Classroom Analysis of Data: Once back in the classroom the group members share their data observations so each student can fully complete his or her own Snow Observation Journal. Each student completes a graph of the relationship between snow depth and temperature. A gradient of 0.18° F / cm is sufficient to drive the vapor transfer process. "Was a significant temperature gradient present?"   The next step is to calculate the density of the snow samples found at the top and bottom of the snow pack. The sample volume is constant, the size of the box, 200 cm3. A simple Density = Mass/ Volume will give the required result. A teacher should do an example of these calculations for the class on the board using data collected from one group. Caution! Do not mix data from different pits or you will have trouble interpreting the results.   The Snow Journal with data and observations from the Snow Pit

As a class, determine:

• Was there a significant enough temperature gradient to drive the process of constructive metamorphism? (0.18°F/cm is needed.)
• What different types of crystals did you identify?
• Where were the crystals located in the snow pack?
• Can you use the three different concepts of metamorphism to explain the types of crystals found?
• Do you have any direct personal knowledge of the past winter's weather that could help explain the types of crystals found in the snow pack?
• Was the snow more dense at the top or the bottom? What processes that we have learned can explain your observations?
• From what part of the snow pack is water traditionally collected from and why? (This can be kind of tricky. The loose pack depth hoar is usually thought to contain less water. Although the crystals are not tightly packed, the individual six-sided cup shaped crystals actually contain more water than a destructive metamorphism snow crystal. A bucket full of them melted over a fire will produce more water for your effort.)

 

Galena Project Education Residential School results from our classroom analysis of data

ACTIVITY: Building a Snow Shelter Galena PECS students in the Snow Shelter This is a fun way to use up some excess energy while doing a practical lesson in destructive metamorphism. Building a quinzhee (snow shelter) is a traditional survival skill. As snow crystals are physically broken they form rounded ice grains. With less space between the individual ice crystals, the disturbed, broken crystals pack together tightly. These ice grains will rebond upon contact with each other, a practical example of sintering. Snow resulting from the processes of destructive metamorphism is dense and strong. Not only is this good snowball snow, but it makes an excellent building material for a winter shelter. This exercise is best done on two separate days so snow can recrystallize and bond overnight. Five to eight students is a nice size group for each quinzhee. Day One: Depending upon the location of your quinzhee, one or several students may have to break trail to the construction site. Breaking through uncompacted snow versus easy walking on a well worn snow trail is another excellent example of destructive metamorphism rendering snow dense and strong. It is best to stomp down the snow on the bottom layer of your quinzhee before beginning to pile snow on top. Next let the students shovel their hearts out. A typical quinzhee can be up to 10 feet in diameter and 10 feet high. Your finished size will vary depending upon the time allotted, available snow, and the energy level of your group. You can insert sticks approximately twelve inches long, perpendicular to the mound surface to use as a thickness guide when excavating. Day Two: Students begin by digging an entrance from either side, as well as a top hole. Once the initial tunnels connect the interior space is excavated. A consistent wall of about 12 inches thick is sturdy. It can be incredibly informative to have the students destroy the quinzhee at the end of the exercise. It may not be safe to leave the quinzhee unsupervised, and it is usually extremely difficult to break the structure. The snow has become quite strong and dense indeed!

TERMS:

Constructive Metamorphism: The process where a significant temperature gradient causes warm water vapor to rise from the earth upwards through the snow pack. As warm vapor contacts cool temperatures, it sublimates and forms large, six - sided, cupshaped ice crystals. Another name for this process is temperature gradient metamorphism.

Corn Snow: Clusters of large, rounded ice crystals, formed by the processes of melt-freeze metamorphism.

Density: The degree to which the atoms of a substance are packed. Density is a measure of the mass per unit volume. Density = Mass/ Volume. Snow in interior Alaska has a density of 0.18 - 0.24 kilograms per liter. Water has a density of 1 kg/liter.

Destructive Metamorphism: The change in snow crystals from delicate six-sided shapes to rounded, bonded ice grains due to disturbance, molecular motion, and pressure. This process is NOT dependent on a significant temperature gradient.

Depth Hoar: Large, six- sided, cup-shaped ice crystals that grow on the lower layers of the snow pack as a result of vapor moving from the warm earth up through the snow pack. The process of constructive metamorphism depends on a large temperature gradient to move the vapor upward.

Firnification: After one year, snow that has become hard and dense due to the processes of destructive metamorphism is classified as firn. During this transformation to glacial ice, the crystals and the small air spaces between the ice grains are squeezed by the pressure of the weight and motion of the surrounding ice. Typical conditions will yield glacial size ice crystals with distinct air bubbles in about 5 years.

Glacier Ice: Well-bonded ice crystals compacted from snow with a bulk density greater than 860 kg/m3. Air becomes trapped inside the crystal fabric in the form of bubbles.

Melt Freeze Metamorphism: The process where ice crystals form clusters of large crystals due to the melting and percolating of water through the snow pack, and then refreezing. These conditions typically occur in spring during warm sunny days and cool nights. If this layer becomes buried, it forms an ice slab, that may present avalanche danger.

Metamorphism: To change in an important way.

Pressure Metamorphism: The process where ice crystals become larger, rounder, bonded due to weight from above in the snow pack.

Sintering: The process by which two particles weld together without liquid present.

Subnivean: Below the snow cover.

Sugar Snow: The common name for depth hoar, the large, cup-shaped crystals that form in the bottom layers of a snow pack due to process of constructive metamorphism.

Sublimation: The direct passage of a substance from solid to gas or gas to solid without appearing in the intermediate liquid state.

Temperature Gradient: The relative difference in temperature between the earth, the snow layers, and the outside air. The process of constructive metamorphism prevails when the gradient exceeds 0.18°F / centimeter.

 

Observing Snow
Introduction
The Four Corners of Life
Water: the Stuff that Makes Snowflakes
Snow on the Ground Changes Through Time
Exploring Native Snow Terms
Glacier Investigations
Open Note Review
Conclusion
Bibliography & Resources

 

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