Snow in Science, Culture, and Climate

HOW MUCH WATER IS IN SNOW? Snow Density and Snow Water Equivalent

Overview

This activity may be done in conjunction with the snow pit study or as a separate lesson. Students use a snow sampling tool of established volume to explore snow density and snow-water-equivalent (SWE), the amount of water that is contained within a given volume of snow. Older students may calculate density and SWE values, whereas younger students will learn based on observation and comparison of different snow samples. 

In addition to gaining an introduction or applied context for the concept of density, students will gain an appreciation for the importance of snow as a water source.

Background/reference

What is SWE? Snow Water Equivalent. 2021. MissoulaAvalanche.org. https://missoulaavalanche.org/2021/01/what-is-swe-snow-water-equivalent/ 

What is Snow Water Equivalent? No Date. Natural Resources Conservation Service, U.S. Department of Agriculture (USDA) https://www.nrcs.usda.gov/wps/portal/nrcs/detail/null/?cid=nrcseprd1314833#:~:text=Snow%20Water%20Equivalent%2C%20or%20SWE,the%20snowpack%20when%20it%20melts

Snow Density. Avalanche Encyclopedia. 2021. American Avalanche Association and National Avalanche Center. https://avalanche.org/avalanche-encyclopedia/#density-snow-1 

Materials 

Included in kits unless otherwise noted

● snow sampling tools (black PVC tubes with handles) (2)

● foam stands for sampling tools (2)

● square wooden rulers for sampling tubes (2)

● large metal spatulas (2) – from snow pit study materials

● extendable utility shovels (2) – from snow pit study materials

● Snow density sampling how-to video clip LINKED HERE

Procedures 

1. Activity set up

At least twenty  minutes before starting the activity, place the snow sampling tubes and spatulas outdoors to cool down, so that they don’t melt the snow when they come in contact with it during the sampling process.

There are only two sets of sampling tubes. If you would like more students to be able to participate in the sampling activity at the same time, the activity can be done by collecting snow in other types of containers of known volume (e.g. plastic containers provided in the kit) with some modification in procedures. To be able to compare results from different locations, all students should use the same type of container and follow the same procedures.

2. Activity introduction and demonstration

If students have already completed the snow pit study, they will have noticed that there are differences in snow grain size and shape, how much the snow grains stick together, how much space there is in between the snow grains, and how hard or compacted the snow is among different snowpack layers. One characteristic of snow that scientists measure when doing a snow pit study is the density of the snow layers and the overall or bulk density of the snow at that location.

If students have already been exposed to the concept of density in math or science, ask a student to refresh everyone’s memory of what it means and how it is calculated. 

  • Density is equal to mass (usually measured in grams or kilograms) divided by volume (usually in cubic centimeters or cubic feet). 
  • If you have two objects of identical size and shape and one feels “heavier” than the other, it is the denser of the two objects. (It contains more mass or material within the same volume.)

You might also ask guiding questions to find out if students have an intuitive or experiential understanding of what density means and how it is different from but related to weight. 

Do students have any ideas about why someone would want to know about how dense the snow is? They are not expected to have answers in advance, but they might suggest implications for snow stability/avalanche risk/ “post-holing” in snow; determining which snow is best to melt for water; or other ideas from personal experience.

Another way we can think about density of snow is to think in terms of how much water it contains. Snow is made up of ice particles and air, so snow with more ice and less air is more dense than snow with less ice and more air. By collecting a known volume of snow and letting it melt, we can measure how much water it contained, which is related to its density. 

3. Activity procedures

If available, show the video clip of UAF snow scientist Charlie Parr demonstrating how to collect snow samples using the density sampling tool.

Note that only two students will be able to collect samples at a time if using the snow sampling tools. Designate two students to take the samples. Distinguish the tools by writing each students’ initials on a piece of tape and affixing the tape to the wooden handles of the tools.  You might suggest that the students collect samples from different layers in the snowpack or from different locations. 

Steps:

  1. Use the shovel to cut vertically into the snowpack and remove snow from the area where you want to collect your sample. (See first snow pit study video clip for a demonstration.) Expose a vertical wall of snow at least 0.5 meters wide so that you can access it. Use a whisk broom or a gloved hand to brush loose snow from the exposed vertical surface so that you can see the locations of the different snow layers.
  2. Decide which layer you want to sample. The layer must be at least as thick as the diameter of the tool opening. If it is not thick enough, choose a different layer. 
  3. Insert the tool.
    1. If you choose to sample the top layer of snow, hold the wooden handle and insert the open end of the tool horizontally into the layer exposed in the face of the snow wall, being sure to keep it horizontal. Push it into the snow gently but firmly until the bottom of the black cylinder is flush with the face of the snow wall (with just the handle sticking out). 
    2. If you choose to sample a layer that is lower in the snowpack, use the shovel or metal spatula to gently remove (by scraping away) the upper layer(s) of snow. Once you have removed the snow above the layer that you would like to sample, repeat the process described in step 3.a.
  4. Use the large metal spatula to cut down vertically into the snow at approximately the location where you expect the open end of the tube to be located. (It might be helpful to use a ruler or tape measure to find the approximate location by measuring horizontally 13 centimeters back from the exposed snow wall.) Move the spatula around or cut down from the top again to try to find the position at which the spatula will cover the open end of the tube. Be sure not to push the spatula toward the tube opening, as this will push more snow into the tube, compacting it, which will affect the results.

The spatula can be done by the student who is still holding the sampling tube or by another student if more convenient. 

  1. When the spatula is touching the open end of the tube and covering the opening, gently and slowly pull the tube out of the snow horizontally, being sure that the blade of the spatula remains fully covering the opening of the tube the whole time.
  2. Once the tube has been extracted from the snow, slowly turn it to the upright position, keeping the spatula blade over the opening.
  3. Once the tube is in the upright position and being gripped by the handle, the spatula can be removed, and the sample should be carried inside, being careful not to spill any of the sample.
  4. In the classroom, insert the handle of the tube into the foam stand so that it remains upright while the snow melts.
  5. Place the tools in their stands on a stable surface close to a heat source, if possible, to speed up melting.
  6. When the snow in the tube has completely melted, insert the square wooden ruler, with the zero mark down, vertically into the center of the tube so that it touches the bottom. Then remove it. The part of the ruler that was submerged in the water will appear darker.
  7. Measure how deep the water was by referring to the darker colored, wet portion of the ruler.

4. Calculating density and snow-water equivalent

This portion is optional and is suitable for students who are more advanced in math, including geometry.

Steps:

  1. Calculate the volume of the cylinder, which is the volume of the snow sample if collected properly. 

Volume of the cylinder in cm3 = pi x radius of the cylinder squared x height of the cylinder

Vcylinder = π x r2 x hcylinder

The number pi (π) can be abbreviated as 3.14

V = volume, r = radius, h = height

  1. Calculate the volume of liquid water contained in the snow sample.

Volume of water in the cylinder in cm3 = pi x radius of the cylinder squared x height of the water in the cylinder

Vwater= π x r2 x hwater

The number pi (π) can be abbreviated as 3.14

V = volume, r = radius, h = height

  1. Determine the mass of the water in the cylinder.

Water has a known density of 1 gram per 1 cubic centimeter (1 g/cm3). Therefore:

Mass of water in the cylinder =  volume of water in the cylinder times the density of water.

mwater= Vwater x 1 g/cm3

m = mass, V = volume, 1 g/cm3 = density of water

Density is represented by the symbol ρ (Greek letter rho)

  1. Determine the density of the original snow sample.

Density of the original snow sample = Mass of water in the cylinder divided by volume of the cylinder.

ρ snow = mwater ÷ Vcylinder

ρ = density, m = mass, V = volume

5. Comparing and discussing results

Compare the depth of water that resulted from the two different samples. Was there more water in one sample than another? Were the samples taken from the same snowpack layer or a different snowpack layer? If from different layers, which layer contained more water?

We can infer from the amounts of water that each sample contained whether one snow sample was more dense than the other. Which sample was more dense?

6. The importance of water from snow

References to help connect the activity content to critical freshwater resources from snow:

Importance of snow. 2020. Our Winter World website. http://ourwinterworld.org/importance-of-snow/ 

Importance of snow: Supplying Water. 2020. Our Winter World website. http://ourwinterworld.org/importance-of-snow/supplying-water/ 

Snow Program Overview. National Water and Climate Center,  Natural Resources Conservation Service, U.S. Department of Agriculture (USDA). https://www.nrcs.usda.gov/wps/portal/wcc/home/aboutUs/snowProgramOverview/
Program History. National Water and Climate Center,  Natural Resources Conservation Service, U.S. Department of Agriculture (USDA). https://www.nrcs.usda.gov/wps/portal/wcc/home/aboutUs/programHistory/

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