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The Insulative Value of Snow

Introduction:

During the cold winter months, plants and animals must have ways of dealing with subfreezing temperatures. We know that some animals dig deep down into the mud at the bottom of a pond or find shelter in burrows beneath the earth's surface. Certain plants "hug" the surface of the ground and in this manner stay out of the drying winds of winter. In this field exercise, you will learn how a layer of snow actually helps these animals and plants stay warmer than the surrounding air temperature.

Terms:

Subnivian - The environment that exists beneath the snow layer
Termal Conductivity - The ability of snow to conduct heat through its layers over time
Thermal Index - A scale of the insulative capacity of snow based upon its thickness and density

Materials:

Garden trowel
Metric rule
Soil thermometer
Zip-lock bags
Spring scale
Soil auger (core sampler)

Part 1: Examining the Thermal Gradient of a Snow Pack

In this section of the field exercise, you will determine the temperature at different depths of a snow pack. To accomplish this, simply follow the procedures described below:

Using a garden trowel, carefully dig a 30 cm wide trough from the surface of the snow down to ground level.
Using a soil thermometer, record the temperature at ground level by carefully inserting the tip of the thermometer into the snow so that it lays upon the ground surface under the layers of snow.

3. Next, record the temperaturs of the snow at 3 cm intervals, beginning at 3 cm above ground level and working up to the surface of the snow pack.

Record your results in the table found below. Denote the surface of the snow pack using an arrow to indicate the correct thickness of snow.

Depth (cm above the ground	Temperature (^oC)
Air temperature =
18
15
12
9
6
3
Ground level

5. Now, graph your results on the grid found below:

Part 2: Determining the Thermal Index Scale of a Snow Pack (after Peter Marchand: Life in the Cold, University Press of New England, 1996)

In this section of the field examination, we will determine the Thermal Index Ithe snow layer found on the school grounds. The Thermal Index is a number which integrates depth and density of snow. As the snow pack becomes thicker, its insulating capabilities become greater. As snow density increases as packing occurs, its ability to conduct heat increases, and therefore its ability to insulate the ground below decreases.

As the Thermal Index which we will determine approaches "200", the snow will insulate the ground below to the extent that the ground temperature will remain fairly constant even though the air temperature may fluctuate considerably.

Determine the mass of an empty zip-lock bag using the spring scale.
Cut a new 30 cm trough into the snow. Determine whether the snow pack is made up of one homogeneous layer or several discrete layers representing a number of different snow events. Measure and record the layer thickness in the table found below.
Using a soil auger, carefully extract a core of snow from the center of the bottom-most layer and empty it into the plastic bag. Record the mass of the snow in the table below (remember to subtract the mass of the bag alone to get the mass of the snow only)
Repeat this procedure for each discrete layer in the snow pack. (If we've had only one significant snowfall so far, there will be only one layer)
Once you return to the lab, determine the volume of the auger by filling it with sand and emptying it into a graduated cylinder. (Remember, 1ml = 1cc)
Complete the table below, filling in the volumer determined in step 5and then determining the density by dividing the mass of the snow by the volume of the auger.

Layer #	z = thickness (cm)	Mass (g)	Volume (cc)	G=Density (g/cc)
1
2
3
4
Ground layer

7. Finally, calculate the Thermal Index using the following formula:

Show your work in the space provided below.

Analysis:

How did the temperature vary within the snow pack from the ground level up to the surface of the snow (air temp)?
Packed snow with few air spaces is a poorer insulator than freshly fallen dry or "powder" snow. How is this similar to the way in which birds and mammals use their fur and feathers to protect themselves from the cold?
What was the value of the Thermal Index you calculated in the field?

How does this compare with the "benchmark" figure of 200?

Will the ground temperature around your school at this time be affected much by varying air temperatures according to your findings?
In your own words, explain the relationship of the insulating value of snow as related to its depth and density.