PURPOSE:  To observe the effect of snow cover on frost depth.

MATERIALS: Field -- 3 frost depth gauges, meter stick, snow scoop, numbered plastic bags (about 30 x 30 cm), snow shovel, clipboard or other firm writing surface.  Lab -- triple beam balance.

INTRODUCTION:  One of the important sources of warmth in the winter environment is the ground surface.  The soil on the earth's surface has a net gain of heat from the sun during the summer.  When the temperature begins to drop in winter, the soil at the surface cools and eventually the water in the soil freezes.  The depth to which water freezes in the soil is the frost depth.

Frost depth is influenced by the snow cover, since the soil is cooled from above.  The insulation value of the snowpack is directly proportional to snow thickness; that is, thicker snow insulates better than a thin snow covering.  The insulation value of the snowpack is also inversely proportional to snow density; that is, light fluffy snow insulates better than the same thickness of heavy wet snow.

Although the frost depth may seem insignificant for creatures above the ground, animals and plants below the soil surface are greatly affected.  For those organisms, the snow cover limits the frost depth.  In this activity, you will measure frost depth under different snow conditions using water-filled frost gauges placed in the ground in autumn. You will remove the snow around one gauge, trample the snow around a second gauge to compact it, and compare both to an undisturbed site.

PREDICTION: Use what you know about the insulation value of snow and heat transfer to make a prediction:

1.  Will the frost depth be greater under bare ground, undisturbed snow, or compacted snow?  In which area will frost depth be least?  Explain your ideas.

FIELD PROCEDURE:

A. In autumn, place 3 frost depth gauges about 3 m apart in an area in which you expect snow to collect.  About a week before you begin to collect data in winter, the snow should be kept compacted around one gauge, cleared away around another, and should remain undisturbed around the third.

B. Collect data on the depth and density of the undisturbed snow near the three gauges as follows:

C. Collect data on the depth and density of the packed snow around the compacted gauge as follows:

D. Measure the frost depth in each location as follows: Remove the cap of the gauge.  Lift the tube inside and measure the distance from the soil surface mark to the bottom of the frozen column of water.  Record your data.  Replace the frost gauge tube and cap.  Replace any snow you may have disturbed.

LAB PROCEDURE:

E. Weigh each bag of snow.  (You can ignore the weight of the plastic bag because it is constant and small compared to the weight of the snow).  Calculate the density of each layer, using the formula density = mass/volume.

DATA ANALYSIS: The snow thermal index (STI) is a number which describes the insulation value of the snow on the ground. The STI is calculated by dividing snow depth by snow density.  For example, if the average density for a snow pack 30 cm deep is 0.3 g/cm³, then:

STI =  snow depth  =   30 cm   = 100 cm4/g.
      snow density   0.3 g/cm3 

When the value of STI reaches at least 200 cm⁴/g, the ground is fully insulated from the air and additional snow makes little difference in insulation.

The snowpack on the ground consists of one or more snow layers which may differ in thickness and density.  Sometimes the layers represent different snow events.  However, the snow at the bottom and top of a single snowfall may also change with time into distinct layers.  If the snowpack has several layers of different thicknesses and density, then the STI for the snowpack is calculated by summing the STI for each layer. 

F.  Calculate the STI for undisturbed snow as follows:  Divide the thickness of each layer by its density, and then add the values for each layer together.

G.  Calculate the STI for compacted snow by dividing the thickness of the compacted snow by its density.

CONCLUSIONS:

2. Which condition resulted in the greatest and the least frost depth?

3. What is the relationship between the insulation value of the snow (STI) and the frost depth?

4. How do your results compare with your predictions?

DISCUSSION:

5. Explain why air temperatures above 0^oC do not always result in thawing of ground frost.

6. Why is the frost depth usually greater under compacted snow than under loose fluffy snow?

7. Why does dense compact snow have less insulation value than the same thickness of loose fluffy snow?  (Hint:  Consider the relative amounts of ice crystals and air spaces in each.)

8. According to your data, what impact do you think snowmobiles and skiers may have on the winter environment?

9. What impact do you think increased frost depth has on plants and animals in the soil?

10. Often the ground will freeze to a certain depth early in winter and later will partially thaw even though the air temperature continues to drop.  Explain this observation.

FROST DEPTH DATA SHEET

Volume of Snow Scoop:___________cm³

UNDISTURBED SNOW NEAR GAUGES:

Frost Depth:_______________ STI:__________

COMPACTED SNOW GAUGE:

 Frost Depth:_______________

 SNOWLESS GAUGE:

Frost Depth:_______________