TOPICS:  Heat transfer, insulation, earth surface heat

LEVEL: High school and advanced placement

TIME: One period to collect data; additional time to clear and trample snow daily prior to data collection and discuss predictions and conclusions.

ADVANCE PREPARATION:  Construct frost depth gauges as described below. Frost gauges must be in place before ground freezes in early winter. Collect empty soup cans, removing both ends to construct snow scoops. A wooden handle may be bolted on to make sampling easier .

MATERIALS: Use small masking tape labels or permanent marker to number 30-40 plastic bags consecutively.

FROST GAUGE CONSTRUCTION:

3 pieces 1" metal electrical conduit pipe

3 pieces 3/4" OD clear plastic tubing

9 rubber stoppers

sturdy string

flagging tape

hammer

nail or awl

food coloring and water

permanent marker

The pipe and tubing should be long enough to extend above the expected maximum snowpack and below the expected frost depth.  Your local cemetery association or agricultural extension agency should be able to give you guidance.

Crimp the bottom of the metal pipe with the hammer.  Use the nail or awl to make a hole near the top of the clear tubing for the string.  Fill the tubing with colored water to within 10-15 cm of the top.  Run hot water over both ends of the tubing to make it easier to force both stoppers several cm down into the tubing.

Hammer the metal pipe into the ground in the fall.  Mark the soil level on the inside tube before sealing the pipe with a stopper.  Use brightly colored flagging tape to mark the top of the frost gauges .

PREDICTION: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students: The bare ground should have the greatest degree of frost depth; the ground beneath the undisturbed snow should have the least.  Snow is a good insulator when it contains a great deal of trapped air .

PROCEDURE: Students should work in groups.  One group can be assigned to collect frost depth data (step D).  Other groups should make replicate snow layer depth and density measurements (steps B-C). Students should be instructed to disturb the snow as little as possible when taking measurements .

STEP A. Students should be assigned to clear the snow away and pack it down each day for about a week before data is collected.  To insure accuracy, clear or pack an area with a 1 m radius around the frost tubes.  Wait at least 24 hours following a snowfall to collect data .

STEP B. If layers are not distinct in the snowpack, have students measure the total depth of the snowpack and take snow samples at the top, middle, and bottom.  Use the total depth divided by three as the thickness of the three layers for STI calculation .

STEP D. When the water begins to freeze it will force the dye into the unfrozen part of the water.  When checking for frost depth, lift the inner tubing and measure the distance from the soil surface to the dye .

STEP E. Volume of a 10.75 oz size soup can is 365 cm³.

QUESTIONS 5-10 require higher-order thinking skills.  We recommend that you discuss these questions before having students write out their answers .

2. Undisturbed snow should result in the least frost depth; the cleared site should have the deepest frost. The compacted site should be intermediate .

3. Low STI value of compacted snow should be associated with deep frost; higher STI value of undisturbed snow should be associated with more shallow or no frost in soil .

4. Answers may vary .

5. Ground that is well insulated by snow will not respond to daily temperature flucuations .

6. Compacting the snow eliminates most of the pockets of trapped air which provide insulation.  For this reason the ground beneath compacted snow will be more likely to have deep frost than that beneath loose snow .

7. Compact snow has more ice crystals and fewer pockets of trapped air than loose fluffy snow.  Heat is transferred from soil to atmosphere better through ice than non- convective air .

8. Snowmobiles and skiers will compact the snow and may increase frost depth compared with adjacent areas.  This may affect plants and animals within the soil by destroying the pockets of insulating trapped air .

9. Increased frost depth may cause the death of plants and animals in the soil.  Animals that are able, may burrow deeper within the soil .

10. The ground initially freezes due to constant exposure to low temperatures early in winter before there is sufficient snow cover to insulate.  Later in winter enough insulating snow accumulates to allow the ground to thaw due to conduction of heat from the deeper layers of the ground.