TOPICS: Crystal structure, weather
LEVEL: Middle and high school
TIME: One period for field work; 20 minutes or more for lab work and discussion.
ADVANCE PREPARATION: Purchase several cans of clear plastic spray (KrylonTM #1303 or similar product) at art supply store OR prepare formvar solution (1% by weight polyvinyl formal in ethylene dichloride) in dropper bottles. At least an hour before class, put all materials outside to cool in a protected place. OPTIONAL: Purchase 2 x 2 inch glass photographic slide covers on which to collect and project snow crystal replicas for class viewing
. MATERIALS:
Wooden (not plastic) clothespins
are useful for holding slides while collecting snowflakes. One
possibility for protected cold storage of slides is metal microscope slide
trays placed in a large cardboard box outdoors. Formvar solution can be
made by dissolving 5 g formvar (polyvinyl formal) powder in 400 ml heated
and stirred ethylene dichloride (1,2-dichloroethane) to make a 1% solution
by weight. Be sure to use a hood.
.
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: Falling snowflakes may be hexagonal and symmetrical in shape (see Snow Crystal Chart). Snow crystals on the ground lose delicate appendages, becoming smaller and rounded. Later, fusion of crystals and other types of crystal metamorphosis may result in larger granules, sometimes round or cup-shaped (see Snow Teacher Background
.
PROCEDURE:
It is important to review general procedures for collecting and preserving snowflakes, and specific field procedures with your class before going outdoors. Have students work in groups of 2 or 4 students, dividing up the field procedures (steps A-C) within the group beforehand. To observe different environmental conditions, you may wish to collect samples from various snow storms
.
STEP C:
If the preservative does not cover the snow crystal when using plastic spray, have students spray another slide and drain an additional drop of plastic off the corner of the slide near the granule
.
STEP D:
Especially good student slides can be saved for future reference
.
STEP E:
Do NOT distribute Snow Crystal Chart until students have examined their own snow slides.
QUESTIONS 4-8 require higher-order thinking skills. We recommend that you discuss these questions in class before students write out their answers. Students will need the Snow Formation Chart to answer question 8. You may wish to omit question 8 for younger or less able students
.
2.
Answers will vary. See Prediction note
.
3.
Randomly collected crystals are likely to be broken. Selected crystals are more likely to be hexagonal and symmetrical
.
4.
Collisions caused by wind can break crystals. In addition, crystals can fall through air masses with different conditions, building combination crystal shapes (see Snow Teacher Background
.)
5.
Crystals can fall through air masses with different conditions, building combination crystal shapes (see Snow Teacher Background
.)
6.
Wind and the weight of snow on the ground can break crystals. Crystals can also fuse
together, shrink, or grow from humidity inside snowpack (see Snow Teacher Background
.)
7.
Stellar crystals with interlocking appendages are most likely to form conglomerate flakes
.
8.
Answers will vary depending on environmental conditions.