How Clean is your snow?

Freshly fallen snow makes your backyard took clean and bright. But how clean is that snow? If you live in a city, it may contain particles of pollution. Even snow in rural areas may have some contaminants, as air pollution can travel many miles.

Collect about ten cups of fresh snow and let it melt.

Cover the mouth of a measuring cup or jar with a paper towel or coffee filter.

Then pour the snowmelt through the filter.

Examine the filter paper for tiny particles – you might want to use a hand lens to get a better look. How clean was your snow?

For further investigation you can measure the amount of pollution in your snow. Weigh a filter paper before you pour the melted snow through it. Let it dry with the particles and weigh it again (you’ll need a sensitive scale).

You can also compare snow from different areas: an area with lots of traffic, a field far from a city, your school playground, neighborhood parks.

There's No Business Like Snow Business...

not an actual snowflake...

The northeast has had its share of snow this winter. Where does all this white stuff come from?

A snowflake starts as water vapor that condenses on a dust particle. At cloud temperatures of 23 degrees F, the water vapor begins to form ice crystals. As long as the cloud remains supersaturated with water vapor, the crystals grow. When they get too big and heavy, gravity takes over and the crystals fall toward earth.

Air temperature determines the basic shape of the snowflake, and th4e degree of saturation in the clouds determines how fast the crystal grows. Snow crystals tend to grow larger and faster when the air temperature is higher and there is a lot of water vapor.  Lower air temperatures and less water vapor means smaller snowflakes.

Though each snowflake is unique they generally reflect six-sided symmetry. Here are some of the typical types of snowflakes:

• Star-like crystals that have six arms that extend from the center;
• Plates that are flat six-sided crystals with no obvious projections;
• Columns that may be hollow or solid, with flat or pointed ends;
• Capped columns with flat plates on each end;
• Needles - skinny columns that end in points;
• Spatial dendrites that have fern-like branches; and
• Irregular crystals that live up to their name.

Go outside next time it snows and take a good look at the flakes that are falling down. And if you need an expert flake guide, check out Ken Libbrecht’s Field Guide to Snowflakes (2006, Voyageur Press) or check out his website, Snow Crystals. He’s got lots of flake photos online, and even some advice on making snowflake fossils.