Everyday Science: Snowflakes

Snowflake photo through compound microscope by Wilson “Snowflake” Bentley, from snowflakebentley.com

Snow. It’s that time of year. Some of us have received a whole bunch so far this winter, and others… none at all. Still, everyone knows about snow. When it’s warm and the clouds come, it rains. If it gets cold enough… it snows.

Snow is the result of the saturated clouds of water vapor whose droplets of water attach themselves to ice particles. They attach and they freeze. Frozen ice particles grow when they encounter more water vapor. Sometimes they mix with other particulate such as dirt, smog, dust.

When the frozen ice particles grow so big that their weight is too heavy to be tossed about within the cloud, they fall out of it. Another way to say this is that the fall speed of the ice particle exceeds the upward air drafts within the cloud.

Note that for the snow to hit the ground as snow and not rain, the temperature all the way down must be below about 32° F. That snowflake has to stay below freezing or it will turn to rain. Even if it is above freezing where you sit, at ground level, if the majority of the air on the way up to the snow cloud was below freezing, chances are that snowflake will remain a snowflake long enough for it to hit the ground.

And the shape of a snowflake? It has six sides, a function of the molecular composition of the snowflake. Snow is water, and the H2O molecule consists of a central oxygen atom with two hydrogen atoms joined to it on either side at 104.5° angles. This V-shape with a negatively charged oxygen atom and positively charged hydrogen atoms means that additional water molecules that attach will do so in a particular shape. A 6-sided one.

It follows that larger snowflakes are formed in higher water-vapor conditions within the snow cloud. Smaller more compact snowflakes mean lower water vapor content aloft.

The actual shape of a snowflake can even be predicted by the temperature, as seen on this chart from Chemistry.com:

  • 32-25° F – Thin hexagonal plates
  • 25-21° F – Needles
  • 21-14° F – Hollow columns
  • 14-10° F – Sector plates (hexagons with indentations)
  • 10-3° F – Dendrites (lacy hexagonal shapes) 

As an activity, you can bundle up and head outside and see what shapes of snowflakes you can find.  Take this printable resource with you on snowflake types (or leave it inside and come back and refer to it over a hot cup of cocoa).  And for more fun activities and resources check snowcrystals.com.

    For fascinating reading about Wilson “Snowflake” Bentley and his snowflake photography, check out the website here.

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