Note: This piece first aired in January 2008
If you were paying attention to the immediate environment anywhere in Maine during December, you know it was covered with snow. That may be hard to remember during the current January thaw, but December 2007 was record setting for snowfall. Snow is frozen water, formed through a remarkably complex process in the clouds. We've talked about clouds before, they are formed when water vapor, the invisible gaseous form of water, condenses out of the air to form liquid water—that's what condensing is-going from gas to liquid. So clouds are masses of tiny drops of liquid water, suspended in the air. When the cloud gets cold enough, the droplets of water can freeze, forming the foundation for a snowflake. Then water vapor that is still in the air as gas freezes directly onto the ice nucleus—going directly from gas to solid—this process is called deposition-it's also the same process that causes freezer burn. So snow isn't frozen rain—that does occur but we call it sleet—solid ice pellets that form when rain falls through a cold air layer on its way down to the ground. Snow is the direct deposition of ice crystals onto an ice nucleus, and the shapes and forms it takes are highly dependent on both the temperature and the relative humidity of the air where the snowflake originates.
We've all heard the saying that no two snowflakes are alike, which is really neither here nor there, as it would be impossible to prove or disprove anyway. What is certain, and easily observable for most of us, is that snowflakes come in several distinct shapes. Almost all snowflakes have some sort of 6 sided or hexagonal structure. This comes from the way the water molecules arrange themselves when they freeze and form crystals, due in large part to the unique qualities of the water molecule. But that's another story.
When it is very cold, the air does not hold much water vapor, and the snow that forms tends to come in very small, simple shapes, plates-flat hexagons, or small columns (like a stack of plate snowflakes). As the air warms up more complex forms show up, stellars—which are star shaped, and dendrites—which are lacy or leafy—the classic snow flake we all imagine. Snowflakes also come in the form of needles! Any of these forms can show secondary effects as well, like when the snow flake falls or gets recirculated through a higher humidity cloud or fog. When it makes it down to the ground it looks like little bits of Styrofoam and is called graupel.
So you have an assignment. The next time it snows, go outside with a dark jacket on. Catch some falling snow flakes on your arm and look closely at them. See if you can determine their general form. Don't be discouraged if you can't tell at first, when its windy snowflakes get broken up on the way down, and it its warm they stick together. You will probably be able to identify parts, and if it's a nice calm snow event, you should be able to see snowflakes in their entirety.