On the Edge

This morning I drove toward the edge of the world. As I approached the end of the pavement, moment after moment, the world emerged from thick, white nothingness and revealed itself before me.

A bit dramatic, perhaps, for describing a foggy morning in late fall, but such was the mood of the light. The morning was part beauty, part eerie, and part mystery. Frosty fields and forests zoomed by on either side, and a pallid sun, somewhere above the world’s ceiling, found the strength to make them sparkle.

I’ve had the feeling, lately, that we are living on the edge of the seasons. Any moment now (I hope!) the puddles will freeze and stay frozen. The frosty grass will not darken and melt under the low-slung sun, and will instead disappear under thickening layers of snowflakes.

Water itself is living on the edge these days—the edge of phase transitions from gas, to liquid, to solid. Each of these transitions results from changes in temperature and the amount of energy. Heat is the energy of molecules in motion, and thus the phase of water is directly related to its temperature, an indicator of heat energy.

Our daily temperatures fluctuate from just below to just above the freezing point of water (which is the same as its melting point). Each night when Orion presides over icy stars, the temperature here on Earth drops. At the dew point, water molecules in the gaseous state slow down enough to condense into liquid water droplets, suspended in the air. Clouds of fog appear near the damp earth. The invisible gas becomes visible, but it conceals the solid world. Gas to liquid.

The same thing happened as my tea pot whistled cheerily, expelling hot water vapor that condensed into visible steam in the chilly kitchen. Taking my mug of tea, I went down to the lake where a fragile layer of ice hugged the shore. It is so thin that I can barely see the ice itself. Only the unnatural stillness on the surface, or a tap with my toe, betrays the ice’s presence. The lake is on the edge of something, too.

Most of the lake is now at about 39 degrees Fahrenheit, the temperature at which water reaches its maximum density. Under Orion’s watchful gaze, the surface water’s temperature will drop farther – and begin to float. This colder water will accumulate until it reaches the magical 32 degrees. Then molecules align, crystals form, and the lake receives its winter lid. Liquid to solid.

When the dew point is below the freezing point, water vapor transitions directly from gas to solid, in the form of frost on solid surfaces. High in the sky, where the only solid surfaces are grains of dust, the frost becomes flakes. Ice crystals form directly from water vapor in the air, using dust as condensation nuclei. A snowflake’s complex sixfold symmetry is guided by the chemical properties of water. As the incipient snowflake travels around a cloud, more ice crystals condense on its facets, and their growth is influenced by the temperature and humidity of the air. The best snowflakes grow at 5 degrees Fahrenheit inside dense, humid, winter clouds. Gas to Solid.

No snowflakes fall today. Blue sky is becoming visible as the fog bank lifts. Energy from the sun heats the air and it expands, increasing the amount of water it can hold. Water droplets suspended in the fog evaporate and become invisible again, revealing that my path leads not toward nothingness, but into a fantastic and almost magical world. Even more amazing is that, as I tell my students, “It’s like magic, but it’s actually chemistry.