The Shark and the Jellyfish: More Stories in Natural History

Window on the Sky

183 Window on the Sky The flattened crystal facets of a free-falling snowflake form a hexagonal window. That window looks out over a panorama of unparalleled depth. If you could tumble across the sky beside a snowflake, hovering just above its surface and looking through, the vista you would see filling the frame would be breathtaking. The view swings in continuous motion through 360-degree spins and reversals, flipping and rolling as the flake flies, panning across the cloudscape canyons inside a thunderhead. One instant would focus on the heights miles above—with blue showing through the cracks in the ascending ramparts—and the next instant would look down into the abyss, darkening far below toward the storm-shaded ground. Bolts of nearby lightning fill the frame with momentary flashes of pure white. Then the panes of ice glow in shafts of sunlight that chase prismatic bright lines around the margins of every facet. The snowflake’s six arms frame a constantly changing picture, while the framework of the snowflake itself also changes constantly, all the arms evolving in unison. A snowflake is a manifestation of the fleeting microclimate in which it is immersed. It is not an impervious diamond, aloof from its surroundings; its structure reflects an intimate relationship with an ever-changing environment of which it is an integral part. The snowflake continually grows new projections and then retracts them, according to conditions around it that alter with every passing instant . Snowflakes live on a thin edge between extremes of temperature, pressure, and humidity. Water does not exist as a liquid in this coincidence of circumstances but converts directly back and forth between crystalline solid and invisible vapor—forging the snowflake’s feathered blades to reflect the changes of the moment. The snowflake is a single crystal of millions of millions of water molecules , each one fitted into the geometric ice lattice in its turn. These miniscule building blocks maintain their geometric order as their formations grow—until the molecular geometry is manifest in the symmetry of the visible flake. Variations in the chill environment dictate whether growth will be at the cusps of primary arrow points or along rows of secondary 184 projections—on the edges of internal margins or thickening filigrees on terraced surfaces. Rising and falling humidities move the structure either to spread its frosty tips or to reverse the process, pulling in its branches and contracting into a simple hexagon that shrinks until it vanishes. The dynamic flecks of ice sublime away to nothing, but a moment later the process reverses. Countless minuscule hex crystals flash into being in midair, forming around invisible motes of airborne sea salt or dust powder, bits of diatom skeleton, or smoke particles. The branches of the new flakes bloom into arrays of struts and braces as free-floating water molecules condense directly onto the ice matrix, following a formula for endless variation. The frozen asterisk is only a millimeter or two wide, small enough that each of its rays experiences the same microclimatic shifts. Conditions vary widely between wind-shear zones, inversion layers, and wisps of cloud reaching into sunlight. But any of these unique patches of air is broader than a snowflake is wide, so the consequences of each applies uniformly over every flake—the embellishments on each arm are repeated exactly on all the others. Countless gradations of microclimate inhabit a single cumulus cloud. Each snowflake ricochets along a chaotic trajectory from one chill eddy to the next. The thunderhead may span temperatures ranging from zero to fifty below, and pressure gradients between five thousand and fifty thousand feet—all connected by shafts of wind whistling past each other at five thousand feet a minute. Two snowflakes do not fly side by side for long under these conditions. Snowy columns of wind collide head on and shatter into countless diverging currents, scattering flakes in every direction. Despite their vast numbers, no two snow crystals follow the same path, so no two of them ever grow to be exactly alike. Calmer circumstances anticipate the creation of snowflakes—conditions that foretell the arrival of a storm still hours or days away below the western horizon. The leading edge of such a cold front advances along the top of the stratosphere—through an environment like nowhere below. The air pressure there is very low—breathing would not be possible. Water at the temperature of a human body would quickly boil away to vapor; then the vapor would radiate...