Flakes on the Shoulder Season
OK, it's time to face facts. Winter is upon us. Look up: every cloud you see is pregnant with white stuff, ready to litter all over the landscape.
Since this is the shoulder season, that hump between the softness of summer and the bone-hardness of winter, not all those cloud-borne ice crystals will make it to the ground as snowflakes. In this unpredictable season, variations in air temperature can lead to a range of precipitation types, from frozen and fluffy to liquid but slushy.
Even though it's a frozen form of precipitation, hail is the least likely to fall at this time of year. It starts off like all the other forms, as an ice crystal that gains weight as it fails through supercooled water droplets in the dense mass of vapor making up a cloud. (Supercooled water is liquid at temperatures lower than 32oF or 0oC. It's not as unusual as you might think, because water droplets need something to freeze around, such as a bit of dust. Slice open a hailstone, check it with a microscope, and you'll likely find a speck of dirt at its core.) Then, however, hail floats back up.
"Float" is probably too gentle a term to describe the upward whoosh of an infant hailstone, propelled by the surging updrafts in a thunderhead. But float or fly, up hailstones go, then back down they fall when their weight overcomes the uplifting forces of the moving air or when the updraft dies away. Sometimes they catch several such upward rides, growing on each pass as new water droplets and ice crystals freeze on.
The kind of round-tripping turbulence hailstones need for growth generally comes only in hot-season thunderstorms. When icy blobs fall out of the sky during autumn's cool but not frigid weather, they're usually sleet. Sleet is fairly rare in the Interior, but can show up any time from fall through spring near the coast. A sleet storm is proof that layers of air with differing temperatures lie overhead. This sort of precipitation starts off as snow at cloud level, melts as it falls through warmer air, then refreezes when it passes through cold air on its way to the ground.
Freezing rain also shows that different temperatures can be found at different altitudes. The falling icy crystals melt but stay liquid as they descend through cooler air, supercooling as they go. The supercooled drops freeze as soon as they hit something, such as the cold ground or a chilly airplane wing. Snow shows that the air overhead is cold, all the way to cloud level. The form of the snowflakes (more properly, the snow crystals, since flakes can include many crystals jammed together) tells something about the temperatures at which they formed.
The first constraint on form is set by the electrical bonds between water molecules. That, in a nutshell, is why all ice crystals show some kind of six-sided arrangement. The classic six-pointed star with varying degrees of lacy complexity forms near 5oF. A few degrees colder or warmer, between about 3oF to -8oF and 10oF to 14oF, and the crystals are flat plate-like forms somewhat resembling a child's drawing of a six-petaled flower. Hollow hexagonal columns appear at temperatures between 21oF and 14oF and again when the temperature is -8oF and lower. Six-sided needles appear at temperatures in the low twenties, and flat hexagonal plates show up nearer the freezing point.
Snowflakes fall, or ride updrafts, through different densities of water vapor and air masses of different temperatures, and their travels cause variants on those basic themes...as we'll be able to observe and observe and observe over the next several months.
