A year ago, right about now, I headed for a weekend at the Pacific Northwest Sportsmen Show in Portland, and a visit with South African friends Richard and Ruth Lemmer. You may recall the intense snowstorm which knocked a lot of travelers off their pins and locked up a fair amount of the Pacific Northwest. Through hours and miles of amazement at the physics of snowflakes and temperature and wind and icy roads, I made it to Portland’s Expo Center and our reunion.
This weekend, I am headed for a similar reunion in Portland. So far, the weather is cooperating, but after a bit of snow falling around Paradise, and a few dicey moments on Snoqualmie, I think a primer might be in order. Then, too, there are my responsibilities as Science Education Chair of the Reecer Creek Rod, Gun, Working Dog & Outdoor Think Tank Benevolent Association.
As moist air moves off the Pacific, up the west side of the Cascades or up the Columbia and Yakima, it lifts and cools below its point of saturation so that excess water vapor condenses. Our clouds will have billions of tiny water droplets formed around microscopic nuclei of dust or chemical particulates, all floating in saturated air. The droplets may not be much larger than two microns (two millionths of a meter) in diameter and may remain liquid to temperatures far below freezing. (Most clouds in our latitudes are such “supercooled” clouds.)
Enter Tor Bergeron and W. Findeisen (who suggested the process in the 1930s). They theorized that, until an actual ice crystal forms, the water in most clouds will remain as liquid.
Bergeron first, then Findeisen and others, found that the vapor pressure of water vapor around ice is lower than the vapor pressure of water around a liquid droplet – even at the same super cold temperatures. Ice may form if the supercooled droplets jar each other (causing them to fuse into ice), from an ice nuclei passing through, sudden cold, or even (according to some) from the vibration of a nearby lightning bolt. Once an ice crystal forms, the water vapor almost instantly flows from the vicinity of the droplet to the vicinity of the ice crystal. The ice crystal grows as the water droplet evaporates. The movement from that process seems to trigger more ice crystal formation, and an entire cloud can change to ice crystals in seconds.
Thus, common forms of cloud seeding involve shooting or dropping dry ice or phony ice nuclei such as silver iodide into clouds of supercooled water droplets to force ice crystal growth.
Through sublimation of the water vapor onto the ice crystals, or bumping, or both, the ice crystals may grow big enough to fall. If it is cold enough at ground level, we get snow; if not, then the ice crystals melt as they fall and we get rain. As we commonly see the cooler foothills get snow, while we get rain down here where it is warmer.
Got kids? Keep a metal tray in the freezer. When the snow starts, take the gang out and catch flakes. Then explore the mysteries of the flakes you have captured. A magnifying glass makes it a simple, rich process.
Japanese scientist Ukichiro Nakaya raised the study of ice crystals to an art form in northern Japan in the 1930s. Snowflakes are hexagonal, and Nayaka found a huge variety of forms. He demonstrated that individual flakes often grow concentrically, one six‑sided ring after another, beginning at less than 1/100 inch across.
Almost anything can happen with an infinite number of temperature and moisture combinations, of course, but Nakaya found somewhat predictable patterns.
In general, colder air holds less moisture and produces smaller, more perfect crystals. In high clouds, we may find tiny hexagonal plates at -35°F, and similar quarter-inch plates at ‑5°F in lower clouds. Tiny six‑sided, hollow needles may occur at or below ‑39°F in high cirrus clouds.
Moderate temperatures (-5° to 12°F) give us individual dendrites (leaf- or fern‑like). Stellar crystals are the tiny stars, with six long points or arms, sometimes called “diamond dust.”
Relatively warm air (14° to 28°F) holds more moisture and produces the biggest flakes( and the most slippery snow). The largest single-crystal flakes on record exceeded five inches across, reported in England in the 1950s, and in Virginia in spring of 1900. With this much moisture, we may find needles or cones a half-inch long, or even hollow, six-sided columns, capped with hexagonal plates.
Aggregations of crystals happen in relatively warm air, occurring from collisions and/or electrostatic charges. Huge aggregated snowflakes, as large as 38 centimeters (15 inches) across, and 10 centimeters (four inches) thick, fell on January 28, 1987 at Fort Keogh, Montana. They were said to look much like milk can lids.
Check the web for more – you will find an almost limitless supply of photos and beauty. We owe it to the skies to make the most of whatever snow they send us this winter.
