Our fascination with snow is rooted in centuries-old lore and modern-day fear; the lore of the wonder and beauty of it all, and the fear of, well, lots of things.
As an example of enduring tradition, one need look no further than the popularity of the Old Farmer’s Almanac. Dating back to 1792, the iconic handbook is still coveted for its winter weather forecasts. See the word snow in its pages and quiver with either childhood delight or curmudgeonly grumbling. Either way, the mysteries of snow — “Will it happen? How much?” — solidify the allure.
As for that fear of snow, whether it’s getting stranded, losing power, slipping and falling, getting into an accident, or whatever other malady might ensue, there’s no doubt we perceive the risk of snow as significant. Our anxiety is often deserved though sometimes overwrought. Understanding the science behind snow might settle our nerves a bit.
Snow has character. Each flake is different, of course, no two crystals the same as most of us learned when young, but its broader personality is what dictates human responses. Ranging from powdery to slushy, sleet-like icy to grainy dots; each kind of snow brings dramatically different impacts, each the result of ever-so-slight temperature variations in the air, from ground level to miles above. As humans, we can hardly perceive the difference between, say, 33 degrees and 31, but for snow, that gap makes all the difference.
Those temperature disparities make the physics of snow a tale of how water and ice temperature interactions within clouds fashion the connection to what we eventually see fall to coat our landscapes. Crystallization along with melt-freeze cycles define the process in forming the different winter precipitation types, each bringing varying consequences.
Powdery snow, the kind you can clear with a leaf blower, forms when critically thick air layers aloft are saturated enough and cold enough for highly efficient snow crystal growth. As long as temperatures all the way to the ground are cold, too, this snow can pile up quickly due to its low density, and lack of compaction and melting.
Slushy snow results from snow aloft falling through lower layers with temperatures a little above freezing. The flakes start to melt a bit, which gives the characteristic wet feel. The thicker that lower-level warmer layer, the more the slush-factor and the heavier the waterlogged snowflake. This snow often melts some on impact, but still accumulates based on intensity.
A sleet-snow mix and plain old sleet occurs when an above-freezing air layer aloft resides below the snow layer, but the air closer to the ground is colder, causing refreezing of melting snow before getting to the surface. This brings icing and attendant risks.
Those tiny pellets of snow we sometimes see that don’t look quite like sleet can be graupel, the result of supercooled water droplets freezing on snow crystals. Like sleet, they can rapidly cause slick conditions.
The dissimilarity of impacts from each type can be significant. While powdery snow will accumulate quickly, the other varieties bring their own kind of misery. One of those is power outages, always looming as one of our greatest storm fears. The weight of the snow defines the threat.
Jeff Mock, senior meteorologist with Dominion Energy Virginia, says the company uses snow types to assess outage risks.
“Ice and wet snow events are similar,” he said. “The amount of snow accumulation matters to us much less than the wetness or stickiness of that snow. If the temperature is 20 degrees and we’re going to get a foot of snow, there’ll be some trouble, mainly from vehicles hitting poles, but it usually doesn’t cause large numbers of outages. But if we’re getting just 3 inches of snow and the wet bulb temperature is just about freezing, 32, then it’s going to be a sticky snow, and it’s like an ice event … you reach that threshold where you go from no power outages to a lot of them.”
Travel, accidents and personal health risks are also directly related to the type of snow. Powdery snow is usually easier to clear and often yields better road traction than slushy snow or a mix with sleet. It’s also less stressful when shoveling and clearing off cars. The lighter consistency and lower water content of the snow (less weight) is key, whereas slushy snow is the opposite. Sleet or snow mixed with sleet obviously exacerbates icy conditions of walkways and roads, and poses significant hazards.
The people who make travel safe again after snowstorms, our highway departments and road crews, use assessments of the type of snow (will it be powdery or icy, for example) to determine how they assign crews for clearing and what situations will be most appropriate for pretreating. Important to their decisions are road surface temperatures, with networks of sensors set up nationwide on most major highways.
From businesses and municipalities, to each of us individually, the consistency of the snow is what controls how we’re affected. As this winter rolls in, use an understanding of the science to appreciate snow beyond the often scary power of that word.
Not all of our snowfalls will be those pretty powdery ones, but perhaps we can find some positives in the rest; like great snowball-making, or … well, that’s one anyway. Have a safe winter!
Jim Duncan recently retired from his 40-year career as chief meteorologist with NBC12 WWBT-TV in Richmond. He runs his own meteorological consulting firm, Jim Duncan, LLC, serving clients in media, education and other industries.