Speaking of Science: Apropos of snow, wind and rain

As tempestuous as this week's weather has been, it's not entirely without precedent. The occasional rain or wind doesn't phase us Seattlites; it's the extraordinary that catches our attention.

Seattle received its first widespread snowfall of the year Saturday morning. While areas near Puget Sound and Lake Washington saw only an inch or two, people at higher elevations like Capitol Hill and Bothell found 4 to 5 inches on their doorsteps.

Starting as a mid-morning steady rain, soon the precipitation turned to snow. As the flakes fell with increasing vigor, a collective buzz began to fill the air, fitting for a city caught up in the event's novelty.

It takes a special set of circumstances to get snow in the Puget Sound lowlands. Perhaps to no one's surprise, the critical ingredients that comprise such circumstances are cold air and moisture.

On an ordinary day, the prevailing winds blow from the west, bringing moisture inland from the Pacific Ocean. However, those winds also carry warmer air, giving Seattle its moderate climate.

"To get cold and wet at the same time is difficult," said Cliff Mass, professor of atmospheric sciences. "You need a very specific setup."

Mass said the setup that most favors snow in Seattle puts high pressure in the eastern Pacific and lower pressure at the coast. The high pressure aloft directs winds out of the north and the low pressure at the surface draws cold continental air out to sea and then back into the Sound region.

The trouble is, air pulled off the continent is relatively dry. Snow can only begin falling after the atmosphere is sufficiently cooled and moistened via evaporation. This is why the snow Saturday began as steady rain and quickly changed over to large, aggregated snowflakes.

Enchanting as it was, Saturday's snowfall was marginal in comparison with past events. Lynn McMurdie, a senior lecturer in atmospheric sciences, recalled skiing into school on the Burke-Gilman Trail after past snowstorms of 12 inches or more.

"Whenever we got a good dump of snow, I'd don my Nordic skis and head in," McMurdie said. "It was by far the best way to travel given the conditions."

Mark Albright, a research meteorologist in atmospheric sciences, reminisced over being out of school for weeks during the winter of 1968-69. That year, Seattle received nearly 68 inches of snow, competing with the likes of Minneapolis and Denver.

"The median annual snowfall in Seattle is much different than the mean snowfall," Albright said. "This is because there are big events in the climate record."

Indeed, the long-term average annual snowfall for Seattle is about 12 inches. The median snowfall over the same period is closer to 9 inches. This illustrates how big events, which McMurdie and Albright alluded to skew the average far above the median, or what is most commonly observed.

As soon as thoughts of snow had sunk in our minds, Monday arrived, bringing record rainfall to the region. Sea-Tac Airport saw its second rainiest 24 hours on record, with 4.11 inches falling. The record of 5.01 inches, set in October of 2003, still stands.

"It's 72 degrees at the airport, which is pretty weird because I don't know anyone who lives at the airport," George Carlin said in a well-known quip.

Seattlites may have felt similarly upon hearing that the record at the airport failed to fall. Northern portions of the city saw their rainiest day on record. The National Weather Service office at Sandpoint received 5.07 inches in 24 hours, and surrounding areas reported even higher totals. The rain wasn't even the worst of it. Hurricane force winds knocked out power on the coast, major river flooding was reported south of Seattle, and avalanche warnings were posted throughout the Cascades.

Just when the attention given to the storms reached a climax Tuesday, the skies parted, paving the way for more benign weather, receding rivers and a chance to pause and catch our breath.

Ever changing and sometimes fierce, the winters here bring with them a certain dynamism that has the potential to create and destroy, aggravate and enliven.

The weather need not be a foe from which to insulate ourselves. Rather, our sense of humor and pathos ought to create the understanding that we are irrevocably entangled with the tumultuous atmosphere. The next time stormy weather descends on Seattle, grab a coat, relish in the moment and smile knowing that brighter skies aren't far away.

[Reach columnist Brian Smoliak at news@thedaily.washington.edu.]