Precipitation: a deposit on the earth of hail, mist, rain, sleet, or snow; the quantity of water deposited.
I wrote about our Pacific Northwest weather on April 30, but that was actually wussy weather. This time the combination of snow (8″) and freezing rain (1/2″) has the Portland metro area at a standstill. More snow and freezing rain are expected.
In fact the whole I-5 corridor, from the California border to Seattle, is an icy snowy precipitous mess and is supposed to remain so for the better part of a week. This bodes well for a white Christmas and poorly for a visit from my children and grandchildren, who were planning to drive up I-5 from the Bay Area for the holiday week with us.
My own car, which normally snuggles cozily in my garage, sits abandoned at the bottom of the hill on which I live. Yesterday morning, just before the weather made driving impossible, my son went to fetch our Christmas tree, and simply slid down the hill past our driveway and couldn’t get unstuck. The car is now blanketed in snow and sealed with a substantial layer of ice.
As is my entire yard, and my driveway. Very photogenic, but treacherous.
Until yesterday I didn’t really understand the weather terminology that describes what we’re now experiencing. So here’s a primer.
Precipitation comes in three forms:
- liquid (drizzle, rain – our usual PNW fare);
- freezing (freezing drizzle and freezing rain); and
- frozen (snow, snow grains, ice pellets/sleet, hail, snow pellets, ice crystals).
Snow begins in the clouds as water condenses into a tiny droplet. The droplet grows as more and more water vapor condenses onto its surface, then cold air freezes it into an ice crystal. As long as the air temperature remains at or below freezing the snow will reach the ground.
Freezing rain happens when it’s cold (below freezing) on the ground and in the upper atmosphere but there’s a warm air mass in the middle (temperature inversion). Snow formed up in the clouds melts when it passes through the warm layer then freezes when it hits the cold ground (roadways, trees, wires, shrubs, airplane wings) glazing them with ice. Freezing rain is notorious for causing travel problems on roadways, breaking tree limbs, and downing power lines.
Sleet also happens when snow melts as it travels through a warm layer of air, but because the warm air layer is thinner or higher, the thawed snow refreezes before it hits ground, so it looks like small hail and doesn’t glaze surfaces unless, as sometimes happens, it’s combined with freezing rain.
So I’ll carpe diem and catch up on household chores. Could be worse.