Thursday, January 13, 2011
Freezing Rain Hits the Northwest
It almost belongs in Ripley's Believe it or Not
Can water be below freezing and still be liquid? Liquid water at 30F or even lower?
The answer is yes and we call this supercooled water.
It turns out that the atmosphere is FULL of supercooled water, most frequently in middle clouds in the atmosphere.
See this cumulocongestus cloud? The upper portions are undoubtedly mainly liquid water even though the temperatures are well below freezing. Sometimes rain is supercooled and when it hits the ground it freezes on contact--this is called freezing rain and we had some this this week in the Gorge and in the passes (and even a few reports over the lowlands)
So how is this possible?
Well you all know that water is made up of water molecules that are weakly bonded together, yet able to move around. Ice occurs when the molecules get locked into a hexagonal crystal structure and weakly vibrate in place. Now if you have clean water in the middle of the atmosphere, not in contact with any particles or surface, the water molecules are in no hurry to get all lined up as a hexagonal crystal when the temperatures cool below freezing. Think of trying to get some jumpy kindergarteners in line--it takes some doing. Only when temperatures get to -40F (-40C) will you be guaranteed that the water turns to ice. The transition to ice can be helped along by freezing nuclei (little particles than might encourage the transition at say -10 to -15C) or by contact with some surface (which encourages this to happen at freezing). Most of the time you experience water in the latter situation so you might conclude water HAS to freeze at 32F. But you would be wrong.
This week we had some freezing rain at the surface in our area and the set-up is similar at most locations. Here is a little figure that shows how it works. You start out with cold, subfreezing air at the surface. Then you have warmer air move in overhead, warm enough that the precipitation aloft turns to rain (snow really high up melts into rain). For us this is usually associated with warm, southwesterly flow associated with approaching Pacific weather system. Then the rain falls into the subfreezing air near the surface and cools to below freezing and yet is still liquid. Magic! You have freezing rain. And serious danger.
Freezing rain is like liquid dynamite...freezing on contact with the surface. Freezing rain not only makes roads treacherous but can take down powerlines or close down airports.
So what about this week? Portland had a "good" freezing rain event as cold air moved westward out of the Gorge, while warmer rain feel into it from aloft. Hours of freezing rain--something known as the "silver thaw." The Gorge is freezing rain central. Here are the observations at Troutdale in the Columbia Gorge for earlier this week. You can see the transition from snow to freezing rain (ZR stands for freezing rain, F for fog).
The Cascade passes get a lot of freezing rain as shallow cold air from E. Washington moves westward into the passes when a warm Pacific system moves in.
And eastern WA also "enjoys" plenty of freezing rain. Cold air gets trapped in the basin centered on Hanford and the Tri Cities and warm rain from an approaching Pacific weather system falls into it.
So we know freezing rain in the NW, but it is relatively rare over the Puget Sound lowlands.
For you video lovers...here is an example of what freezing rain can do:
Posted by Cliff Mass at 8:44 PM