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:


  1. We had unusual freezing rain in around Yakima. What was different than usual is the temperature was near 20. Almost always here in sheltered valleys, that cold of air results in sleet, not freezing rain.

    By the way, computer models really struggle with warming up E Wash. too fast.

    But the process in which it scours out is fascinating. The hourly observations alone tell the story.

  2. That is a frightening video. I hope the people in the car were OK.

    Overall, thank you for the clear and detailed explanation of freezing rain and the atmospheric conditions which cause freezing rain to occur.

  3. We are a new small family owned business in Auburn Wa.. We do liquid snow removal (mag chloride) in winter and landscaping in the summer, so weather has a huge influence in our lives...!! If you decide to use advertisers we will try and support you with a small tastefull ad of some nature. You can see us at seattlesnow.net.

  4. There was a significant freezing rain event in Black Hills between Aberdeen and Olympia 8-10 years ago- The upper third of all the Alder trees in the area snapped off by the weight of several inches of freezing rain creating quite a scene- It appeared as though there were whole groves of sharpened stakes.

  5. We had freezing rain in Yakima yesterday. All of the local schools were 2 hours late, and several closed. Driving on it was worse than driving in snow! Plus it took me nearly 10 minutes to chip the stuff off my windshield and windows. Give me snow any day -- this freezing rain is awful.

  6. And that video was a good example of what NOT to do! Why would anyone open their car doors and jump out? Wow!

  7. Thanks for all these awesome blog entries. This is such a cool blog! A simple blog fee of $10 or something would be fine with me! Keep up such awesome work! And thank goodness we never got freezing rain! Just 5.5" of pure fun in Silverdale.

  8. My neighborhood in NE Portland was hit pretty hard late Tuesday into Wednesday morning. I have yet to find anyone else in Portland who experienced it.

  9. When did Towering Cumulus (old weather symbol TCu) start getting called "cumulo congestus". As a total cloud freak when a pre-teen 55 years ago, I thought I could name all the clouds from stratus through noctilucent, but I never heard of cumulo congestus.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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