January 18, 2024

Freezing Rain or Rain That Freezes? They are NOT the Same!

On Tuesday there was a lot of talk about freezing rain events, ice storms, and the like.   There was also a lot of confusion and misinformation about what happened Tuesday evening as precipitation moved in.

As I will describe, there was a lot of above-freezing rain falling on cold surfaces, but very little freezing rain.  Few locations experienced freezing rain, but some locations had rain that froze on localized cold surfaces.


So what exactly is freezing rain?

Here is the definition provided by the National Weather Service:

Freezing rain occurs when snowflakes descend into a warmer layer of air and melt completely. When these liquid water drops fall through another thin layer of freezing air just above the surface, they don't have enough time to refreeze before reaching the ground. Because they are “supercooled,” they instantly refreeze upon contact with anything that is at or below freezing (32 degrees F), creating a glaze of ice on the ground, trees, power lines, or other objects.

The Wiki definition is pretty much the same:


The figure below illustrates a freezing rain situation (from NOAA). 

Aloft, where the air is below freezing, all precipitation is in the form of snow.  Certainly true this week.   

Then the snow falls into a layer above freezing and the snow melts into raindrops. The water in these raindrops is above freezing.

Then the raindrops fall into a modestly deep BELOW FREEZING layer and raindrops cool below freezing (32F, 0C) but remain as water.

Yes, believe it or not, liquid water can cool below freezing and stay liquid.  This is called supercooled water.

Supercooled water has been described as "liquid dynamite" because if it hits a surface below freezing, it freezes on contact.  Instant and profound icing that can be very dangerous.

Freezing rain can accumulate on trees, causing branch failures and power outages.  It can fall on subfreezing roads causing near-instant glazing, resulting in accidents.


And it can cover aircraft with ice, halting operations.

This is NOT what happened on Tuesday evening over Puget Sound country.  Very little freezing rain occurred.  Very little impact occurred.

I can prove that this was not a freezing rain event by showing you the temperatures above SeaTac Airport on Tuesday.  The figure below presents winds and temperatures at the airport based on sensors on incoming and outgoing aircraft.

At 7 PM, the freezing level was ABOVE 5000 ft.  Temperatures were above freezing all the way to the surface.   A narrow band of precipitation was just reaching Seattle at this time (shown by radar image below).

 The snow aloft fell into the warm air below and rapidly melted (it takes about 1000 ft to melt snow into rain).



7 PM weather radar

By 10 PM, the precipitation band was over Puget Sound (and would be through by about 1 AM)--see below.  Temperatures were still above freezing through much of the lower atmosphere, except for a narrow layer where evaporation of falling rain cooled the air to near freezing.  The surface air temperature at SeaTac at 10 PM was 31F.   

10 PM radar

However, such a shallow layer of near-freezing air did not warm the rain by much and it hit the ground above freezing (I measured the temperature of the rain at my house that night and it was roughly 35F).

Thus, this was not a freezing rain event.  

Freezing rain sensors at both Olympia and Boeing Field did not report freezing rain at any time.   At SeaTea (which is relatively high at 452 ft) it reported light freezing rain for one hour only (which may be a sensor issue).

Unlike true freezing events, there were few reports of accidents, roadway closures, falling trees, or power outages.

But some folks reported localized icing.  What happened if this was not freezing rain?

Answer:  It was rain, relatively warm rain, the froze eventually in some locations where the surface was below freezing.

We had just finished one of the coldest periods during the last 30 years in our region  Looking at 3-day stretches, the period ending January 14th had the coolest average high temperatures (26F) in the period.  Many surfaces were cooled down.


But now it gets complicated, December was relatively warm and the soils below the surface were above freezing.  For roads in contact with warm soil, many were above freezing on Tuesday evening, with the above-freezing air temperatures that day helping to prevent freezing surface conditions.   With relatively warm rain, there was no ice.

Thus, most/roads were fine.   But in places that were particularly cold (e.g., shaded locations) and where there was not good conduction of heat from below, the previous cold spell left a vulnerability to icing.

In short, this was not a freezing rain event where supercooled water freezes instantly on cold surfaces, but a situation with warm rain freezing on some chilled surfaces.   

A subtle distinction perhaps, but an important one.



14 comments:

  1. It is super slippery! Our sidewalk didn't even look wet but I still had an ice skating almost fell moment.

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    Replies
    1. It's kind of lost its glamor, for me, anyway!

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    2. But my favorite type of cool precip, is sleet! We seldom get to see this weirdly gentle type of "snow". Sleet looks like pills, sort of like the larger Tylenol tablets...They can cover the ground more quickly than regular snowflakes, but the sleet only forms for a few minutes,,.. it is a fascinating situation to see, while it lasts.

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  2. Well, I learned something new. - I appreciate your objective and science-based explanations of weather issues.

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  3. Thanks for pointing out this distinction, Cliff. Tuesday night I observed that our sidewalk and driveway were just wet, but our car and grapevines were coated with ice. Also, icicles were forming along the bottom of the car even though the temperature here according to my Ambient Weather Station was 30.4 degrees. I suspect this means the rain was running down the car and then freezing rather than freezing on impact.
    I'm hoping you will blog on the weather the last couple of days and the difficulty we've seen forecasting warming. Thanks!

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  4. I'm in North Seattle in the Haller Lake area, the elevation here is close to 450 feet. I woke up Wednesday morning to not only the rain on the ground being frozen, but bushes and tree branches had a layer of ice and some were leaning over, pulled down by the weight of the ice. I have no doubt the ground was cold enough to freeze the rain once it landed, but I have a hard time believing these branches were so cold that they froze above freezing rainwater on contact. Is there any chance that this area, being at the elevation that it is, that this was actual freezing rain?

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  5. No rain yet on San Juan Island @250' elevation. Snow fall "6 on the ground from today and yesterday. What a nice surprise. Break out the skis, quick, before the warm front arrives!

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  6. We had freezing rain in North Bend, everything was (is) coated in 1/4" of ice. Trees, street, sidewalk, houses, slow moving animals, etc....

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  7. Very well explained! We did - very briefly - have a wave of bona fide freezing rain here on Tuesday night, 30F and just ahead of the approaching blizzardlike front...only lasted 40 minutes (thank goodness). I phoned-in a Skywarn report to NWS Seattle. I keep a small panel of plastic for snowflake observations, and (wow) the phenomenon (looked like jacketed crystals) was textbook.

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  8. We most certainly had TRUE freezing rain in Monroe and Sultan. LOTS of downed trees due to the weight of the accumulated ice, LOTS of power outages across the area. The photo of the tree you posted has icicles on it. That's not what we had, we had ice-coated branches. Check again.

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    Replies
    1. That is true...and that is because of cold air moving westward in the Skykomish Valley. Completely different situation on what happened in Seattle and the south Sound.

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    2. Thank you for the clarification. I'm 3 miles NE of Monroe - posted some pics to twitter and you were tagged by someone else - so it definitely was freezing rain. It was the worst I'd seen since early 2004 when I had an ice storm much further west in Snohomish.

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  9. I believe that almost everyone here in Portland understands all too well the differences between the two weather idioms. Unfortunately both kinds caused widespread havoc and power outages, for some lasting over five days (over 180,000 homes at one point). First we had the hurricane force winds coupled with crippling cold and snow that felled hundreds of huge Ponderosa pines falling and taking out large amounts of electric wires, followed up with an ice storm. During clean - up operations we had another ice storm that took out even more homes, then finally we had yet one more ice storm that affected locations such as mine, which were subjected to even more cold winds blowing through the Gorge.

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  10. That was a strange one for sure that night. In Maple Valley we had an ambient temp of 34, strong east winds and a dewpoint of 29. The roads were fine, but cars, trees, bushes and other things were getting coated in ice. I was thinking we had some low level evaporative cooling that was causing the rain to freeze on contact.

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