Tuesday, January 20, 2009

Black Ice


What is the the weather condition that kills and injures more Washington State residents than any other? Floods...no. Windstorms..no. Tornadoes or Thunderstorms. No way. I am pretty sure it is roadway icing...and that threat is very real this week.
Roadway icing is often called "black ice"--but it really isn't really black...it can look dark at some angles and twinkle in others. Roadway icing usually is associated with high pressure situations, with no rain and light winds. There are several ways you can get ice on the road:

1. Frost--skies clear under high pressure and the earth radiates heat to space via infrared radiation. The surface thus cools sufficiently that frost forms on the ground. Frost can be slippery, but it is generally thin.

2. Fog. The big threat. If the ground cools to below freezing and if some nearby fog passes over the ground, it can rapidly and severely ice up.

During the past few days both of these have occurred. Two days ago, I had frost on the street outside my home. Then the fog bank moved in and a 1/4 of ice covered everything...just treacherous.

Keep in mind that on cold, clear or nearly clear nights the temp of the ground can be 3-6 F cooler than the air temperature at 6 ft, where official measurements are made. So if you hear that temps have dropped to 36F or your car thermometer is down to 35...you should be worried, particularly if fog is around.

Once in a while I have testified at legal proceedings about icing accidents, and many of them have occurred under clear nights, with nearby fog, a bend in the road or a turn, and often someone driving to fast for the conditions.

If you want to review a detailed tutorial on this subject, check out:
http://www.atmos.washington.edu/~cliff/Roadway2.htm

By the way, the inversion has strengthened over the central Sound and in some places the fog remained most of day (like at the UW). (see graphic of inversion from the profiler). You can easily get out of the murk by going up (above 600 ft should do it). Air quality has declined as result, so no burning if you don't have to.

Finally, a reminder that I will talk about Lowland Snowstorms at Third Place Books in Lake Forest Park tomorrow (Wed) at 7PM.

25 comments:

Anonymous said...

I got such a laugh when I blew up the picture of your “High Accuracy Thermometer” and saw that it was a Radio Shack indoor/outdoor digital. That was what I used to make the temperature measurement that I sent you from my Tiger hike last week, although I think that my unit is a few years older than yours. I mounted the sensor on the back of my pack to isolate it from my body heat. Also the sensor has low thermal mass so it tracks pretty well when you are on the move. I took care to keep the sun off of the sensor so I felt pretty confident about the numbers that I got.

Cliff Mass Weather Blog said...

Don't laugh! I checked it out against very expensive thermometers and it was very comparable. Who says cheap can't be good?

Josh said...

Any ideas what's going to happen next week? The weather channel is showing rain/snow is in the forecast for wednesday and Thursday of next week. 60% chance?

andycottle said...

Hey you know what? As long as it`s reliable, works good, and for a well reasonable price....then cheap is good. On some things that is.

andycottle said...

Hi Josh. I would`nt depend very much on TWC. It`s best to with your local NWS forecast.:o)

Anyway, hard to say what will happen. Short term for late this weekend into early work work week shows mainly dry N-NELY flow and 850mb temps lowering about -9, -10c with surface thicknesses in the upper 510`s(517-518m). So probably some cool/cold days with VERY frosty nights. On the flip side, 0zGFS wants to have a cut-off low south of our region for late next week with a weak ridge over WA/OR, while 12z/00z EURO models show a few cold troughs dipping down over our region for the same time period. So will have to see what really happens.

Snowlover said...

Are we ever going to return to winter? The past two years have seen more "winter" activity happening in Fall and Spring than winter. That's weird.

Josh said...

You dont call the snow storm and rain storm we had "winter activity"?

natchrl8r said...

Brightest, sunniest day yet in Bellingham today. We have yet to see really persistent daytime fog here. Beautiful white frosting last few mornings. Around here, anything can be winter weather. I am no longer surprised by the rare 60 degree December or January day. I also look forward to the reliable warm, spring days in February, April snow storms, 80 degree May days and 50 in June. Interesting always...

SnowLover said...

@Josh Yes, we had snow, but remember Winter began in the middle of that. So we had about 2 weeks of "winter" then we went back to spring like weather. I seem to remember this last year too with November being more "winter" than winter was, then spring being more "winter" as well.

Anonymous said...

We have had crystal clear skies in east Woodinville, elev 530' for the past few days, no fog to speak of at all, that's a switch. And you lowlanders are not only in fog, but yellow, nasty, weird fog...

Begreen said...

There is confusing information coming from Puget Sound Air Quality. Although news reports are indicating that the air quality has deteriorated to a condition that may affect the health of sensitive persons (orange on the alert status), the metrics on pscleanair’s website do not support this claim. Since Jan. 17, the particulate matter graphs at: http://www.pscleanair.org/airq/aqi.aspx show the air quality never getting into the orange zone. Today, half of the reporting stations are showing the air quality as good. Yet we remain under a stage 2 burn ban. Is the equipment in error? Is there a reporting error? Or is this a case of over-zealous authority? Why is there a stage 2 burn ban with these air quality metrics?

Anonymous said...

would love to know what next weeks weather could possibly do.....I know the models are all over the place but do any of them hint at any snow that would impede travel or any wild winds, etc? I have to travel for work next week....always seems like the weather acts up when I have to travel!

Anonymous said...

I've been wondering about that, myself -- regardless of what the Puget Sound Clean Air Agency says about the air quality, I can tell you it's not very good; folks I know with lung diseases like asthma and reactive airway disease are having some difficulty.

The Puget Sound Clean Air Agency is very clear, though, that there's a burn ban in effect -- stage 2, so no indoor burning, either ("unless this is your only adequate source of heat"). We're making do, but won't be able to get it above 60 F inside without the wood stove if the weather gets any colder.

At this point, I'd trade the sunshine for rain if it meant easier breathing and a warmer house!

- Morgan

mjgrota said...

Begreen's comments convey the complexity of air quality forecasting. Forecasting air quality levels is interesting and counterintuitive. The stage 2 burn ban was issued once measured criteria was met. With no change in the synoptic pattern to break the inversion and restore effective mixing and dispersion expected for >48hrs. Controls are necessary to slow or reverse pollution trends. If you are successful, air quality concentrations should be below the forecast. Forecasts are indicators of expected air quality levels based on predicted meteorology and expected public behavior. For clarity burn bans are only issued on a county wide basis. Fortunately it seems that the public is responding to all of the great coverage of this event and many are choosing to comply with the burn bans. There is a real awareness that our laws permit the lawful use of wood burning devices and PSCAA promptly lifts bans when well mixed weather patterns return. It is noteworthy that the new law permits the issuance of stage 2 bans at levels below the EPA standard. It will not take another decade before the next one is issued.

Henry Romer said...

There is some important physics going on near the surface in black icing conditions. Near surfaces, a film of still air accumulates that acts as an insulation barrier between the general air mass and the surface. It acts very much like the insulating glass cover space of a single glazed solar collector. On a clear night the surface is seeing a cold night sky temperature, determined by the infrared absorption in the water vapor content in the air column. The radiation balance for the surface is the difference between the warm surface radiation and that of the cold air column. Remember that this radiation occurs as the 4th power of the absolute temperature. Insulated by the surface air film, the differential radiation transfer will cool surfaces substantially below air temperatures. I expect that some of the cooler air temperatures measured at the surface is conduction cooling from the surface back into the near surface air. Some, of course, will be density gradient flow as well. This radiation cooling effect is also responsible for fiberglass and metal car porch roofs dripping from the underside in clear nights. The radiation cooling cools the roof surface and condensation from the air forms on both surfaces. The condensate on the bottom surface drips off and is annoying. An insulated or less conducting surface will not experience a temperature difference on the lower surface and will be dryer.

Anonymous said...

Depending on which model you believe, Sunday could deliver a couple inches of snow to the area.

Brian said...

I am wondering the same things as a few other commenters are about the long-er range forecasts. Any wild weather to speak of at all? Wind, lots of rain, more snow, etc. or are we going to stay in this dry period for a while?

I think its time for a good, old-fashioned forecast :)

John F said...

Any chance you talk about lowland snow storms will end up in a podcast?

Cliff Mass Weather Blog said...

All..yes, some cold air is returning this weekend, but at this point I don't see much lowland snow. More on this tonight...cliff

Anonymous said...

Any chance this fog will end, or at least burn off for a few hours a day ?

Teresa said...

How about "high land" snow, such as at 580 feet on Demery Hill in Sammamish?

andycottle said...

Here on Hollywood hill, Woodinville(Elv roughly 300ft), I had some early morning fog, but then skies were partly cloudy for rest of the day. My high today was 40 with low of 25.

Anonymous said...

Andy,

Any ice fog out there in Woodinville? Sunny and warm in North Bend today!

TT Sea

andycottle said...

Well Hi Tim!:o) Nice to see you on Cliff`s blog as well.

No ice fog today that I could tell, as for was mainly gone by about 930-ish this morning.

But geez...yesterday? It was PEA SOUP fog last night! Could barely see the trail on my bike ride home from work last night.

Anonymous said...

The burn ban isn't all that well publicized, unless you're a weather geek (like us) or own a tv. - Morgan