Monday, December 7, 2009

Why no frost?

It was quite cold this morning, with temperatures dropping to the low 20s and even teens in some western Washington locations. You don't want to know how cold it is in eastern Washington! Tonight will be even colder.

Several people have asked...why was there little or no frost this morning, even though we had the coolest temps of the year so far? The answer: low humidity.

Our dew points have been very, very low. (Remember what dew point is? The temperature to which you have to cool the air down to get saturation. Below 32F we really should call it "frost point" but we never do). The dewpoints this morning were in the single digits and even below 0F for most of the region. So you would have to cool the air down that low to get frost...and we didn't. So the temperature remained well above the dewpoint and we didn't see any frost or fog. Here are the current observations....same story tonight...LOW dewpoints

That is good thing. We had lots of frost on Saturday morning and slippery roads and accidents were everywhere. I almost spun out myself. One woman was killed. Never forget, roadway icing is the number one weather killer in our area. More than floods, more than windstorms. I put a whole section on this in my book for a reason. I have also created a web page just on this topic (sponsored by Washington DOT):

http://www.atmos.washington.edu/~cliff/Roadway3.html


Most of the time such accidents can be avoided by driving slower. And make sure you have decent tires and get Vehicle Stability Control on your new or used car if you can afford it. Remember: the road temps on clear nights can be colder by 2-5F than what you see on your car thermometer, the official weather reports (which are for locations about 6 feet above the surface), and certainly the often inaccurate bank thermometer. And bridges ice up soon because they don't get the heat conducted from the soil.

Air quality is still quite good, but it may decline in time. A good place to check on it is the web site of the Puget Sound Clean Air Agency (http://www.pscleanair.org/)

Finally, some of your have noted that PROBCAST is going 7F for Seattle tomorrow. Probcast is wrong, trust me. And I know why...we are using an early version of our current computer models to drive it and it is not dealing with this cold period correctly. As soon as I can secure funding for new computers I will upgrade the whole system--so don't trust the low temperatures. Probcast is an experimental system.

Finally, what about snow? Well, to get snow in our area you need cold and wet and it is very hard to get both together. Well, we have cold now...all we need is wet. The big snow threats are moving into and out of cold periods. We got in without much, but what about the exit period? Thursday could have some scattered snow showers, but the real threat is over the weekend as we transition to a warmer, wetter regime...more on that tomorrow.

24 comments:

HarrisonCZ7 said...

Great entry!

Absolutely bitter cold over here in Spokane.

Right now it's 2F outside my dorm at Gonzaga U. Brr...

JewelyaZ said...

Thanks for the Probcast news... what can we do in the campaign to get the money for the computers you need? :-)

IF we'd managed to get frost or even light snow early last night, it would have sublimated (evaporated) into the atmosphere by morning, right? We had a very lively conversation about this in the hallway this morning and would love your expert declaration one way or another.

So glad it hasn't been very icy... it really is so treacherous and it can sneak up on you in the early morning darkness!

Corie said...

At 4:45 am in West Olympia it is 8 degrees. I don't think it got that cold last year!

Stine said...

Thanks for the explanation. I was out on my bicycle Saturday AM (my neighborhood didn't seem so frosty), and it was quite a commute. Yesterday was fine, and I'm gearing up to get out and pedal this morning.

Kenna Wickman said...

Lots of frost here in Kingston both yesterday and today. Its actually amazing in the dark with a good halogen flashlight. The frost is quite crystalline and sparkes in this light. Individual crystals appear about 1/8" across.

KW

Vince said...

It is 6 degrees at the Olympia Airport....on the very cold end of the probcast for last night

K said...

I like not having the frost and icy road conditions. Is there a website that has predicted dew point?

natchrl8r said...

Why YES frost? Here in Bellingham we had a low temperature of 18F and dewpoints in the single and negative digits. But there is new frost on the grass and rooftops this am. Why?

Christopher said...

The NCEP analyzed fronts page from the UW website, link at
http://www.atmos.washington.edu/cgi-bin/latest.cgi?fronts-ir
shows a low pressure area just off Vancouver Island and two lows up in the Yukon area, but doesn't show the strong high which is supposed to be flooding this cold air into us, and indeed doesn't show any highs at all. How come?

shari said...

As a survivor of 17 years in the high mountain desert of Idaho, I had to argue w/my Chicago-native husband about the whole frost thing. He refused to believe it was below freezing, until I showed him our pond (frozen solid). Time to pull out the Chapstick!

todd said...

how come it has been so much warmer in hanford over the last few days than anywhere else in the pnw?

Kevin Fink said...

Re: "As soon as I can secure funding for new computers I will upgrade the whole system"

What kind of computers do you need? I wonder if you could get Amazon to donate some EC2 time - they could write it off and it wouldn't actually cost them anything if you were only using excess capacity. Plus it would be great marketing for them. If they aren't interested, I bet you could get one of the other cloud providers to do it.

Cliff Mass Weather Blog said...

Yes, some locations have had a slight frosting. How is this possible? Some areas are moister (high dewpoint) than NWS locations. Like if you are near water or moist areas. Also there can be more moisture right at the surface. Second, temps can be much colder at the surface than at 2 meters where official temps are taken. Probably other factors I haven't thought about!..cliff

Rus said...

8* in East Olympia at 0830.

Andrew Poultridge said...

So now I see Olympia has both triple digit and single digit temperatures within the same year. Has this ever happened before? if so when? where would one search for this information?

Joseph Ratliff said...

The later part of the week now seems to be trending colder than it was...more towards mid 30's highs and mid 20's lows instead of 40's and 30's...and it seems the snow chance is creeping up. Hmmmm....

zmb said...

Cold enough this morning to have ice on the interior bus windows (with heat on full blast)on my way to school!

theartist said...

Hmmmm, latest forecasts look like they're bringing snow showers in on Friday/Saturday and some out and out snow on Sunday. Rain in time for the Monday commute.

Wonder if it'll come off?

athos said...

Komo is saying we may get snow this weekend. Others are saying rain. Thoughts?

Must read blogs said...

hubby's car would not start this am. It was 8 this am her in chehalis, I put 2 sweaters, a jacket and a parka on the 8yo with a hat and mittens. I also put thick moisturizer on her face so it wouldn't chap. Yes I am a protective mom, but its worth it!

RobbyRob said...

Cliff,

It seems that with each run of the GFS it keeps the Northwest colder longer, are the other models trending the same way?

Cliff Mass Weather Blog said...
This comment has been removed by the author.
Tom said...

As a motorcyclist who rides in the "off" season, road ice becomes a major concern for me. Is there some kind of a road ice risk map online? I usually go to a weather site to look at recent air temperatures and relative humidity, and then make a guess as to risk of ice. When I guess there is some risk, I drive the car instead of the motorcycle. Just wishing I had something more substantial than my guessing skills to depend on!

Joe Anderson said...

The icy roads probably aren't the source of killing people. A lot of people seem to think they can drive the same way on icy roads that they do on dry roads (which is over the speed limit, hence illegal and unsafe even for dry conditions). So if "accidents" happen, this isn't the "fault" of the icy roads or even the weather, but rather the people driving themselves.

If people are being cautious and driving slowly, and still slide on the road, then you can perhaps call it the "fault" of the icy roads. But for the most part, it is the fault of the drivers themselves. Weather really doesn't 'kill' many people; people just need to learn better to adapt to different weather conditions, that's really the main "problem."