January 04, 2017

Dry Air Storm Hits the Pacific Northwest

You thought the BLOB was bad, with its gentle warming....

But now Northwest residents are facing a greater challenge:

An invasion of very dry air that will crack skin, irritate throats, increase itchiness of the eyes and skin, and enhance the risk of colds and flu.  Sparks will fly when you touch doorknobs and metal, and don't even think about getting near combustible or explosive substances.

What do we call this menace?   El Arido.  
The dry one.

The origin of El Arido is two fold:   cold, dry arctic air has spread over our region, with easterly flow moving down the Cascades resulting in even lower humidities.

A plot of the surface air relative humidities at 8 PM Tuesday shows low values of around 30-40% over NW Washington to around 50-60% around central Puget Sound (click on plot to expand).

El Arido brings splendid visibility, particularly over the western slopes of the Cascades.  This is particularly true viewing the Cascades from the Puget Sound lowlands (to illustrate see the picture below from Seattle's Space Needle cam around noon on Tuesday).  The mountains look so close you could touch them.

Why such good visibility with El Arido?   Visibility is reduced by small particles in the atmosphere, with a number of them (hygroscopic nuclei) absorbing water vapor and growing when relative humidities are high.  El Arido takes away much of the water vapor  and thus the particles stay small, improving visibility.   In addition, offshore flow prevents the large, water-loving particles of the ocean (including big salt particles) from moving inland.

But now let me show you the future, if you have the nerve to view it.  First, the UW WRF model forecast for OUTSIDE air relative humidity at 4 AM Wednesday.  Values below 35% (brown color) over much of Washington State and particularly over the western sides of the Cascades.

 At 1 PM Wednesday, El Arido is really cooking, with the entire state dry and the western part desiccated.
 By 4 AM Thursday, the situation has improved slightly over the West, and the Columbia Basin is a bit humidified.

A plot of relative humidity over time at Seattle (blue line) show values getting down to around 20%.

But folks El Arido is even more severe than that.  These are OUTSIDE relative humidities, the air is MUCH drier inside of homes and buildings.

Why?   Buildings are leaky and outside air diffuses inside and is warmed by heating systems.  

Imagine outside air of 30% relative humidity and a temperature of 32F.    The air is brought into the house and warmed, resulting in a further drop of relative humidity.   To understand this, one must think about the definition of relative humidity.

Relative humidity=  amount of water vapor in some air
                                 maximum amount of water in that air sample

Air moving indoors retains the moisture it started with, but the amount of water vapor the air can hold increases radically because the temperature has warmed (warm air can "hold" more water vapor).  Thus, relative humidities inside are far less than outside during the winter--sometimes less than 10%.

Tuesday afternoon, while the relative humidity outside my building was 40-50%, the relative humidity inside my building was about 20%.    With drier air outside today (Wednesday), the relative humidities inside heated buildings around Seattle will be around 10-20%.   A lot of sparks will fly and for some skin their skin will become dry and itchy.   You may develop a dry cough.  Cracks may occur in your skin.  You may pick up the flu or a cold.  Icy conditions may endangers those cycling to work.

Thus for some, suffering from El Arido will be much greater than any BLOB.  But these physical challenges will be balanced by bright sunshine and great visibility.   For those inflicted by seasonal affective disorder (SAD), the sun of El Arido will be welcome.

El Arido is thus a health paradox that will require further analysis of the most perceptive among you.


  1. Having grown up on the Front Range in Colorado and in Montana, these low humidity levels remind me of home and childhood. I feel for all the native north westerners for whom this air feels alien. I just hope this extra cold and dry air will help reduce the number of slugs next season.

  2. CO front ranger here... is there a simplistic explanation for how when you are at higher and higher altitudes 100% humidity doesn't *feel* humid? Growing up in texas 100% humidity was OPPRESSIVE (and any time i feel 70+% at sea level for that matter). But at 10,000 feet 100% humidity seems less... humid. So what gives? I can even hang clothes out to dry at 100% humidity and they'll still dry relatively quickly (outside under eave). Have myriad of thoughts swirling on this but never really arrived at anything conclusive.

  3. humidity is relative Marky! I'm sure CM can explain it far better however

  4. Brutal headwind biking east to work yesterday and today.

  5. @MarkyV, at high elevations the air is less dense and therefore will hold less water vapor. That's likely why it "feels" drier.

  6. Interesting, seems no one saw the high winds in south sound coming?

  7. I spent 20+ years living in Colorado Springs, and I'm no fan of that charged, dry air.

    Sometimes, when the Chinook winds blew, the air was so charged that people got irritable and crabby and you got shocks from just about everything. My wife had trouble breathing when the air was filled with charged ions. Even my dogs would freak out and not want to touch their noses to anything (and my dogs have always been big sniffers) because of the shocks. They would shy away from touching people. Cats would turn into balls of electricity, fur raised and tempers short. It was ugly.

    Thankfully I'm in southern CA right now, near the ocean, and the air is humid and warm. Rain several times a week. Feels like May in Sequim. Not what you usually think of for S Cal. By the time I get home in 2 or 3 weeks, I hope the PNW has regained its sanity and gone back to its moist and lovely self. I'd like to enjoy the last bit of winter.

    1. Wow. That's the worst part of dry air. The shocks. Cats are always electricity magnets even when the air isn't that dry. I'm afraid to touch mine now.

  8. Carmex is our friend at home for the Family.
    My wife and kids just got back from Hawaii for a week, talk about a climate shift!

    We still have about 2" of the 5.5" snowfall on New Years Day in Normandy Park. Going from Normandy Park to SeaTac airport, the snow drops off in amounts pretty fast.
    Its been a while since we have had a heavy snowfall and then went into the icebox!

  9. Any chance we will get any moisture during these frequent Frazier river outflow events, and actually end up with SNOW? Theyre getting dumped on on Oregon, while snow lover's here are high and dry (and cold).

  10. In Chicago we call this kind of weather "late November."

  11. The strong easterly wind off Baker and other high Cascade peaks this morning at sunrise was impressive. Huge pink spin drifts perfectly clear with the El Arido.

    Snow is pretty.... On mountains... Ready for warmer temps in the lowland. (Camping has been rough.)

  12. Our first bout of cool/cold air a few weeks ago was drier. I have kept an eye on RH this week expecting to see numbers in the teen as they were the first bit of winter air, but have been surprised that the air mass hasn't been drier. Dew Points have been pretty low, but not RH. My house is in the mid 30s for RH right now. I know I saw low teens in my office a few weeks ago, but it was in the 20s today. Yes, it's dry for us and I was into the Carmex today for the first time.
    I heard Crystal Mt. had a gust to 107mph? last night. Anyone remember, roughly, when Mission Ridge had a gust to 127mph this past year? Just trying to remember if it was a mountain wave or westerlies that produced it.

  13. Green Lake was frozen shore to shore this morning. That doesn't happen very often...

  14. Yes, El Arido indeed. Up here in Whatcom County Wednesday, sparks from a passing semi on I-5 caused a chain of brushfires next to the highway near Blaine. Brushfires in January on the west side? Impossible.....

    Yes, very arid.
    Abe Jacobson


  15. Brutally cold this morning in Eastern Washington. My neighbor's weather station on the Swauk Prarie outside of Cle Elum registered -12F at 8:24 this morning! I'm sure there were other spots that were colder...

  16. So we warm enough for rain.....then mid next week cool again for possible lowland snow? I need to move to Hawaii.....much easier forecasts

  17. I am new to the Bellingham are and this is my first winter. Is this normal to have this much cold/dry air or is there something happening that is different? Are the Fraser outflows that we are experiencing normal?

  18. Under 20f in Auburn this morning, coldest we've been all week, but we were suppose to be a lot warmer.

    Am I mistaken, but shouldn't the cloud cover today help trap the cold air at the surface? I'm doubting the warmup for Saturday, or at least not as warm as forecast.

    KOMO weather this morning said less sun means more heat, but yesterdays temp was quite high (just under 40, highest it's been all week I think), I've always been under the impression cloud cover with cold air will help keep the cold air near the surface.

    Why are we not expecting an overrun event on Saturday like we usually do with cold near the surface when a wet system comes in?

  19. I just had the flu last week. I wonder if it's related to this weather.

  20. Dry cracked skin this week, just as Cliff said. Even my feet. But the bright sunshine has been a great mitigation for short, dark day moods.

  21. On January 1st, we experienced strong Arctic Outflow winds where I live on Bowen Island, BC (northwest of Vancouver- sort of like Bainbridge is to Seattle). Howe Sound acts as a terrific funnel as the arctic high moves air out to the coast. Winds blew 55 knots, knocking out power along the corridor.

    This followed by the very dry air that spread into Washington. Humidity levels as low as 37%.

    Now (Jan. 7), cloud is moving in and humidity up quite dramatically. Freezing rain in forecast.

    The temperature gradient has been quite remarkable- just below freezing in Vancouver, and low 20's (Fahrenheit) just a few miles away.


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