Tuesday, March 5, 2019

The Driest Air in the U.S. is Found in SEATTLE

You may not believe this.  But the driest air in the U.S. Monday was in Seattle!


So why go to Palm Springs, Phoenix or Tucson when you are looking for nice, dry air?
The relative humidities are much lower right here in Seattle the last few days.

I suspect this Emerald City aridity is not a surprise to you.   Notice the scratchy throat?  Cracks in your skin?  Or the urge to drink more water than usual?   Or perhaps a spark when you touch a door knob?

Let me show you how dry it is.  Below is a map of the surface (2-meter) relative humidities around the nation at 7:40 AM yesterday morning.   24% in Seattle, 20% in Bellingham.  But 55% in Palm Springs, 89% in San Diego and 61% in Las Vegas.    Seattle beat them all for dry air...no one is even close.


This morning at 5:30 AM?   Seattle still has the driest air!


To appreciate the situation, here are the relative humidities for the past 12 weeks.   The last two days had the lowest relative humidities for the entire winter--around 24%.   I am getting thirsty even thinking about it.

What about dew point temperature, a measure of the absolute amount of water vapor in a sample of air? (lower dew point means drier air).  Monday had the driest air of the LAST SIX MONTHS, with the dew points getting down to a stunningly low -1F.

A forecast map of relative humidity at 1 PM Monday over the Northwest shows very low relative humidity over Washington State and southern BC, extending eastward towards northern Idaho and Montana.


So why has our air been so crazy dry?  A combination of cold air, mountains, and easterly flow.

The interior of the region Idaho and particularly Montana has had EXTREMELY cold Arctic air over it, with many sites breaking long-term low-temperature records for the month.  Such cold air is also very dry (cold air can not hold much water vapor). 

Relatively humidities are low, but not as low as Seattle's.  Why? Consider the definition of relative humidity--water vapor content  of an air sample divided by the maximum water vapor it can hold at a temperature.  Since cold air can not hold much water vapor, even a slight amount of water vapor produces a modestly high RH.

But with high pressure inland, there is offshore (easterly flow), so that the air is heading westward.  As it sinks over the western slopes of the Rockies it warms by compression and that causes the RH to plummet (since warmer air can hold more water vapor).  And there is even more warming as the air sinks again over the western slopes of the Cascades (see figure).  So we start with very low amounts of water vapor and then warm the air, resulting in very low relative humidities.

The bottom line of all this:  we have experienced amazingly dry air for the last few days, with many locations around western WA getting down to 20-25%.   And the air INSIDE buildings is even drier, getting down to under 10%.





20 comments:

  1. And here I thought I was getting (yet another) winter cold!

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  2. Super interesting to know how relative humidity works in Seattle, Cliff!

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  3. The lowest RH I measured in NW Bellingham was 20%. Lowest dew point -1F.

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  4. Are you saying Seattle had the lowest water vapour content?

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  5. I've get a cold too, and I suspect the super dry air has something to do with it.

    I'll repeat my question for before: Please tell us when the moist, mild sea air will return. The sun is great but with this cold I find air this dry uncomfortable. My house plants need to be watered about twice as often as normal! Even my outdoor pond has evaporated down a couple inches. This almost never happens in winter.

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  6. The static electricity has been crazy. Can't take my jacket off without creating an EMP. :)

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  7. Drier in Bellingham than Seattle. The wind & dry air is quite tedious. Count me among the real Western Washingtonians yearning for some rain!

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  8. Cliff, I always get confused with what is written about dew point temp. Every reference says a high dew point temp makes the air feel "muggy". But is it that simple? If we have a high dew point temp, but also a sufficiently high air temp, then the relative humidity is low. In this case there is plenty of room for evaporation to occur. It has been explained that, the ability for moisture to evaporate off our skin, or not, is what triggers the feeling of "mugginess". So, we could have a high dew point temp, but if the air temp is also sufficiently high, will the air not feel "dry"? This would occur because even though there is a high absolute amount of water in the air, as long as there is room for evaporation, we sense "dry" air. Yes/No/Other?

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  9. The air is drier than my sense of humor.

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  10. John K,

    Yes- up to a point. A person's skin had better not get much above normal body temperature! If the dew point were, say 85 degrees (a situation I have experienced in July 1992 in the Bahamas) the air will feel muggy whether the air temp. is 85 (saturated) 90, or 100 degrees. If it were 95 degrees at a dew point of 85, raising the temperature even further is not going to be a good thing. So yes, there might be more evaporation from your skin at 100 degrees, but there comes a point where it cannot compensate for direct overheating.

    People do perceive things a little differently. I say that it feels warmer at 65 degrees with a high dew point than low. Not everyone agrees. But assuming there's no standing water on your skin, I think you'd find 65 degrees fairly comfortable at a dew point of 60, but would feel cold in dry air like today.

    Although a dew point of 65 and air temp of 70 would feel good to me about now...

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  11. I did a quick trip from Wenatchee to Seattle yesterday and it could have fooled me the air was dry. Never fails to smell so good and fresh, compared w/ E. Wa. Can you explain why Wenatchee still getting snow?

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  12. Most people will find dew point temperatures of 75F or greater to be uncomfortably "muggy" regardless of the ambient air temperature and relative humidity. For instance, at an ambient temperature of 110F with relative humidity of only 34% the dew point temperature is 76F. My experience is that ambient shade temperatures of 110F and above are uncomfortably warm regardless of the humidity though perhaps those thoroughly acclimated to very hot desert climates would be less bothered. The flip side is that at ,say, an ambient temperature of 25F and relative humidity of 34% the dew point is a mere 1F and as those of us in Whatcom County in particular can attest, conditions this dry tend to result in rather unpleasant drying of the mucous membranes. Essentially, relative humidity in isolation is unimportant with regard to human comfort unless the values are particularly extreme. For example, I grew up in the deep south, an area famous for its oppressive "heat and humidity". However, it's not the humidity per se which is especially important in producing the "muggy" conditions, but the high dew points. Seattle generally has higher humidity than Atlanta but Seattle dew points rarely exceed 65F while dew points in excess of 70F are the norm in Atlanta. Basically, dew point is the primary determinant of comfort except in the case of extremely high ambient temperatures (>110F) or extremely low relative humidities (<10%).

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  13. Ansel - thanks, that is very interesting. So you feel that if the air temp is not "too high", a higher relative humidity can make it feel warmer. I agree. I hear people say they don't like "damp cold", but I think it's actually easier to take than frigid, dry air. But yes as you are saying, if your skin is actually wet and the air is cold, then you are going to feel very cold - no way around it.

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  14. It looks like the chance for a major snowfall event is over for the year, but it seems like Thursday could be interesting. The national weather service no longer mentions thunderstorms, but the weather channel still mentions the possibility of thunderstorms Thursday evening with a 40% chance of snow overnight and a low temperature of 31 degrees as a cutoff low moves eastward. However without a strong flow of arctic air out of the interior of BC it doesn't look like a major snow situation.

    It seems as if there could be isolated areas of heavy snow during thunderstorms, but I don't know enough about forecasting to guess how much snow will fall and if it will be cold enough for snow to accumulate significantly in areas that get thundershowers. Maybe the weather channel will send Jim Cantore to Seattle just in case we get heavy thundersnow so that he can jump up in the air and shout out "we've got thundersnow!" just like he did in Boston in 2015. However in terms being spectacular it will be hard to top February 27 2017 when lightning hit the space needle during heavy thundersnow.

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  15. No wonder my hands feel like leather

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  16. My personal experience is that dewpoints above 60F start to get uncomfortable, and below 55F feels really good, but anything below 40F dewpoint feels too dry.

    Thankfully, dewpoints between 40F and 60F are remarkably common here in Western WA. Low 50's seems to predominate most of the year.

    But right now, dewpoint is 25F at my house in Sequm, and my dogs don't want to be petted. If I run my hand down their backs in the dark right now, the flashes are shockingly bright. Not comfy for dogs or me.

    Looking forward to Wed and Thurs moisture, if we're lucky enough to get any.

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  17. OK, so when it is said that "a high dew point temp makes us feel uncomfortable" what that really means is 2 things.. the air temp is high AND the relative humidity is high. The high air temp makes us sweat, but the high relative humidity impedes that sweat from evaporating. Thus we feel hot, covered in salty sweat that is not able to evaporate and cool us off.

    I still don't like the statement "a high dew point temp makes us feel uncomfortable". It understates what s actually taking place.

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  18. @John k

    I mostly agree with with that. Two points though:

    a) Part of the “uncomfortable” from humid conditions is tactile. Damp, clingy clothing against your skin. Sweaty legs sticking to the chair you’re sitting on.
    b) 35F damp air would feel colder 35F dry air. It seems you were comparing “cold, damp” to the much lower ambient temperatures associated with frigid dry air.

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  19. You probably know this, but many local operating rooms had to be shut down for elective cases because the humidity was so low. This snow has helped though.

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  20. Snape,

    Yes I agree with your first statement. Your second is where I disagree: Assuming you are wearing typical clothing, and the "damp" air is still not quite saturated, for example 90%, I maintain that it feels slightly warmer than 35 degrees and "dry" air, say, 30% humid (which would correspond to a dew-point around 15 degrees I think). In the first case, The moisture still slightly retards the evaporation from your skin.

    But I realize not everyone agrees. It is a subtle difference, and it assumes other factors such as wind, sun, clothing type, etc., are the same. In my example above, I think it makes more difference when we are close to normal comfort range and the air holds more moisture. When the air temp is just a little too cool for comfort, I find higher humidity helps, and dry air makes me cold.

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