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.