The Driest Place in the U.S. is 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.

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%.

from Cliff Mass Weather and Climate Blog https://ift.tt/2HlxgyS