The Driest Air of the Nation is Over the Inland Pacific Northwest

People think about the Pacific Northwest as being a moist place, but during mid-summer we are often one of the driest.

An excellent measure of the amount of water vapor in the air is the dew point temperature, the temperature to which the air must be cooled at constant pressure to become saturated.  The higher the dew point, the more moisture air contains. 

Here is a map of dew point at 9 AM this morning.  The lowest dew points in the nation was in the Pacific Northwest east of the Cascade crest (around 40F).  Middle of the country and in Florida?  Around 70F--very sticky.

Another measure of humidity is relative humidity, a measure of moisture that also involves temperature.  Relative humidity tells us how much moisture is in the air compared to the maximum it can "hold".   Since warm air can hold more water vapor than cool air, relative humidity tends to drop as temperatures rise.  

Here is the relative humidity at the same time as the above figure (9 AM Saturday).  The lowest relative humidities were over the  inland Northwest (both little water vapor in the air and high temperatures) and over the interior of California and eastern Texas (very high temperatures).

Relative humidity varies greatly over the day, declining substantially during daytime as temperatures rise.    Let me illustrate this for today (Saturday) using forecasts from the UW WRF model. 

At 5 AM, when temperatures were relatively cool, the driest air is over the lower elevations east of the Cascade crest (brown colors), with higher humidity along the coast.

Big changes by 11 AM, as warming causes a huge decline of relative humidity over most of the region.

And by 5 PM, near the time of max temperatures, the region was pretty much desiccated--with relative humidities below 30% in most locations away from the immediate coast.   No wonder your mouth felt parched and you grabbed a cool drink.

10 PM tonight?  Cooling temperatures resulted in an increase of relative humidities.

In contrast, dew point hardly changed during the day.  To show this, here are the dew points for 5 AM and 5 PM today.  Pretty similar. With drier air (dew points less than 40F) over much of the the high terrain and regions east of the Cascade crest--see below.

So when outsiders tease you about Northwesterners having webbed feet and other jokes of our region always being wet, feel free to correct them, noting our wonderfully dry conditions during our near perfect summers.

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