July 14, 2018

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.


  1. If people wanted to open their windows to cool off their home in the PNW would there be an average time of day that would be best to do this?

  2. @Benjamin. On Saturday morning at 5 my living room temperature, in Portland, was 70°. I opened windows and doors and went back to bed. When I got up at 7, the temp was 66°. No A/C needed until much later in the day. By the way, and Prof Mass might confirm this, Portland summer temps run warmer by 3-5° than the Seattle. (Seattle's my home; sure miss the beauty of Puget Sound and its moderating effect on temperature.

  3. "The higher the dew point, the more moisture air contains."

    Well, yes but the dew point temperature in and of itself is meaningless unless one compares it to the current temperature. Only by comparing it to the current temperature will one know how "dry" the air is.

  4. I assume it varies wildly by microclimate.

    In marine areas, I would say the breeze intensifies around 3pm. It rarely gets over 80f and even during our 5 week long summer, nights are in the low 50's. Our brief perfect summers are well earned.

  5. John...that is not correct. The dew point is extremely valuable as a measure of the total amount of moisture in some air. It also helps one know when dew and god will form. We have a RH of near 100% in the winter, yet we are comfortable...because the temps are low...cliff

  6. Cliff - thanks, my post may have been misunderstood. I was afraid some of your readers might think "oh, the dew point around here is temp X".. year-round!

    That 100% RH winter day.. is it comfortable because the temp are low, or because the air, although saturated, contains little water vapor?

    How about a 120 degree day in the desert? Many times the day is very dry, ie, it's a "dry heat". Is that because the difference between the temp and dew point is so large, or because the dew point temp is low? Asking another way, if I raise the air temp high enough, can I have a high dew point temp and still have dry air?

    1. In theory, yes, but you’d be dealing with temperatures beyond what is earthly possible. At least in the lower 48.

      In general, to the human body, a dew point above 60 starts to feel “muggy”.

      You’ll notice it whether the temp is 70 or 90.

      This is because regardless of the air temp and the RH, dewpoint indicates how much moisture is in the air.

      Also, much like the science behind being able to put a styrofoam cup full of water in a fire and it not melting, it’s difficult to heat air with a high moisture content, which is why it’s rare to see humid air reach the same temps as desert locations. The east coast and areas in the Middle East sometimes see extreme dewpoint/temp combinations, but those are extreme examples mostly due to geographical and climatological reasons.

  7. Renton dew point late last night, 57. Now it’s 57 again. EVERY other place has few points in upper 40s-mid 50s. What gives?


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