July 11, 2019

Sticky Air and Record-Breaking Moisture over Western Washington

How do I say this politely?

A number of people of been complaining to me about their discomfort the past few days and, yes, their profuse sweating.

The air has been humid and sticky.  Uncomfortable, even with relative modest temperatures.

The reason?  The air has been unusually moisture laden, particularly for the normally crisp, dry Pacific Northwest.

A good measure of the moisture content of air is the dew point temperature. The maximum amount of water vapor that a sample of air can "hold" depends on temperature.  Warmer air can hold more water vapor than cooler air. 

Imagine you start with some air with some water vapor in it, but the air is not saturated (the relative humidity is less than 100%).   If you cool that air down, you get to a temperature in which the air can just barely hold the moisture it started with and thus the air is saturated.  That is the dew point temperature.

The more water vapor in an air sample, the less you have to cool it to get saturation. Thus, moist air has a higher dew point than dry air.  To put it another way:  dew point temperature is a measure of the water vapor content of air.

Typically, the dew points in western Washington in July are in the 40s and low 50sF (see map below).    Why low dew points over the West Coast?  Because our air generally comes over the cool Pacific Ocean, whose low temperatures prevent the air from picking up a lot of moisture.  In contrast, over the SE U.S., the air comes off the very warm Gulf of Mexico and south Atlantic--so dew points are in the 60s to low 70s.  Miserable.

So here is the rule of thumb.  With dew point temperatures in the 40s and lower 50sF, you feel quite comfortable for normal summer temperatures.   You start to notice the moisture when the dew points get into the upper 50s and it gets "sticky" when the dew points rise into the lower 60s.  Mid-60s results in more sweating and by the upper 60s and lower 70s, the dew points are associated with real discomfort.  And wet clothing.

Now you are all dew point experts, it is time for the big reveal:  yesterday and parts of today, the dew points got into the mid and upper 60s in portions of western Washington.  

People noticed.  And many of you had glistening skin after the most minor of exertions.

To illustrate,  here are the dew points at 4 PM yesterday (Wed)--click on image to expand.  OMG!   69F at Tacoma's McChord AFB and low to mid-60s all over Puget Sound and southwest Washington.  No wonder we were all sweating.

And these high dew points were quite unusual.  Take a look of a plot of dew points at McChord over the past ten years.  The maximum dew point yesterday was a torrid 70F--the highest observed dew point  OVER THE ENTIRE PERIOD.

Or check out the surface dew point climatology at Quillayute, on the WA coast, which goes back to the late 60s.  Today's value at 5 AM (12Z, gray dot) was an all-time record for the date (the red line shows the records).

Why so humid yesterday?  A perfect set up.  We had an upper level trough moving moist air in from the southwest (see plot of water vapor in the air column--colors--and winds at 5000 ft).

And there was light rain, falling into the low-level air, that moistened the surface layer.  At low levels, we did not have much onshore flow at low levels, which would have brought in drier, cooler air from off the ocean.

So take a shower and get a cold drink. Or find a room with AC.  Thankfully, the dew points should slowly drop over the next day.


  1. Such wimps we are! Mill Creek has been running 70-ish with a dew point of 60-ish. This would be considered sweet relief on the East coast.

    1. All in what you are used to...

    2. Homes in the Midwest and East coast have AC. We don't have that here, so there's no way to escape it. Completely different situation for people in the PNW.

  2. Yesterday, I measured a maximum dew point in NW Bellingham of 64F. Last month's maximum dew point was 69F - the highest that I've measured.

  3. I remember humidity in the 90+% range and temps in the high 80's/low 90's in northern Illinois as a kid.

  4. yes, the weather the past couple of days has reminded me of summers in the Midwest as a kid. Now what we are experiencing would be considered good by Midwest standards due to the lower temperatures, but for us its really, really humid.

  5. What happened to the big storm you were talking about Wednesday?

  6. Thanks. Always enjoy your science Cliff. And the lack of science from the "sky-is-falling" commenters.

  7. Cliff, I've loved your latest posts, but could you perhaps explain what's causing this unusual weather from a macro perspective? Ex, it seems to me the North Pacific High is stuck off the coast of CA/OR, not in its normal summertime position in the Gulf of Alaska. This causes low pressure fronts and southerlies to get through and hit Van Isle. But the question is why is the High so late - or are its movements too complex to understand? And are there historical records of the High's movements that would allow us to say more precisely how abnormal this year is?

  8. Is it hot in here? Oh good it is not just me! I have been monitoring my home hygrometer here in Anacortes,on Fidalgo Island, surrounded by water, and noticing not just being sweaty, but being unable to dry out! That is the uncomfortable part! Indoors 72 degrees and 53% humidity is too much for me!

  9. As an aside, one of my pet peeves is people claiming it's "90 degrees with 90% humidity". From what I understand the only place this ever happens (rarely) is around the Persian Gulf. When the temperature is 90 degrees and the dewpoint is 75 degrees it's extremely humid but the relative humidity is still only about 61%.


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