July 27, 2016

Are Pacific Northwest Summers Becoming More Humid?

A number of people have told me that Pacific Northwest's summers are getting more humid.    Is this true?

Let's examine this issue by looking at trends of dew point, which is probably the best measure of stickiness and unpleasantly moist conditions.  Dew point, the temperature at which the air becomes saturated when it is cooled, is a good measure of the amount of water vapor in the air.  If there is more moisture, you don't have to cool as much to get saturation.  Thus, high dew points mean more water vapor in the atmosphere. When dew point gets into the 60s F, we start to feel uncomfortable. 70s is unpleasant.  80s are oppressive.
I will begin by showing you the daily dew points over the past 20 years at Seattle Tacoma Airport.   You don't need to be a meteorologist to see there is no apparent trend, either up or down.   You will also notice that Seattle dew points rarely rise to the mid-60s, reflecting our very pleasant climate.

A longer period plot  (35 years) for summer dew point at Seattle is shown below.  Again no trend.

What about Yakima, along the eastern slopes of the Cascades during the past two decades?  As shown below, there is no temporal trend there either.

Next, let's consider summer precipitation, could that be increasing?  More humidity might be expected to enhance rainfall.  Here is plot of Washington State precipitation over the last century--no apparent trend is obvious.  During the past 30 years, there has been a drying trend, if anything.

The bottom line of the above information is that there is really no evidence that Pacific Northwest summers have been getting more humid.


  1. I love the smell of data in the morning. And the sound of anecdotal evidence crushing under its weight. :)

  2. I've noticed this summer that on the days that feel more humid, there is little or no breeze here in Bellingham. When the breeze picks up it feels less humid. Have we had more days with less wind velocity this summer?


  3. Dr. Mass
    NOAA has selected FV3 over MPAS as the new GFS!

    Who would've thunk that?

  4. This screams for null hypothesis testing

  5. Funny Cliff. I think some folks imagine stuff and then convince themselves it is true.

    Or, these are folks that have lived in the Seattle area for four, or less, years.

    In the 54 summers that I have lived in the Seattle area, I have always thought our summers have been pretty low humidity. I have always noticed the dew forming towards the very late summer, when the temperatures drop in the high 40s and very low 50s. Yes, dew starts forming on the windshields that late in the summer. But high humidity in Seattle, and getting higher, yeah, you betcha, NOT.

  6. Good to know--I admit I was wondering. We humans do forget things quickly and then get struck by them when they come around again. Similar to how every late summer and fall, people say, "I swear there are more spiderwebs than usual this year! I keep walking into them," though spiders haven't actually been getting any more numerous. And every spring in the NW they say, "The weather's been crazy this year! Rain, then sun, then rain, then sun," even though that's what spring is always like around here. :)

  7. Humans and their confirmation biases. Small sample sizes and recent feelings/negativity bias really contribute to these issues most likely.

  8. Similar plots for heat index and temperature-humidity-solar-wind index would be interesting.

  9. Unpleasantness now almost always feels worse than unpleasantness in memory. When you're sweating through a sticky summer evening that feels like it will never cool down, the same weather last summer is a distant memory.

    Meanwhile, it seems the impression I hear far more often is not that summers are getting wetter in Washington, but drier.

    Of course, that sentiment was repeated most often last year, when we really were having an unusually dry summer. There is that 30 year possible drying trend Cliff noted, but that comes on the tail of an equally large 50 year wetter trend.

  10. If the dew point is the same but the temperature is rising, it must mean it is getting less humid, not more.

  11. Thank you for your non-hyperventilating climate commentary. It's refreshing.

  12. I would like to see longer time frame. In theory, if the temperature is rising, the atmosphere will have the capacity for more water content. There would also, in theory, be a shift in the state of the earth's water: less ice, more water, more water vapor. I'd think, with global warming, if we accept that is really happening, higher dew points, i.e., more atmospheric water content--- would be expected. This could of course vary dramatically dependent upon climatological patterns and specific locations.

  13. I would have bought into the "it's getting more humid" school of thought too. Just more proof that our memories betray us.

    Research on memory, along with my own anecdotal evidence (I'm getting old), say that our memories are for less reliable than we like to think. And the important memories we hold onto the tightest and value the most are usually the most corrupted. Which seems a bit discouraging on the surface.

    Basically, the act of frequent remembering leads to "write back" errors that are influenced by emotion, context and ego.

    In most people, that leads to an increase in pleasant memories (which is a certain comfort) unless one is suffering from PTSD, when it can work the other way. The trick is not judging the present based on memories of the past. A pitfall that many of us over a certain age are prone to.

    Data always trumps memory. Thankfully, data is far more easily available now than it was 20 or 30 years ago.

  14. I was just saying to someone the other day, "I think these summers are getting more humid." What's interesting is that quite a number of people "feel" that way - so what's up with that? I've lived in Seattle since '82 - and we're definitely having warmer, dryer summers than what I remember from the 80's and 90's, so perhaps it's just the increased number of warm days combined with our usual humidity, that makes it feel that way. But I spent my early years in upstate NY - there's no comparison to midwest and east coast summer humidity. This is paradise by comparison!


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