May 21, 2017

Are Air Conditioners Needed in Seattle Now or in the Future?

The Seattle Times published a story on air conditioning in Seattle, suggesting that "as Seattle summers keep getting hotter and hotter" more apartments are being built with A/C.   In fact, they have a plot suggesting that while only around 4-5 percent of Seattle apartments were built with central air conditioning before 2010, during recent years that number has climbed to 25%!

So one might ask:  do you really need A/C in Seattle and have our summers got "hotter and hotter"?

Let's check the facts.  One of the key measures of the need for air conditioning is cooling degree days, which is based on the difference between daily average temperature and 65F.   If the daily average temperature at a location is say 75F, then that day has 10 cooling degree days.  Do this for all days of the year at a location and add them up, and you have annual average degree days.  The idea is that once the daily average temperature reaches 65F (say a high of 80 and low of 50), you start needing A/C.

The map of average annual cooling degree days below shows that western Washington has the some of the lowest numbers of cooling degree days in the nation outside of high terrain.  Much of western WA is white (low numbers) with only central Puget Sound entering the red colors (101-400).  Seattle clearly has the lowest need for cooling of any major US city in the lower 48 states.

In fact, Sperling's Best Places has rated Seattle America's number one summer "Chill City" based on our temperature and humidity.  Seattle Chill--I think I have heard this before.

The reason for Seattle's low number of cooling degree days is clear:  we have some of the coolest summer temperatures in the nation, because western WA generally is covered with cool air from off the Pacific Ocean.  The map of the normal high temperature for August shows this clearly, with our area the only coastal one with a lot of green.   Seattle does better than Portland because we have cool Puget Sound next to the city and there is a sea level passage to cool Pacific air.   Portland has much more need of A/C.

But temperature is only part of the story---humidity is also an important issue for summer comfort.  Higher humidity reduces our ability to cool by sweating, which is a very effective way our species reduces our body temperature.    A good measure of the water content of air is dew point, the temperature at which air becomes saturated (100% RH) when cooled.

Take a look at the dew point map of the US for August below.  The lowland Northwest has LOW dew points (low 50sF)--low levels of moisture in our air.  Which means our relative humidities end up quite low during the day.   Your body can cool effectively from sweating (which may not even be apparent to you).  Compare that situation to the eastern (and particularly southeastern) US where moist, humid air greatly interferes with our natural cooling mechanisms.

Why are our dew points low?  Because of the cool Pacific Ocean....the amount of moisture air can pick up increases with temperature.  Cool water...not much moisture in the air.

But wait...there is more!   Our dry air allows temperatures to cool rapidly at night (water vapor is a powerful greenhouse gas), so our typical daily low temperatures even during a warm spell almost always drop in the 50s.  Open a window or use a window fan, and comfortable sleep is almost always possible.   Finally, even on the warmest days, cooler temperatures are generally very close....just head to the water, since Puget Sound is generally around 50F.

The bottom line of all this is clear:  Seattle is one city where A/C is a luxury that is not particularly needed for buildings in which windows can be opened, except for the most unusual and exceptional days.  Apartments with poor circulation and facing the sun can get warm (and could use A/C).  Typically, Seattle has two days a year when the maximum temperature reaches 90F. 

But what is the trend of Seattle temperatures and how about the future?  Will A/C become a necessity for most, like in Houston?  

To "warm up" the discussion, here are the average June to September temperatures over the Puget Sound lowlands (NOAA WA Climate Division #3) for the past 65 years.  There is a small upward trend with a lot of variability.   Roughly 60.5 F in the 50s to early 70s, and approximately 62F since then.   The transition in the mid-70s may have been due to a mode of natural variability called the Pacific Decadal Oscillation (PDO), which transitioned from the cold to warm phase at that time.  A small human-caused contribution is also possible (from urbanization of our region and increasing CO2).   But the upward trend is modest and quite small for the last 30 years.

There was one crazy warm year recently (2015), which was associated with an usually persistent ridge of high pressure.   Around a DOZEN days reaching 90F at Seattle.  And you will notice some similar heat spikes early in the record.  These transient features were probably the result of natural variability.

The bottom line of this analysis is that there has not been a large warm up over our region during the past decades and increasing temperatures is probably not the reason for more A/C installations.    A/C may be nice on a rare and extreme Seattle hot spell, but generally is unnecessary.  More A/C is like the trend for expensive granite counter tops-- pleasant to have perhaps, but a luxury.  More A/C is probably a better measure of the increasing wealth of our region than of increasing temperatures.

But what about the future?  If A/C may not be necessary now, what about later in the century?
It might be a good idea.   Let me show you the projections from a regional climate simulation that we completed at the University of Washington (credit to Professor Eric Salathe and research scientist Richard Steed).   Specifically, this is number of days per decade that will climb about 90F at Seattle-Tacoma Airport.  This simulation assumes that we keep on burning fossil fuels in a similar way as in the past (which is what is happening I am afraid).

For the 1990s it shows about 50 per decade (4 per year).  The model is overestimating the number of heat waves, probably because it does not have enough resolution to get Puget Sound correct.  2020s?  Pretty much the same story.  2050s--a modest increase to 80 a decade.  But look at the 2090s.  Wow.  A huge increase to 170 a decade (17 a year).  You will probably want air conditioning in Seattle in the 2090s.

Global warming due to human-inspired emissions of greenhouse gases is probably having a small impact on our local temperatures now, but by the end of the century our climate will warm profound.  But we need better information to get a better idea of the local effects.  That includes running regional climate models at higher resolution and running them many times to get a handle on uncertainties.  That is why we are trying to build a regional climate effort at the University of Washington.

And enjoy the perfect weather A/C needed.


  1. My wife and I have lived in the Puget Sound area since 1993. Part of that in Port Townsend, where A/C was neither necessary nor even very desirable due to cost. We've lived in Maple Valley for about half the time; we had A/C installed and have found it very nice to have. I would not say 'needed', save for 2015 as you note. :)

    OTOH, my personal experience doesn't give very well with the averages you cite. The real utility of A/C in the Seattle area are the outliers. Any day with a high temp in the 80s is a better day with A/C. Is that necessary? No. Is that useful? Very much so.

    And keep in mind that extremely local conditions play a role. Concrete and asphalt surfaces retain (and emit) a lot of heat. Nearby buildings can trap heat, creating severe hot spots. Air circulation can be poor for a variety of reasons. Individuals vary, too; as I get older, I am far less heat tolerant.

    I am _amazed_ that the humidity seems low enough to allow for efficient human-body cooling. Spending time in both Seattle and New Mexico as I do, I find those very same days you describe as really moist! :) The lack of efficient evaporation in Seattle is part of my misery, and why I say that once you hit 80F here, it's time for the A/C. I can do without A/C in New Mexico up to about the low 90s. And if there is a breeze down there, A/C isn't even necessary if you have shade.

    So while I agree with your conclusion about a lack of necessity, I find A/C highly desirable and very much worthwhile as an investment. Even a few weeks of easy living with A/C makes it a joy to have. Since our current house has a fair amount of concrete around it, it overheats easily and A/C's value is multiplied.

  2. The nice thing about having reverse-cycle heating with ducted airflow is that it works both ways. On those occaisonal hot or humid days, it's a nice luxury.

    But more importantly, a whole-house fan can rapidly replace the warm day heat with cool evening air... and for far less power. I could live without the AC, but not that big whole house fan in the attic. Turn it on and open the desired windows and you can have a cool breeze in every room.

    These things were huge in Colorado, but they work here as well given our usually cooler evenings and low dewpoints.

  3. Before I moved to Portland I thought it's climate was somewhat similar to coastal cities like Seattle and Vancouver, or perhaps SF. Not even close. The coastal mountains often block the natural maritime air in the summer, leading to variances often of 20+ degrees in the summertime. Many of buildings here are quite old so are not equipped to handle central A/C and have poor ventilation, and there are usually episodes every summer where having at least a bedroom A/C is necessary. I went without any A/C for two summers and it almost drove me insane, the upper levels often got to 90+ degrees by ten o'oclock at night. However, coming from the Midwest the summers here are quite mild by comparison.

  4. When I lived in Chehalis, a little warmer in the summertime than Seattle we needed some air-conditioning. Standards at the time called forca 4-6 ton heatpump unit. I calculated that 2 ton would be more than sufficient, and we seldom ran it more than 4 hours a day in July-September. With a couple of baseboard heaters it was also sufficient throughout the winter.

    West facing condos and apartments can warm up anywhere in the region, and without cross ventilation may need a little AC. I have found that the 5000 BTU units labelled good for 130 square feet are adequate for spaces three times that.

  5. My guess is that the Seattle Times missed the point.

    What is being installed is a heat pump - these do both heating and cooling. When heating, they are a lot cheaper than gas or oil or electric resistance heating. The AC is just gravy for the few days/year that we need it. Cheap heat is the primary goal.

    1. Bingo..
      THE technology is a heat pump for our region. Cheap heat is the primary reason, bonus is dehumidification, and cooling with the same unit.

      Even in urban heat centers nights are cool.. period.

  6. The Seattle climate is just about perfect IMHO. Humans and their diurnal body clocks seem to have annual body clocks too. Many folks have moved to the allegedly perfect climate of LA, only soon to leave because "There are no seasons!" "Xmas in tee shirts is plain wrong!"

    The Seattle winter has just enough cold/snow/ice PITA days such that you know winter has happened. Similarly, the Seattle summer has just enough hot/muggy/no breeze PITA days such that you know summer has happened. Here we seem to have four real seasons without excessive climatic abuse ;-) Still. . . . Nature does bat last!

  7. North West A/C. Open upstairs Windows. Open basement Windows. Turn on furnace fan (not the heat, just the fan), and do it early in the day. Enjoy.

  8. Yes, but...natural ventilation depends on available breezes reaching the open windows. With more high & medium rise construction the breeze doesn't move as it used to do in some locations.

    Our heat pump runs in cooling mode one afternoon a year some years just to exercise the parts. In other summers it runs many afternoons.

  9. Building Standards.
    With the cost of housing in the area going up, you would think their would be improvements in heating and cooling. Sadly, the house built by my parents in 1959 and re-insulated during the energy crisis of the 70's and then double-pained in the 80's stays cooler in the summer and holds heat better in the winter than two newer apartments I've lived in and my current $1/2 Million Redmond condo.
    Housing built not by the future residences, but buy housing speculator always cut these corners costing future owners many times the $$ that should have been spent. Because of this separation, regulations have to stand in place of what should be market forces to build better housing.
    My 15 year old condo has a day in May every year where the sealed off attic stores more heat during the day than can be released overnight even in this area. The resulting heat vault will now operate through October, keeping my condo a minimum of 80 and above 90 during July and August. I live in a thousand unit complex where every unit has the same issue and a building code requiring an attic fan, vent and temperature control unit would save countless thousands.
    Yes, Market forces should do this work, but when supply and demand get out of wack, then those forces stop working and regulation has to step in.

  10. We've had a/c for the last three years and I would not go back. I have tried every energy efficient option but my two year old's room still gets ungodly hot even in the 70s and I am very sensitive to heat. I gave birth at the end of 2015's wretched summer. Basically lived the entire summer naked in the bedroom in front of the a/c

  11. A lot of apartments have exposure on the S, SW, and or SE sides, but no windows on the opposite side of the building (usually just a door to a hallway). On our long, sunny summer days, these can get pretty hot -- even when outside temps are only in the 70s.

    I had an apartment like this in Belltown...even with the blinds down it could feel like an oven on a typical summer day.


  12. I think the answer to the A/C questions is really situational.

    The last 5 years we've had majority of our summer with inside temperatures at or above 80f both upstairs and down.

    The last 2 or 3 years we've hit 90f+ upstairs for weeks at a time.

    A/C is a necessity for us now, but it wasn't always like this. And it also has nothing to do with the economy or having extra money. A/C units can be had for as little as just over $100 for one that can cool a large room. Not exactly a luxury price at all.

  13. I'm in a SE-facing downtown apartment with sash windows and it's an oven the minute it hits 70 outside even with all the windows open. Just miserable at night! So I have a bedroom A/C unit hooked up. It can barely keep up, but it beats lying awake at night in a pile of sweat. So yeah, some people in Seattle need A/C for sure.

  14. I for one am sensitive to dewpoints above 55, and I have installed A/C window units in both my apartment and house for the past 15 years. There have been more summer weeks than I could shake a stick at where it's been 84/60 all week with dewpoints hovering around 58.

    While friends and colleagues would bellyache about the weather during those stretches, I'd get home to a nice cool house and enjoy sleeping with a light blanket over me.

    The bigger issue for A/C in our area is the continuous march ever-upward of Seattle City Light power rates. Over the past ten years, SCL has raised residential rates by a whopping 75%. When power is no longer inexpensive despite the free bounty of the impounded Skagit River, the cost of using that A/C starts to become a real factor.

  15. I just use a window fan in the bedroom starting a 7 pm or so on summer nights. Even on the hottest days, the bedroom is down to the low 70's by 10 PM.

    In Connecticut where I grew up I would run it all night and sleep in my underwear w/o blankets. Or we'd sleep outside.

    Honestly, only a wimp needs AC here.

  16. Heat pump = noise pollution.

  17. "This simulation assumes that we keep on burning fossil fuels in a similar way as in the past (which is what is happening I am afraid)."
    Right, but to list only one assumption among a myriad of assumptions that must be made in any model is tantamount to "leading the witness." And what assumptions are built in tacitly?
    If his model is good, why the need for a NW Climate center-you know what you think you need to know. If the model is bad, it won't be falsified for 20 or 60 years! In the broader picture, so far the climate models have failed to predict temperatures.
    And doesn't your series of slides suggest an urban heat island affect at SeaTac? How couldn't there be one with new runways and more development of land around the airport?

  18. Cliff, Chart 1 doesn't show the inclusive years for total cooling degree days, and an annual mean is less representative than one just for the summer months, July - Sep.. Moreover it has one color for lowland Puget Sound, but the discussion is about Seattle. Cities are heat sinks compared to surrounding suburban and rural areas. The increase in tall buildings decreases wind circulation adding to the heating effect. Fig. 5 shows a regular increase in average temperature from 1978 - 2015. I think that validates the impression that summers are definitely getting warmer.

    As pointed out in several comments, a house allows one to open windows and doors for air circulation, whereas apartments are limited to windows, many of which do not open, leading to a greenhouse effect, especially those facing south and west. So the indoors of new buildings are definitely getting hotter and require AC.

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  20. Cliff you going to talk about the high wind warning for the Admiralty inlet?

  21. I moved up here in 2001 and lived in Port Angeles and then in Burien since 2013. Both houses might have been ok without air conditioning except for one fact. Designed for the Northwest, very few windows open and both houses have very poor cross ventilation. The first house had a heat pump, so just used it to air condition. The second house had gas heat, so I bought a heat pump which is much more efficient in the winter and cools the place in the summer.

  22. 2015 wasn't the only warm (hot) year.
    Average # of days above 80 in Seattle: 26
    2014: 45 days
    2015: 49 days
    2016: 38 days
    And temp, of course, is taken in the shade. Good luck if your place faces west and gets blasted by the sun.
    Plus, we don't always get cool Pacific air. High pressure - summer or winter - means winds from the east. Absolutely no ocean cooling.
    AC can be awfully nice, and is almost a necessity in some areas.

  23. So, any idea what this summer will look like?

    I do not do well in heat and was Miserable the summer of 2015. We almost got air last year but then I read a post from you saying that the summer would be cool, and it was!! And I loved it!!

    So, I'd love to know your thoughts on this year.

  24. I am glad my office has a/c right now. Also glad that our house has a cellar to retreat to. It has never been too hot to sleep in our cellar.

  25. I love our condo's amazing views of downtown and the Olympics, but during the summer the accumulation of heat is a massive PITA. As others have pointed out, in a building like ours, all of the windows face one direction; except for some totally inadequate bathroom and kitchen fans, it's impossible to generate any cross-circulation.

    The killing joke, however, is the huge picture window in our living room. It sports a single, puny, crank-operated window that opens facing down maybe 10-12 inches max. That same window catches the afternoon sun full-blast for 6-8 hours a day in the summer, and being on the top floor means we catch more heat from the roof of the building.

    The window opening is also too high to be practical for connecting an AC vent hose, so while portable ACs help (a little) in the office and BR, the living room can hit 85-90 multiple days during the summer.

    Add it all up, and you get pretty typical modern, mid-range condo construction practices here in Seattle. I'm going to approach our HOA board this year to start a conversation about allowing the installation of ductless AC units, at least in west-facing units. Barring that, and view be damned, I might consider selling. . . and you can bet that I'll put it on the market in the fall rather than explain to buyers why only "wimps" need AC in Seattle. Yeah, right.

  26. I am assuming that your article is only for Seattle. I feel that Bellingham is also a candidate for someone who does not like air conditioning. Where I live in Bellingham (intersection of E. Victor Street and Meridian Street where my west side is protected by another house to my west) I have not needed any fan during the day and have needed at least one blanket almost every night for the summer of 2016. Compare that with 2015 in Portland where I moved from! I think that Bellingham can be at least 20 degrees cooler than Portland, especially at night.

    In fact, one evening last summer (2016), there was a NWS Heat Advisory for Western Washington, including Bellingham. It included the evening of the day. That evening, I almost had to turn my heat on in my house because the night temperatures dropped through the floor!

  27. Cliff - the averages and means as you cite them don't correlate well with the actual lived experiences of summer here in a nearly treeless corner of Ballard. I've lived in Seattle since the early 80's and the number of summer days above 80F (especially in the last 8 years or so) has definitely been on the uptick. As others have indicated, how well we tolerate it depends on our housing setup.

    Granted, we have it better than most places - I grew up on the Great Lakes, it was night-time cooling. That's the saving grace of PNW summers - low night time temps. I'd never care to live anywhere where AC is necessary for survival all summer - the South & SW - no thanks!

  28. so global warming is the PDO, Cliff? with only a "small human-caused contribution" come on, whose payroll are you on? you can't even predict a rainstorm, and now you are attributing 30 years of increasing temps to oscillations. But you really bolster your position as a leader for climate science by finishing with this gem: "you will probably want a/c in the 2090s." Wow. Trump is going to fall over himself trying to prepare for that terrifying prediction. Stop hiding behind your models, Cliff - take a stand to help address climate change now!

  29. The split air conditioner is mainly a economic budget unit. Which saves and maintains your home very well. This guide is so detailed and helpful. Heating and Cooling London


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