A Record Warm Night In Portland’s “Urban Heat Island”

6pm Monday…

Do you remember WAY BACK in time the common saying that “you don’t need air conditioning in Portland“?  By “back in time” I mean at least 20 years ago, and really back in the 70s/80s and earlier. And that saying applied to any area west of the Cascades too.

I think those days are long gone and won’t be coming back for any of us alive right now.


For one, our slowly warming climate (globally and in the Pacific Northwest) means warmer nights in general as each decade passes.  But a larger influence in the urban areas is the steadily growing “heat island” we are creating, especially around Portland.  More on that just ahead.

Last night we broke a “record warm low” record at the airport where it only dropped to 65 degrees.  20 years ago that would have been a real big deal.  Today no one even mentioned it to me here at work, just another “warm night”.

Mark Portland Record Warm Night

We’ve seen a large number of 60 degree or warmer nights in Portland this year.  Here are the numbers compared to recent years; note each count is up to August 5th only.  Apples to apples comparison…

60 Degree Nights Portland

That’s rough sleeping weather if you don’t have air conditioning!  Most of the night was spent in the 70s.  I talked to someone over at PGE maybe 10 years ago and he mentioned only about 30% of customers had air conditioning in the early-mid 90s, but something like 70% do now.  Part of the reason is due to people moving here from hot climates that expect AC.  The other part is due to the rising popularity of heat pumps (I have one).  They heat in winter and then function as an air conditioner in summer.

Now check out other low temps across the metro area last night:

PDX Observed Low Today

Notice all the areas down into the 50s.  Those are generally locations not surrounded by miles of concrete and buildings.

That warm central part of the urban area is called an “urban heat island” and it has been growing over the decades.   In the 1960s the Portland metro area contained around 1 million of us.  Now that number is about 2.5 million!  That’s a lot of new homes/businesses, highways, asphalt and concrete…

Take a look at number of nights above 60 in Portland over the decades.  The 15 year average has gone from around 8 back in the 1950s to 33 now!


That’s quite a rise.  So then I checked Salem airport which has seen a slower urban growth during these decades.  Not quite the same; a far less dramatic rise, until just the past few summers.  Average goes from maybe 4 a year to 13.  Keep in mind that both of these graphs only show the nights this year SO FAR.  That won’t be the final number on the far right side.


I also checked a few non-urban locations west of the Cascades with a long record.  Many of them have seen a SLOW increase in warm nights.  Portland Headworks, around 700′ between Sandy and Corbett has seen a gradual rise from around 1-2 60 degree nights to around 8 now.  That surprised me.  There ARE more warm nights in summertime even in rural areas west of the Cascades.

Coastal lows are rising as well.  Since Astoria almost never stays above 60 degrees I lowered the criteria to 55 degrees.  There were about 30 lows that warm in the 70s, but that has risen to around 45 the past 15 years.

But the warming doesn’t cover areas east of the Cascades.  Pendleton sees FEWER 60 degree nights than it did before 1950.  Baker City is similar.  Interesting eh?

A Quick Summary

  • Summers are warming in the Pacific Northwest, and that season has been changing more west of the Cascades than any other season.
  • Summer nights are gradually warming for all of us west of the Cascades
  • Summer nights are rapidly warming in urban areas (mainly Portland Metro)
  • Summer nights have seen little/no change east of the Cascades at night.  Just open your windows at night and you should be good!

Chief Meteorologist Mark Nelsen

30 Responses to A Record Warm Night In Portland’s “Urban Heat Island”

  1. W7ENK says:

    Severe Thunderstorm Watch now in effect through 10pm for most of Central and Southern Oregon.

  2. Snape says:

    Hi Mark
    I like to look at NOAA’s “Climate at a Glance”. It shows most, of not all areas east of the Cascades getting warmer at night during summer, including Pendleton:


    Does NOAA have it wrong?

    • Snape says:

      I see now that the temperature time series’s in your post comes from ACIS, which is also a NOAA product:

      “Applied Climate Information System
      The Applied Climate Information System (ACIS) was developed and is maintained by the NOAA Regional Climate Centers (RCCs). It was designed to manage the complex flow of information from climate data collectors to the end users of climate data information.”

      Now I’m even more confused about the apparent discrepancy with “Climate at a Glance”.

  3. W7ENK says:

    Thunder over the West Hills earlier. Perhaps bodes well for later this evening?

  4. Kyle says:

    Our oceans are f’ed up royal with microscopic pieces of plastic which due to the Bernoulli’s Principle it slows ocean currents down which in term effects the jet streams making them take the easiest path.

    Since 2013 when the trash starting to grow significantly the waters have been ‘red hot’ on the maps where the trash is concentrated. These plastics never dissolve. A few ships are being used to scoop it up but the Pacific ocean requires a lot more. This is the kind of thing the UN needs to get their butts together and fix it! Not argue dumb stuff.

  5. Roland Derksen says:

    I don’t know if “Nosummer” was being serious or not, but for me, this summer has been more like the summers I used to know when I was growing up. Looking back on decades of observations, I’ve seen a lot of summers that had cloudy spells and occasionally rainy days. The recent hot dry summers we’ve had have historically been more of an exception than a rule.

    • Nosummer says:

      I’m definitely being serious. This summers weather sucks. I’ve lived here for 22 years and this ranks up there with the gloomiest summer I’ve seen. Yes there might be averages and I know those last few warm, dry summers aren’t typical. But, this never ending cloud cover and marine layer coupled with forecast busts (rain when it’s supposed to be sunny or vice versa) and other unusual weather isn’t “typical” July and August weather. Seems my opinion is unpopular and that everyone rejoices over this crap weather. I guess actually liking sunshine and summer is a faux pas here….

  6. runrain says:

    Sounds like an exciting day weatherwise in Central Oregon tomorrow. Wonder if it would be worth driving over to watch.

  7. JERAT416 says:

    Felt like October today!

    • Joshua Lake Oswego says:

      Awesome day! Preview of the gloomy, moody days to come.

      • Gene says:

        Yeah . . . gloomy, moody . . . just awesome. Just what everyone wants during our short Portland summers

        • summerbummer says:

          I never understand people who want it gloomy 12 months of the year. We get 3, if lucky, nice, sunny months, why is that such a bad thing. My mental notes ranks this as one of the worst summers we’ve had since I moved here 22 years ago.

    • Tanis Leach says:

      But the rain does feel nice.

  8. Roland Derksen says:

    Cloudy, breezy morning here-nice change from the warm days of late. Speaking of past Augusts(i.e. 1981) , one of the warmest I ever recorded was back in 1977- at least through the first 3 weeks. If that month had continued to stay sunny and hot through to the 31st, it would still stand as my warmest month on record.

  9. Nosummer says:

    This has been such a lame summer. Morning clouds, rainy days, forecast busts and barely warm enough to do any fun summer activities. Portland truly has the worst weather.

  10. Wonder how hot it would be today if we had the same pattern as was back in August 6th through the 12th of 1981.

  11. W7ENK says:

    Large and rapidly spreading wildfire reported near Clatskanie. Emergency crews en route. Local access has been restricted. No official word yet on evacuations. ICS will make determination after their initial assessment.

  12. Anonymous says:

    It was fairly warm here overnight (59F) as well, but my warmest night s ever were during that late July 2009 heatwave, I saw an overnight minimum of 74F on July 30th.

  13. Anonymous says:

    Well done.

  14. tim says:

    What does this mean for the cool season and especially during artic outbreaks in future?, not as cold for the metro and surrounding suburbs? anyone?. peace.

    • boydo3 says:

      If our winters begin to reflect what’s going on in Alaska..
      Start planting some olive trees!

    • Andy says:

      Having moved back here from Minneapolis last summer after 20 years, I can tell you that the Urban Heat Island (UHI) can have a pretty big impact in the winter. Our winter lows warmed significantly more than our daytime highs in the last 15 years. One thing that definitely would help here in PDX is to get more trees planted in yards and planting strips especially in the eastern metro.

      Given that it’s so much less humid here than MSP (where dew points hang out in the high 60s to low 70s for days on end), I can’t find a good reason to spend money on AC when windows open at night works so well.

  15. JERAT416 says:

    I remember opening windows at night with box fans blowing cool air in when I was a kid. Then we would shut the windows and curtains, and usually be ok except in the case of major heat waves and 100 degree days.

  16. PAUL D says:

    Mark is cranking out the blogs!! Keep ’em coming!

  17. ocpaul says:

    When the Holocene Epoch ended, the planet began warming.
    Anything else?

    • Mark Nelsen says:

      We are in the Holocene Epoch right now

      • W7ENK says:

        That’s open for debate. An increasing number of scientists are now calling to close the Holocene Era and subscribe to the idea that we are now in what they are calling the Anthropocene Era, which is defined as the “…epoch dating from the commencement of significant human impact on Earth’s geology and ecosystems, including, but not limited to, anthropogenic climate change.” While not official yet, the process of ratification is ongoing to define a specific date to mark the cutoff, which would be agreed upon unilaterally across the scientific community. Currently, the most popular date proposed is the test of the Trinity bomb on 16 July, 1945, when humans unmistakably unlocked the secrets of the atom, and thus launched our species into a new era.


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