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

August 5, 2019

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