This weekend, I wrote that this "summer" - or at least the heart of it from July 1 to August 31, was the second-warmest on record at Sea-Tac Airport by average high temperature.
Turns out, we also set an all-time record for the period if you factor in average temperature, which is calculated by taking the day's high and low and diving by two. This year's average for the two months checked in at 69.2 degrees, breaking the 1967 record of 68.8 degrees.
Third place? Way back in 2013 at 68.7 degrees. Yes, it's been quite a toasty two summers in a row now -- not just during the day, but especially at night, challenging the notion that Seattleites can do just fine without air conditioning.
The average low over the two months this year was 58.4 degrees, narrowly missing the record warm low average of 58.5 degrees set last year. Third place is 2004 at 58.1 and fourth place is 2007 and 57.4, so it's certainly been a recent trend.
The National Weather Service says there have only been 5 months in Seattle with 10 or more nights warmer than 60 degrees and three of them have been in the past two years -- both July and August in 2013 and August this year. Last year we also obliterated the record for number of nights warmer that 60 at 34 days. The old record was 18 which we would have matched this year so far.
The weather patterns of the past two summers have featured strong ridges of high pressure that have pushed a very warm air mass into the region, but haven't featured as much of a hot, dry east wind-- thus a large number of days in the 80s but not a significant amount of 90s. Sure, the 90 degree days come with really warm nights too, but then they usually eventually trigger strong marine pushes that cool us off for a while, both during the day and at night.
Instead, we've had a string of days with just enough of a weak marine breeze to keep the humidity a little higher than usual. (The coastal communities, right next to that weak marine flow, haven't had anywhere near the hotter summer that the I-5 corridor has had. Hoquiam ended up a little over a degree above average while Seattle was more than 3 degrees warmer.)
Normally a steady marine flow cools us off that the humidity isn't noticeable but it's been a perfect "storm" of having just enough for a bit more humidity and not enough for the cooling. The higher humidity helps keep the temperatures up at night. There is also the urban heating effect where the city's buildings and asphalt holds heat in at night better than outlying areas. The summer of 1967 that we fell behind in high temperatures we beat by full degree in average low temperatures.