12/18/2014

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Many moons since Seattle has set summer low temperature records

Many moons since Seattle has set summer low temperature records
Photo courtesy: Tim Durkan Photography
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In keeping up with the number of weather records and interesting statistics surrounding our current warm spell, I stumbled upon something on the other end of the spectrum: How difficult it's become to set record lows in the summer.

In looking up the record high Monday when he hit 91° in Seattle (it was 95, set in 1993), the record low stuck out: 44° in 1950.

First thought was: Wow, 1950 is really a ball hog when it comes to Seattle record lows. Already home to 18 of the 34 record lows between Jan. 1 and Feb. 3 -- many of them single digits -- it turns out 1950 is no stranger to record lows in the summer either, owning a half-dozen there plus a number in autumn too.

But second thought was: When was the last time Seattle set a record low in the summer? The answer on the surface doesn't sound so unusual: July 13, 2010, when it dropped to 50°. But then you look at the rest of the record books and...it's been a while besides that. Surprisingly, the only other record low set in the 21st Century in July and August was... July 13, 2001, when it tied at 50, subsequently tied again in 2010.

Before that? You have to go all the way back to 1986 -- Aug. 16, 1986 also tied at 50 degrees. There are two other record lows in 1982 -- July 14-15 at 50 and 49 degrees respectively, then you have to go back before 1975 to find the next ones. So that means since 1975, there have only been five record lows set/tied in July and August, and four of them were 50 degrees.

The last times we were colder than 50 degrees in July and August? July 31, 2002 (49), July 7, 2001 (49) and then it was back in 1993 before we got there again.

What's the deal? It's likely what's known as the "Urban Heating" effect. As Seattle has grown over the years, the city becomes less and less grass, trees and vegetation and more concrete and asphalt. Concrete and asphalt to a much better job holding in the day's heat and radiating it back during the night to keep temperatures warmer in the urban areas than rural areas.

Seattle has grown quite a bit since the 40s and 50s and thus what is likely the case for the warmer lows today. Just to compare, if you check Hoquiam's record lows in the summer, you'll find a much more even spread across the decades of record lows, although many are still in the 50s and 60s.

Seattle's summertime lows rarely fluctuate because our pattern generally fits into two main categories: Marine pushes, or sunny, warm days. The marine push days are cooler during the day with highs in the upper 60s to low 70s but the cloud cover at night keeps lows from dropping much beyond the mid 50s.

The sunny days that lead to clear nights and better overnight cooling typically reach into the 80s so temperatures have a longer way to fall, typically bottoming out... in the mid-upper 50s. The only tweaks come when there is a really hot day with offshore flow and we get well into the 90s or some monsoon moisture sneaks in from the south, increasing the humidity and making it more difficult to radiate heat at night in which case we'll have some nights stuck in the 60s.

To get "chilly", we need a relatively cool day then a clear night -- that record low of 44 came on a day Seattle hit 71. Another record low of 44 came on a 66 degree day.

Since July 1 this year, we've had one low at 53, one at 54, one at 55, and every other low has been 56-64 with the vast majority 57-59.

So realistically, we could just leave off the low temperature forecasts in the heart of summer because no matter the pattern, the lows end up pretty much the same. And those record summertime lows in Seattle? Essentially they've been granted tenure.

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