Temperature readings at Sea-Tac Airport -- Seattle's official reporting station -- have been reporting cooler temperatures of late, but don't worry, our climate hasn't undergone a sudden cooling event.
Instead, it seems the thermometer at Sea-Tac is finally back on track, reporting temperatures more realistic with respect to other nearby thermometers.
It's been a long suspicion among some local meteorologists that the thermometer at the airport been running a bit warm over the past few years, frequently reporting temperatures 1-3 degrees warmer than surrounding sites. (Both UW professor Cliff Mass and I have done blogs on this apparent warming in the past.)
Here is just one example from July 16 last year when Sea-Tac reported a high of 88 degrees but everyone else around the Sound was closer to 83-86. (KSEA is Sea-Tac, the numbers on the far right are the preliminary highs for the day. This link will help decode the other cities listed here.)
UW research meteorologist Mark Albright has been tracking this anomaly for the past couple of years and has been among the most vocal in this apparent discrepancy. As just one example, he found for those first two weeks last July that the Sea-Tac gauge ran an average 2.3 degrees warmer than four other neighborhood thermometers placed within a couple miles of the airport. And he's found several other instances since where Sea-Tac has been apparently running warm.
But then on March 15, the gauge went bonkers, reporting a high of 76 degrees when it was actually closer to around 60. NOAA went and replaced the thermometer and then, as if by magic, Sea-Tac's temperatures have been more in line with surrounding neighborhoods.
To prove it, Albright just went back and compared a 10 day period prior to the March 15th reading and a 10 day period since. From March 3-12, Sea-Tac was running on average about +2.8 degrees warmer than a Weather Underground thermometer just a few miles away to the southwest (designated SEAT4). That's a fairly significant amount. But from March 22-31, it was only running +1.3 degrees warmer -- within microclimate tweaks you'd expect.
You can see how close the stations are together here (Sea-Tac's reading is the one next to the partly cloudy symbol)
(Map courtesy: Weather Underground Inc.)
And here is more suggested proof -- a plot of Sea-Tac vs. Renton Airport -- the top chart spans March 1-7 (before the gauge spike) and the bottom is April 1-7. The red line is Sea-Tac and the green line is Renton:
So Sea-Tac was running consistently ahead of Renton before, and is now consistently a little cooler -- more in line with what we would expect based on geography.
Even last Monday's warm day was a good example of Sea-Tac now remaining in check. Their preliminary high was 69 degrees amid a lot of 70s and 71s nearby, so pretty close. (Note: Seattle would eventually touch 70 after 5 p.m. so that was our official high).
I suspect had this been 2013, Sea-Tac might have reported a high of 73. Albright says Sea-Tac's observations Monday were similar to a number of other thermometers in the region. We'll see if it holds, but if it does, this summer might end reported slightly cooler than the past few, even if the weather itself isn't any different.
Albright points out Seattle is not alone, documenting similar sudden warm anomalies in Douglas, Arizona and Tucson, Arizona.
Hopefully more attention gets paid to some of these apparent warm biases in temperature records. The sensors -- mainly placed at or near airports -- are what we use for the vast majority of individual city climate records but their primary function is to provide current weather conditions to aircraft.
A couple degrees won't make much difference to an airplane, so a minor error in temperature won't raise eyebrows in the aviation world nor create an urgent need to replace the sensor. But it can skew a climate record a bit.
It seems it took a catastrophic failure of a thermometer to correct Seattle's problem -- a problem that appears to have been around for several months, if not longer. Looking back at 2013, Seattle ran consistently about 0.5 to 1.0 degrees warmer with respect to its average than Olympia (i.e., if Olympia reported a month about a half degree above normal, Seattle was 1-1.5 degrees above normal.)
Luckily, there are several other measurement methods used for overall global and regional climate calculations, but at least for Seattle, the numbers have probably made it seem a little warmer than it actually has been.