April 12, 2014

The Great Sea-Tac Temperature Mystery: Is it Solved?

A great mystery has engaged the local weather community for over a year now:  the Seattle-Tacoma Airport warm anomaly.

Day after day Seattle-Tacoma Airport's temperature gauge was warmer than observations in its neighborhood.   Was it the impact of the third runway?  Or the nearby concrete and gravel?

A National Weather Service field trip to the temperature sensor, using a carefully calibrated temperature standard, suggested that the sensor was fine (at that time).  And how would the runway warm the temperatures at night?  My friends in the National Weather Service wanted to ensure the temperatures were correct, but what was wrong?

The mystery grew. The game was afoot.

And then on 15 March 2014, the Seattle-Tacoma Airport temperature sensor went crazy, reporting a high temperature of 76F while four nearby observing sites had highs of 58, 59, 60, 61.  The next day the NWS meteorological technicians replaced the temperature sensors and some of the electronics, and guess what happened?

Much of the warm bias disappeared.

Here is a plot of the difference of the temperature between Seattle-Tacoma Airport and a nearby weather site (Weatherunderground SEAT4, see map below, that station is SW of the Sea-Tac observation--the one with the cloud).  The vertical red line shows the day the equipment was replaced and the horizontal green lines show the average temperatures before and after the switch.  UW's Mark Albright produced this plot. There is clearly a significant drop in temperature after the sensor replacement.

Further evidence is provided by plotting temperatures at Seattle-Tacoma Airport and at nearby Tacoma Narrows Airport in Tacoma. Before March 16th, Sea-Tac has a higher maximum temperature than Tacoma Narrows (TIW) nearly every day, but after the repair they were consistenly very similar.

Is this an isolated incident?  Unfortunately not.  I can give you a long list of airport weather sensors that were reading warm, and then cooled considerably when they were replaced (e.g., Douglas, Arizona).   Or sensor that began reading warm when their fans failed or slowed.   The trouble is that most failures seems to lead to warmer, not cooler temperatures.   Thus, problematic sensors and equipment are contributing a warm bias in official temperature records.  How big a problem is this?  I am not sure we have any kind of handle on this.

And there is another problem leading to unrepresentative warming:  urbanization and development around temperature sensors.  The Wenatchee temperature sensor, part of the Historical Climatological network, is shown below as an example of poor exposure--with a hot parking lot nearby as well as nearby buildiing.

We are fortunate to have a number of meteorological sensors that are less prone to such problems (e.g., satellites), but their records are quite short compared to the surface instrumental record.


  1. As someone who spends most of their professional life replacing and troubleshooting faulty meteorologic instrumentation, there's plenty of it out there. These are expensive, precision instruments; the data they output is incredibly valuable. Sensor swaps are a bargain.

  2. It surprises me that there are not redundant sensors installed at a critical observation point like this. I have two temp sensors on my site... for another reason, but I do validate their accuracy when the conditions are right for them to match.

  3. I am all in favor of 'official expensive' instruments, but from a lifetime of having monitored various things some sort of elementary error checking is needed. The simplest is cross checking with other instruments. Outliers need to be tossed, or at least investigated. And in any event unexplained outliers should generally not be used in averages and determining trends.

  4. I've noticed for a while now that there are strange readings from the Eastsound Airport AWOS (KORS). Pressure always 0.02" less than surrounding stations, visibility 9 mi when it's clear for a hundred miles.

  5. so did anyone investigate the root cause?

  6. My station is just SW of Seatac too:

    KWANORMA3 or DW1735 if you are wanting to see it under CWOP.

    The temps are much more in line with my station now.

    68.1 at my home
    68 at KSEA

    High temps for 4/13

    I have my thermometer running over grass in a white shield with a solar powered fan drawing air across the thermometer the entire time.

  7. The Bremerton National Airport sensor is the worst! Last month it read a high of 80!!!
    A local newspaper report said that it would not be replaced for at least a year...
    What a shame!

  8. This is a stupid question (like: "..is the appliance plugged to outlet?..."), but is there a chance that the sensors located at airports where large jet aircraft move around get "cooked" or "dirtied" by jet engine exhaust? Look at those dirty tails on some airliners!

  9. Two words. Ice Bath.
    Kids these days are allowed to make psuedo-scientific weathercasts based on bad data and bad 'model-analysis' and scrutinize climate and coin phrases like climate crisis and 'superstorm this or that', when simple fundamental understanding of measurements and calibrations are lost. It's only basic science folks. Initialization is imperative to a sound forecast.

  10. The problem is worse than you describe because the "official" temperature records (Hadley, GISS, BEST) are automatically adjusted to correct for statistically improbably "breakpoints" in the record even when the cause of the breakpoint isn't known. Correction involves adjusting older temperatures upward or downward (and more often downward) relative to current temperature enough to eliminate the breakpoint. Sometimes breakpoints are caused by changes in the time of day min-max thermometers are read and sometimes by station moves or changes in the station's surroundings. Problems that develop slowly and are corrected all at once represent a return to normal observing conditions and their correction biases the record. BEST creates two separate records at a breakpoint, thereby retaining any bias that develops slowly.

  11. It would be interesting to know the degree to which this issue has affected the temperature measurements that have formed the basis for the global warming scare.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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