Why do we measure heat records at Sea-Tac Airport?

SEATTLE -- Seattle is in the midst of two days of record-breaking heat, if you go by the official thermometer at Sea-Tac Airport.

But did the city really experience record-breaking temperatures?

All official temperature and climate data is kept at Sea-Tac Airport. But many might not realize the point of the thermometer there isn't to keep track of weather data, but to provide real-time critical weather data to pilots taking off and landing at the airport.

The climate records are a bonus to having a well-maintained and well-calibrated observational equipment that automatically sends out data 24/7.

But when it gets hot outside, sometimes apparent anomalies appear in Sea-Tac data. A rough glance at the surrounding data would suggest that Sea-Tac's thermometer runs a few degrees hot compared to the immediate surrounding area when the wind blows a certain direction.

For example, on Tuesday, Sea-Tac Airport reported a high temperature of 87 degrees -- officially breaking the record of 85. But University of Washington research meteorologist Mark Albright points out that stations immediately surrounding the airport registered temperatures in the 81-83 degree range while at that moment Sea-Tac reported 85.

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The issue has been percolating in the local weather community for quite some time.

Sea-Tac's gauge is located on the southern half of the field in a grassy spot adjacent to a taxi way between the center and "third" runway to the west.

A popular hypothesis is that since the weather observation equipment is placed at a spot inside the runways on the southern end of the field, wind from certain northerly directions will blow air warmed by the vast amount of concrete to the north of the gauge toward the thermometer, boosting it a few degrees on hot days. That area used to feature a lot more trees, but was cleared for construction of the third runway that opened to the west and northwest of the gauge in 2008.

To help answer questions, especially when hot weather is here and records are threatened and/or broken, the National Weather Service put out a "Guide to the Gauge" at Sea-Tac.

First of all, the weather observation equipment, known as "ASOS" for Automated Surface Observation Systems, were primarily built for supporting aircraft operations and safety.

"The temperature sensor is intended to provide temperature readings of the area right near the runway that will aid in the calculation of Density Altitude, a critical piece of information for aircraft takeoff and landing," the National Weather Service wrote, adding the gauge would not be moved to support climate purposes.

Why? The NWS stresses that the gauge is meant to be representative of the conditions at the airport, not the city.

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"Seattle's geography and topography are very diverse with hills, sheltered valleys, forested areas, residential areas, industrial areas, etc. which all will have varied temperatures and behaviors," NWS wrote. "The temperature can vary widely, sometimes 10-20 degrees across the city."

They say the thermometer is calibrated every 90 days (last done on April 22, 2019; next one due in late July) and all thermometers have an accuracy of +/- 3 degrees.

"Uses of the data for climate purposes was not the primary intended use for ASOS. However, the ASOS network is one of the best maintained sensor networks available," NWS wrote. "As a result the data availability makes it a prime candidate for tracking daily weather conditions and is used by the NWS as a primary source for daily climate information since they are generally located near population centers."

But NWS stresses that for long-term climate research, the U.S. Climate Reference Network uses data away from populated areas to mitigate urban heat island effects, but this is poor data to use for tracking daily weather data and records in populated areas.

So how did Sea-Tac become the epicenter for record-watching and our official recording spot? It's 10 miles south of the actual city of Seattle, and as Carlin joked, why would we care about weather at the airport since no one lives there?

For one, we've got good data there back to 1945.

"But what about Boeing Field?" many wonder. "It's at least closer to the city?"

Boeing Field's "good" data only goes back to 1998. There is less-quality data from 1948-1964 then no data from 1965-1998.

So that's out as we don't have enough of a climate record.

Before 1945, official records were kept at the Downtown Seattle Federal Building and we have good data back to 1894. But observations stopped being kept there in 1972, otherwise problem solved. But to that effect, recently the National Weather Service has "threaded" the Sea-Tac and Federal Building stats into one record set to give a continuous record from 1894 to today -- if you notice of late a lot of communications from NWS on records now includes Federal Building data.

"While we understand there is some apples to oranges comparison here, this is the best available option for a long-term, continuous set of records," NWS says.

They conclude by stating they are exploring additional options to identify a "more representative" location for the official climate site for Seattle. Not mentioned but probably the most likely candidate could be the National Weather Service itself. They have been maintaining records at their office in Seattle's Sand Point neighborhood since 1986. But their office is right along Lake Washington which could present cooling issues relative to much of the rest of the city, and they've only been tracking temperature data, not wind or cloud cover or many of the other items that the ASOS sites measure. They'd have to upgrade their equipment, which of course is costly.

"This all really underscores the reason we focus on impact-based forecasts," says Reid Wolcott, Warning Coordinator Meteorologist at NWS Seattle. "Just like 6 inches vs. 8 inches of snow doesn't make a huge difference for most people, 85 degrees vs. 87 degrees isn't an impactful difference ... It's hot, here's what you need to know and what you should do."

This article first appeared on KOMO NEWS.