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Anomalous DCA anomalies?

Inverted_Trough

By Inverted_Trough,
in Mid Atlantic

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CAPE

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Ten man did not complain about Annapolis and that sort of falsehood is what turns legitimate discussion into food fights. Get your shi$ straight. I use Annapolis as the example of why Dca proximity to water does not work because Annapolis is located due west also of a much bigger body of water. You out of area guys are clueless and Ian's comment No better as dca is 70% surrounded by water marsh and extensive tree scape. The very idea that Nothing was wrong at dca has already been squashed with more adjustments likely on the way

Then you picked a bad example to make your case. DC is an intense UHI. Annapolis is not.

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Nice to see dca responding better since the correction. Within just a few degrees of other stations today instead of +5. Had long been my contention that stout nw breezes in reality would mitigate whatever odd heat source dca manufactures and today was evident of that

So are you convinced there was a long term, existing problem with the sensor(s) and/or the instumentation? Even though the NWS has stated the sensors are checked every 3 months? You see, I know a lot about temperature sensors and measurement(and calibration methods), and my contention has been that the least likely source of these apparent anomalies would be attributed to the sensor itself, or calibration issues with the instrumentation. Most likely source- UHI. Another possibility- the specific location of the sensor, and any potential errors introduced by heating effects in that localized environment.

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Inverted_Trough

Inverted_Trough

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The difference in temperature readings was stark and immediate with the change to the new sensor.  There appear to be quite a few people who are very resistant to admit that there is reasonable - I'd say strong - evidence that the thermometer at DCA was on steroids, and was for quite some time.  

 

While they check the sensors every few months, apparently if it's within 5 degrees of the true temperature, it's considered an "accurate"  reading.  Certainly for aviation purposes - which is the true purpose of the ASOS at airports - being off by a couple degrees isn't a big deal.  It's only a big deal to weather geeks like us.  Does anyone think they'd have changed anything if there wasn't an article about it on the Washington Post website, and the Virginia state climatologist wasn't raising a stink about it?  I doubt it.

 

There are other airports nearby that could have similar issues with their sensor or, more likely, siting issues.  Leesburg, Frederick and even Hagerstown come to mind.  So DCA isn't alone - though it definitely has a long record of sketchy readings.

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The difference in temperature readings was stark and immediate with the change to the new sensor.  There appear to be quite a few people who are very resistant to admit that there is reasonable - I'd say strong - evidence that the thermometer at DCA was on steroids, and was for quite some time.  

 

While they check the sensors every few months, apparently if it's within 5 degrees of the true temperature, it's considered an "accurate"  reading.  Certainly for aviation purposes - which is the true purpose of the ASOS at airports - being off by a couple degrees isn't a big deal.  It's only a big deal to weather geeks like us.  Does anyone think they'd have changed anything if there wasn't an article about it on the Washington Post website, and the Virginia state climatologist wasn't raising a stink about it?  I doubt it.

 

There are other airports nearby that could have similar issues with their sensor or, more likely, siting issues.  Leesburg, Frederick and even Hagerstown come to mind.  So DCA isn't alone - though it definitely has a long record of sketchy readings.

This is an awful post... sensors on steroids? hmm. Actually, its the fault of the sun god. And damn that god of wind too. This is all so mystical and magical.

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“It looked like it [the sensor] was reading 1.5-2 degrees too high,” Lee said, noting the ASOS sensor was reporting 78.7 degrees while the psychro-dyne registered 77 degrees during initial testing.

Lee said he authorized technicians to replace the sensor given the difference along with the fact “a couple of  its [the sensor’s] cooling fins were bent” (though he doesn’t think that was obstructing airflow or affecting readings). The sensor was last replaced in 2010, he said.

Lee said the National Weather Service inspects ASOS sites every 90 days to conduct preventative maintenance. Prior to Monday, Reagan National’s last check was on June 9 when there was only a 0.4-degree difference between the temperature sensor and the psychro-dyne reference measurement.

Lee explained that the temperature measurement at an ASOS site is within technical specification as long as its temperature readings are within (plus or minus) 5 degrees of the reference measurement. Both the ASOS and the psychro-dyne sensors have an error of plus or minus 1.8 degrees. Lee noted that, even though he decided it was time to install a new sensor Monday, Reagan National’s temperature readings had not strayed from specification then or on any prior inspection.

https://www.washingtonpost.com/blogs/capital-weather-gang/wp/2015/08/11/weather-service-installs-new-temperature-sensor-at-reagan-national-airport/

+ - 5 degrees F seems a bit large for an allowable error. More typical tolerance for any "critical" measurement would be 2 or 3 degrees max. But that is what it is, based on the rated accuracy of the specific sensor being used, and the test reference. The bigger take away here is that 2 months earlier, when the sensor was checked against the standard, it was less than half a degree off. This implies that the source of the (minor) deviations are somewhat random, and likely attributed to localized heating effects, including effectiveness in removal of any self heating, and is mostly related to local sensor environment. In any case, both the previous, and replacement sensor tested within the criteria for allowable error. The old sensor was not faulty.(or on steroids)

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Actually I am quite perplexed as to why they would be using a psychrometer as a  temperature reference. I use a platinum resistive element (RTD), of high grade and traceable to an official calibration standard. This would be capable of testing sensor accuracy to under 1 degree C. and so would be better suited  to validate a sensor with an advertised + - 1 deg C (1.8F) tolerance.

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Ian

Ian

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I think I already said this but the period we just went thru typically has the closest range of temps of the warm season across the local area. Seeing the sensors come together should be expected. Seems a few are in one camp or another... Do think there was probably some error at times which seems better now but still no real evidence it was 'years' long or something and it's probably smaller than some want to think.

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famartin

famartin

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Average high temps at DCA vs. IAD since DCA thermometer replacement 8/10/2015 (8/11 through 9/12)

DCA 88.2

IAD 86.6

Difference 1.6

By comparison, difs from previous years (same period)

2014: 4.2

2013: 1.6

2012: 1.3

2011: 1.1

2010: 1.6

2009: -0.4

2008: 2.2

2007: -1.6

2006: -0.1

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Ian

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Last year was pretty wet in that period. I still think the wet had something to do with the range earlier this summer. Harder to warm overall but DCA has less green space in the local area so easier to warm there than Dulles. Big part of the high temp parity this time of year is because it's often dry and sunny. 

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mitchnick

mitchnick

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BWI is only 6 miles further away from DCA than IAD, yet I haven't seen any comparisons between BWI and DCA, or BWI and IAD, as a possible cross check to the 18 months of warm readings the DCA/IAD comparison appeared to show.

living 2 miles from BWI and being familiar where the sensor is located, the 3 stations are very different from one another wrt terrain, elevation, and surroundings

IAD is traditionally the cold spot of the 3, hands down on radiational cooling

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famartin

famartin

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living 2 miles from BWI and being familiar where the sensor is located, the 3 stations are very different from one another wrt terrain, elevation, and surroundings

IAD is traditionally the cold spot of the 3, hands down on radiational cooling

Yes, but for highs, that should not matter much, since the urban heat island at properly located stations is primarily a night time phenomenon. So, one should look at high temperature differences over the past 15 years between the three stations and note if there were any notable deviations in the overall pattern.

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mitchnick

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Yes, but for highs, that should not matter much, since the urban heat island at properly located stations is primarily a night time phenomenon. So, one should look at high temperature differences over the past 15 years between the three stations and note if there were any notable deviations in the overall pattern.

well, I only meant to highlight the radiational cooling nights

IAD remains the coolest around 80% of the time and DCA the warmest 80% of the time wrt highs and lows, with BWI in the middle 80% of the time

obviously a guess, but I usually look at all 3 stations when I check temps

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famartin

famartin

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well, I only meant to highlight the radiational cooling nights

IAD remains the coolest around 80% of the time and DCA the warmest 80% of the time wrt highs and lows, with BWI in the middle 80% of the time

obviously a guess, but I usually look at all 3 stations when I check temps

That is correct and only to be expected given that elevation between the three stations follows the same pattern (DCA 10', BWI 137', IAD 292'). However, since the CWG article showed a sudden spike in the difference between DCA and IAD starting in early 2014, someone ought to check BWI agsinst both DCA and IAD to see if something similar happened... I e, did DCA's highs also suddenly get warmer compared to BWI, or did IAD's highs suddenly cool compared to BWI...
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