Chester County PA - Analytical Battle of Actual vs. Altered Climate Data

[ChescoWx]

6 hours ago, chubbs said:

Nope. The Chesco COOPs cool significantly relative to the SE PA Division data from the PA climatologist. Regression isn't a good tool because the station changes are sudden not gradual and you picked an interval that started after the initial Coatesville move and ended before the West Chester move took full effect. If you widen the interval to 1941-1975, the difference in slope is large.

Per table below, comparing the five years before the first move and the five years after the last move, the Chesco COOPs cool by 1.7F relative to the SE Pa Division. Note that the Pa SE Division includes the Chesco Coops so the two items compared are not independent. For that reason it is not surprising that the delta for the SE PA is slighly smaller than ABE. 

Funny seeing your persistent effort to dismiss what the the raw data from Chester County clearly shows.  The 3 stations each cooled significantly after the 1945-1970 moves: first Coatesville cooled relative to Phoenixville and West Chester; then Phoenixville cooled relative to Coatesville and West Chester; and finally West Chester cools relative to Coatesville and Phoenixville.  Clear as day in the raw COOP data.

 

 

All three stations cooled at statistically the same rate as the surrounding stations. Additionally, in looking closer at potential TOB adjustments one may wonder why the West Chester 2NW and Coatesville 1SW records which predominantly relied on morning (AM) observations...so one would think the Raw TOB Distortion of an AM reading captures a naturally lower minimum, meaning the raw data was technically already too cool correct? The NCEI Adjustment you would think logically would be to correct for morning bias at Coatesville and West Chester by warming the historical record. However, NCEI's automated, grid-wide homogenization algorithms actually did the opposite by applying additional sweeping chilling adjustments to both of these stations data from 1895 through 1982. 

[ChescoWx]

Of course @chubbsWidening the interval to 1941 - 1975 does not change the previous answer. Both series show a real, statistically significant cooling trend on their own — this isn't noise, and it fits the well-documented mid-century cooling that shows up across the eastern US from the 1940s into the 1970s.

Are the two trends statistically the same?

This is the more interesting question, and the answer depends on the test:

• Naive comparison (treating the two trends as independent): the difference in slopes (−0.66 vs. −0.46 °F/decade) gives t = −0.92, p ≈ 0.36 — not remotely significant. By this test they're indistinguishable.
• Paired comparison (the right test here, since both series describe the same regional weather in the same years — they're correlated at r = 0.83, so a paired test on the year-by-year difference is more powerful): the difference series trends at −0.021 °F/yr (−0.21 °F/decade), 95% CI [−0.042, +0.001], p = 0.057.

So the more rigorous test lands just on the wrong side of the conventional 0.05 cutoff — the station data cooled somewhat faster than the division average, and that gap is close to, but doesn't quite clear, statistical significance. The honest read: the two trends are not clearly, statistically different so no need to cool the raw data further as they are already a strong match.

[chubbs]

19 hours ago, ChescoWx said:

Of course @chubbsWidening the interval to 1941 - 1975 does not change the previous answer. Both series show a real, statistically significant cooling trend on their own — this isn't noise, and it fits the well-documented mid-century cooling that shows up across the eastern US from the 1940s into the 1970s.

Are the two trends statistically the same?

This is the more interesting question, and the answer depends on the test:

• Naive comparison (treating the two trends as independent): the difference in slopes (−0.66 vs. −0.46 °F/decade) gives t = −0.92, p ≈ 0.36 — not remotely significant. By this test they're indistinguishable.
• Paired comparison (the right test here, since both series describe the same regional weather in the same years — they're correlated at r = 0.83, so a paired test on the year-by-year difference is more powerful): the difference series trends at −0.021 °F/yr (−0.21 °F/decade), 95% CI [−0.042, +0.001], p = 0.057.

So the more rigorous test lands just on the wrong side of the conventional 0.05 cutoff — the station data cooled somewhat faster than the division average, and that gap is close to, but doesn't quite clear, statistical significance. The honest read: the two trends are not clearly, statistically different so no need to cool the raw data further as they are already a strong match.

Repeating what  I said above garbage in, garbage out.  Your analysis and AI's conclusion is faulty. You aren't telling AI that the Chesco COOPs are included in the SE PA climate division. You can't conclude that Chester County is the same as the non-Chesco stations if you include Chester County with the other stations. Second the regression slopes depend on the end-date. You need to include enough years before and after the moves for the station moves to be fully reflected in the regression. For the period 1941-1975 I get a much bigger difference in slopes -.69 for Chesco and only -.32 for the Division. Excluding Chester County from the Division would lower the Division slope further. Finally AI already has a Pvalue of 0.057, i.e, there is a 94% chance that Chesco is different using your faulty comparisons. Correct the errors and the P value will shrink considerably.

My biggest complaint though is that your statistical model is inadequate. Temperature observations are highly correlated across a region because all stations experience the same weather. The Chesco and SEPA stations should be very close. Instead the chart clearly shows that Chesco COOPS cool significantly vs the SEPA Climate Div. The different temperature response by the Chesco Coops is closely associated with the timing of station moves. End of story.

[ChescoWx]

AI knew the Chesco COOPs are included in the SE PA climate division but did not feel it material enough to change the statistical fact that the raw data at Chesco was cooling the same as the surrounding stations so again no need for further cooling. Plus with all those AM TOB readings at Coatesville and West Chester why didn't NCEI apply some warming adjustments? Again only non-stop chilling adjustments. 

[ChescoWx]

A member asked for an analysis of 100 degree days by decade since the 1890's. This is normalized as days per station per year (total 100°F+ days across all stations in a decade, divided by the number of station-years of data in that decade) rather than a raw sum — otherwise a decade with 22 stations would look "hotter" than a decade with 4 stations just from having more thermometers out there, which would be misleading.

The pattern holds up clearly at the county level, same as it did for individual stations: the 1930s stand out sharply at about 1.1 days per station per year — nearly double any other decade — consistent with the well-documented Dust Bowl-era heat. From there the rate declines fairly steadily, bottoming out near zero in the 1970s and again in the 2020s (0.01 and 0.02 days/station/year respectively), with the 2000s-2010s sitting in a modest middle range around 0.11–0.15.

Worth flagging directly: the two lowest decades in the entire 130-year record are the 1970s and the current 2020s — and the current decade of the 2020s (partial decade).