Troy's Scratchpad

November 4, 2010

Explaining correlation in US UHI tests

Filed under: Uncategorized — troyca @ 1:04 pm

Previously, I’d run a few tests and found a correlation between the change in population density from 1990 to 2000 and the change in temps from 1990 to 2000 in the US.  See here, here, and here.  At first I took this to be a UHI signal.  However, a bit more investigating has shown this is likely the result of a spurious correlation.  To summarize:  a large one-year swing in temps in a portion of the United States from 1989-1990 also happened to be in a similar area of the largest population growth from 1990-2000.  (Data and code available here).

Consider the now familiar graph I’ve shown multiple times:

However, what happens if we move back one year to the temperatures in 1989?  If what we’re seeing is a UHI signal, this should not have much of an effect, but…

Well, now we’re getting something nearly as strong in the opposite direction.  Clearly, much of this can be chalked up to the change in 1989-1990:

An extremely high correlation, but the population change from 1990 to 2000 obviously did not have an effect on the temperature change from 1989-1990.  In this case, I believe the explanation comes from geographic location.  Here’s an examination of how latitude and longitude correlate with the temperature swing from 1989-1990:

And now how lat/long correlate to the population change from 1990 to 2000.

The Story

To me the simple explanation is this — the Eastern U.S. heated up the most from 1989-1990.  This allowed for a generally larger temperature increase in the Western U.S. compared to what could be experienced in the East from 1990-2000 (since we already started on a peak for the East).  Meanwhile, it was the Western U.S. stations that experienced the largest relative population growth from 1990-2000.  Thus, we saw a correlation between the increase in temps and the population increase in the Western U.S., although in fact this had little to do with UHI. 



  1. […] Troy_CA's blog Troy_CA's blog Skip to content HomeAbout ← Explaining correlation in US UHI tests […]

    Pingback by A More Robust Method of Finding a UHI signal | Troy_CA's blog — November 8, 2010 @ 10:44 pm

  2. […] Lower threshold distance is better. This is kind of a no-brainer, since we’d think that nearby stations would have less actual climate-related differences.  We see that in almost all cases the < 0.5 degree distance shows better correlation and a higher slope.  Of course, some of this may be due to fewer observations, which is also a problem…there are not necessarily enough very close station pair candidates here.  I have a few ideas on how we might get more observations for this using a sliding time period window.  Once we get to higher thresholds, we start getting into the problem of spurious correlation due to trends in regions which I discussed previously. […]

    Pingback by AMRMo Finding a UHI Signal P3 – Using NOAA Population data | Troy_CA's blog — November 12, 2010 @ 6:32 pm

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