Watts Up With That?

A guest post by John Goetz

[Update: I cross-posted this on CA and in the process added the brief discussion of night lights as well as made some minor text changes. I have reflected all of those changes here]

In May I began a quest to better understand how GISS does its homogeneity adjustment, also known as GISS Step 2. Steve McIntyre took the ball from that scrum and ran with it, producing a set of R tools that nearly replicate the GISS method. Some of the endpoint cases continue to confound those of us trying to understand the source code and how it reconciles – or doesn’t – with the peer-reviewed literature.

As this was going on, Anthony Watts pinged me several times, asking that I look at the Cedarville, CA adjustment to better understand why GISS would apply an urban adjustment to an obviously rural station, a topic which he explored in a previous post. I hesitated, because Cedarville had a lot of “nearby” (as defined by GISS) rural stations, and I wanted something simpler to look at. However, I did not forget his request and I took occasional peeks at the station and its neighbors. At left is an overall site view of the Cedarville station. GISS assigns a “night lights” value of 2 to this station, which is what causes it to go through the homogenization process.

Here is a Google Earth image of Cedarville and the surrounding area with the NASA City Lights image overlay enabled. I am not sure what the NASA sensors are picking up to assign Cedarville the “2″ rating.

Anthony says this in his post about Cedarville: “a place with a good record and little in the way of station moves”. Generally this may be true, but I personally am suspicious about the fidelity of a station’s record when I see Batman lurking in the 1930’s:

OK, let’s assume for the moment that Cedarville’s record is beyond reproach. Let’s further assume that the Cedarville station is urban, and is cursed with the typical frailties of an urban station: lots of asphalt, little vegetation, and placement near an air-conditioner, strip mall, or jet engine. Certainly the surrounding rural stations are of such pristine fidelity that they can be used to remove the urban noise from Cedarville. Let’s take a closer look at those stations and that homogeneity adjustment. Read the rest of this entry »