A/script>s I noted in the previous post, a new post has appeared at WUWT which talks a lot about the reduction in station numbers in GHCN that occurred between about 1990 to present. This post is based on a paper by Ross McKitrick.
The stream of articles that advance various theories about this reduction don't take proper account of the way GHCN was actually compiled. It was initially a historical process, where in the early 90's with grant funding people gathered together batches of historic records, recently digitised, into a database. After V2 came out, in 1997, at some stage NOAA undertook the task of continuing monthly updates from CLIMAT forms. This made it, for the first time, a recurrent process.
Update: Carrot Eater, in comments, has pointed to a very useful and relevant paper by Peterson, Dann and Phil Jones. As he says, the process wasn't quite as I've surmised. I should also have included a reference to Peterson's overview paper.
The big reduction followed changes of policy in going to a recurrent process. As a batch process, it didn't really matter if the geographic spread was uneven. If the records were available, they could be included. But as a recurrent process, it makes sense to spread the effort of updating reasonably evenly worldwide.
If you look carefully at the time sequence of station terminations, it looks like this:
It clearly consisted of a few major culling events. In the next table, the years with more than 100 stations ending are shown with a breakdown by country:
This makes the pattern clear. Australia, Canada, China, S Africa, Turkey and USA had been overrepresented, and were culled in specific events. Canada in 1989/90, Turkey and China 1990, S Africa 1991, Australia 1992 and the USA in 2004 and 2006. The case of the US is special, because the USHCN database is also used.
So when it is said that the reductions produced a reduction in average altitude or latitude, that reflects the fact that some of these countries are relatively high, and are (mostly) from temperate latitudes.
Of course, explaining how and why the reduction happened doesn't remove the possibility of biasing a trend estimate. That's another story.