If you want one more inhomogeneity in station records, it seems that temperature measurements have been whitewashed – literally. The Stevenson screens used to be painted with whitewash which is a calcium carbonate with very specific infrared absorption properties. I gather that whitewash isn’t used much any more and so screens are routinely painted with modern white paint, which has very different infrared absorption properties. See here. Maybe Phil Jones should be collecting tree rings instead.