The weather data is averaged from reports from stations all over the globe. What has been happening with the US data, is that it is being lemon picked. While more weather stations exist than ever before, the US data sets have been shrinking. And the reports that go away are from high altitude, northern latitude, central continental locations. When data is pulled out, the average increases. Also the data sets that are kept or are added are also suspect. Data sets near airports show an increasing trend just because of development. What was 30 years ago an ocean of grass is now covered in asphalt. But they don't normalize for that, and when they do, they often normalize the wrong way. Lies damned lies, and global warming.