|Global Historical Climatology Network|
There are three main global average-temperature histories: HadCRUT (CRU-Hadley), GISTEMP (NASA-GISS), and the NCDC (NOAA).
All three depend on the GHCN's land-data archive. For an explanation of how it is compiled, click here.
1. It peaked with about 6000 weather stations in the late 1960s. But by the late 1990s, the number of quality stations had dropped to less than 3000.
2. Click here to read an example of the kind of conspiracy theories that can be evoked by anything in this field that is in the least bit complex. After that, click here for Gavin Schmidt's (vain?) attempt to rebut it.
3. For a more rational skeptic critique on how this data is compiled and used, click here to read Ross McKitrick's essay A Critical Review of Global Surface Temperature Data Products.