Why different 'global temp' measurements?

Earlier this month,GISS announced that November was the warmest such month in its record books...NCDC, part of NOAA, has published the results of its own calculations, showing that November was the second warmest, not the first.

Such conflicts in global temperature rankings aren't terribly unusual. In fact, NASA-GISS and NOAA-NCDC rank 2005 as the warmest year on record. But a third group...known as "HadCRUT," gives the title to 1998.

Each of the three groups calculates temperatures at the surface of the land and sea. But two other groups, UAH and RSS use microwave sensors on satellites to estimate the temperature of the lowest part of the atmosphere.

And guess what? Their findings differ a bit from each other, and from those of the other groups as well.