The latter, its simply weather station data carefully examined, see my other comment. I guess you could do an actual global measurement if you put a satellite

The latter, its simply weather station data carefully examined, see my other comment. I guess you could do an actual global measurement if you put a satellite fairly far out with capable of measuring the IR and optical. Then you could define a black body effective temperature, but I'm not sure that straightforwardly tells us about surface temperatures.