It’s true that pilots have to worry about more than just the surface temperature. We move in three dimensions, and need to know the forecast temperature thousands (or tens of thousands) of feet above the airport as well as on the ground — we get that from the FD (digital winds/temperatures aloft) forecast, as well as the freezing level in the FA/GFA (wide-area forecast). Any risk of temperature-dewpoint convergence (i.e. mist or fog) is already taken into account in the TAF.
Still, surface temperature has value. For example, it can tell me whether I’m likely to find frost on my wings when I arrive at the airport, and it can tell me whether the forecast includes an inversion (which might not show up in the FD if it’s low level). It also gives us a good indication of where forecasters expect local variations from the area forecast. The main proof that surface temperature forecasts are important is the fact that FSS briefers routinely look up the surface temperature from the general public forecast and read it to me (it has limited value, though, since it’s only the high and low, without time information).
It turns out that the international TAF format does include an optional surface temperature group that looks like this:
T17/20Z - forecast temperature of 17° C at 2000 UTC T08/21Z - forecast temperature of 8° C at 2100 UTC T00/18Z - forecast temperature of 0° C at 1800 UTC TM10/07Z - forecast temperature of minus 10° C at 0700 UTC
(I took these examples from a U.S. web page, INTERNATIONAL TERMINOLOGY AND FORECAST GROUPS NOT USED IN NWS TERMINAL FORECASTS.)
Why not include temperature? Does anyone know the history of this one?