//You were right the first time by following the mixing ratio line
(blue). The
example shows just how easy it is to saturate a dry airmass by deep
mixing/lift. Granted this much lift (900-660) is quite excessive, even
for
the US High Plains.//

Thanks, wxforecaster

//Furthermore, whoever posed this problem needs to check his/her own
work. How
can fog, a ground based phenomenon, occur at 660mb in this scenario? //

As I R A Darth Aggie noted, the meaning of this fog: "parcel reaches
saturation".


//Honestly have never heard of a "Tephigram" before, in reference to a
SkewT/LogP diagram. Interesting. More interesting how you're using what
looks like official British stationery from the 1950s. Hope that's not
an
original :-D //

To say the truth , I never heard that "fog" is tied in some way to the
ground, but hell I've got an excuse: I'm not english native speaker ;-)

Cheers