Your version of 39-degree fog sounds much more appealing than our version would. (Not that fog is really possible at 39C; the highest temperature at which I've experienced fog is 29C in Beijing in 2005, which at least gave me some good stories to scare aspiring Olympic marathoners with).
Ug, yeah, 39 deg fog would be gross- but droplets too quickly evaporate at that temp no matter the humidity? Is that why not possible?
Last week, here in Central Michigan, there was a fog at -2F or -19C.
To get fog at 39C, you would need a dewpoint of 39C, which in turn needs a source of moisture (not necessarily nearby) at 39C - usually a body of water (although sometimes, in place like the Midwest corn belt, transpiration from plants can contribute).
However, the upper limit for surface temperatures in the open ocean (for energy balance reasons) is around 31-32C; slightly higher values can occur in shallow enclosed bodies of water (peaking at around 35C in the Persian Gulf). The only bodies of water in existence at 39C or above would be those which are geothermally heated (Yellowstone is a good place to find those), but those are too small to have much effect on the wider atmosphere.
Highest dewpoint I've experienced myself is 30C in Qatar; have seen reported up to 33C, also in the Gulf.
Think I need to understand this word 'dewpoint' better... =)
We learn stuff on AP. :-)
Ali, it's easy if you consider it in terms from your own field. A dewpoint is an Aries and transpiration is what happens when Venus transits Aries.