theresalwaysone
As it happens I do know what a tephigram or a skew-t is (and use them for flight planning) but it is laughable that you write such a pontificating piece first not knowing this is far divorced from not only PPL met syllabus but from any practical VFR or IFR met understanding requirement. It also barely answers my original question. Finally, a tephigram is hardly relevant that close to the ground; the distance I refer to is barely resolvable on the diagram and is not a lot more than the length of the piece of string the baloon is initially tied to...
This needs somebody with a PhD in physics. I did physics to somewhere part-BSc level but we didn't do atmospheric physics...
skyhawk
I don't think fog and stratus cloud are related, other than both having 100% RH. Fog is a local thing, but stratus can form over vast areas. It was stratus I was referring to in my original Q - why does it (almost) never go all the way to the ground.
The answer could be that the humidity falls near the ground. I don't think the temperature rises near the ground, to the very specific extent required for this to work the way it (almost) universally does, regardless of the surface covering, vegetation, etc.