"Jim Macklin" wrote in message
news:%mqNf.107786$QW2.7653@dukeread08...
It causes some vertical motion which stirs the air a little,
casing the fog to form.





Yes!

The very coldest air at that time of morning is right at the surface of the
earth. As it starts to mix with air above it, it cools a whole layer of 50
or a hundred feet or so, instead of just the thin surface layer. If the
average moisture content through that layer is such that the cooling drops
the temperature down to the average dew-point, bingo.

If you watch weather reports carefully, you will find that in those
clear-morning situations, the lowest temperature occurs *after* sunrise,
when you would have expected that warming is started. But when this mixing
starts, that coldest air at the surface (as part of the slight turbulence)
reaches up to the 1 metre level where the "official" thermometers are kept.

If you check a weather observing site which observes "grass" temperatures,
such as an experimental farm or the like, you will see just how great this
temperature difference between the grass level and the official 1 meter
level can be at sunrise... exceeding 5 degrees Celsius in clear-sky cases.