On Mar 15, 9:21*am, "Dartmoor Will" wrote:
"Graham Easterling" wrote in message

...
On Mar 14, 9:05 pm, John Hall wrote:









In article
,
Graham Easterling writes:
On Mar 13, 9:58 pm, John Hall wrote:
In article ,
Colin Youngs writes:

UK max. temps on Tuesdayhttp://tinyurl.com/826vk5a

Glen Ogle 6.5°C, Spadeadam 6.9°C, Bingley 7.1°C, Fylingdales,
Waddington
and Portland 7.2°C.

That Portland temperature is a surprise. If correct, it suggests that
the water of the English Channel must still be pretty chilly.

9 - 9.5C SST in that area. However, I've noticed that during sea fog,
the air temperature is typically around 2C below that of the nearby
SST.

Over open sea the air temperature is normally lower than the SST . See
Sevenstones todayhttp://www.ndbc.noaa.gov/station_page.php?station=62107
No sea fog, but air temperature below SST as normal. Under sea fog
conditions this phenomena moves over the coast.

That surprises me, as I thought that the mechanism for the formation of
sea fog was the sea cooling the air to its dewpoint. For the sea to cool
the air, the SST would surely have to be lower than that of the air? I'm
clearly missing something, as the measured temperatures can't lie.
--
John Hall
"The power of accurate observation is commonly called cynicism
by those who have not got it."
George Bernard Shaw

I've never had it explained to me convincingly, but it is a fact. I
assume that under normal conditions, evaporation of the sea surface
lowers the air temperature just above. With sea fog, the sun is
commonly visible just through it, if not it's vertical extent is still
normally very limited. Perhaps evaporation from the top of the fog
bank cools it?

Any explanations welcome. *Most reports from buoys show the SST above
the air temperature around 95% of the time, throughout the year.

Graham
Penzance
=======================

It is complex. As the fog forms an isothermal layer is set up within it at
around the skin SST. As it deepens and ages radiation from the fog top
lowers the temperature and also as you say insolation would cause
evaporation adding to cooling as well. In time this generates a weakly
convective over-turning within the bank, this mixes the air which will be
then cooler than the SST. This process can continue as the the SST itself
will then supply a heat source to drive the convective over-turning more
efficiently. So I think the main mechanisms are long wave radiative cooling,
evaporation at fog top and convective over-turning.

Cheers,

Will
--

Thanks, that process may well help explain why virtually calm
conditions often exist within the fog bank, with breezier conditions
on it's edge.

Graham