On 12 Aug, 11:19, Steve Willington wrote:
On 12 Aug, 09:26, Dawlish wrote:



On Aug 12, 8:18*am, Paul Hyett wrote:

On Mon, 11 Aug 2008 at 09:55:06, Dawlish wrote in
uk.sci.weather :

This forecast is still in the balance for next tuesday - but there's
something that we haven't seen on any chart, even at T264+, for weeks.
A North African plume!!

How is that different from a Spanish one?
--
Paul Hyett, Cheltenham (change 'invalid83261' to 'blueyonder' to email me)

It has it's source in North Africa - as most plumes that reach the UK
do. The hot, dry, often Saharan air becomes substantially modified as
it crosses Iberia, on its Northward passage to the UK. "Spanish" plume
is a misnomer, as regards the actual source of the hot air, but would
be accurate if applied to the source of the modification of that air
to produce the potential for thunderstorms in the UK.

Look at the source of this hot air in this possible example at +264 on
the gfs.

http://www.wetterzentrale.de/pics/Rtavn2642.png

It is then shown to cross Spain and if it did get to the UK (which I
think is unlikely) I'm sure the press would call it a "Spanish" plume;
but the source of the hot air would not have been Spain.

http://www.wetterzentrale.de/pics/Rtavn3122.png

Only the modification would be Spanish - and French too! Of course,
pedantically, air has no real "source". It just moves from area to
area, but this air mass lingered far longer over North Africa than it
is likely to linger over over Spain.

Paul

There are a few things here that need clarifying:
Spanish plume is a generic term for an elevated mixed layer (EML)
that, in NW Europe, usually has its origins over hot arid surfaces to
the south, often elevated. In the UK the most common source regions
are Iberia and the Sahara. More often than not they remain separate on
their track to the UK during which time they undergo isentropic up-
gliding conserving their potential temperatures. EMLs of Iberian
origin occur a little more frequently across the UK and France than
those from the Sahara (which, interestingly, are far more commonly
found at mid-level above the Caribbean). On many occasions both EMLs
will be present clearly identifiable in soundings from their
respective potential temperatures which will be closely aligned with
source region temperatures, approx 35 to 38C for Iberian and 42 to 46C
for Saharan (dependent on time of year), the EML of Saharan origin
sitting at a higher level than that sourced from Iberia. EMLs occur in
many regions throughout the world and do not necessarily require
elevated arid regions to form, some forming over regions where there
is a large horizontal gradient in soil moisture.
In themselves EMLs are not that exciting, why they are exciting in NW
Europe during the warmer months of the year is on occasions when they
are advected northwards across France they provide a lid allowing the
BL air below this to take up latent and sensible heat elevating WBPTs.
The longer an EML sits over France for instance the higher the BL WBPT
that can realised and the more energy available for convection if the
lid is weakened, due *to increasing baroclinicity ahead of an upper
trough, or if under-running occurs. IOW without the EML you would not
attain the high BL WBPTs seen, this is one way of identifying the
region overlain by an EML.
BTW Ac Cast occurs (normally northern edge) when the EML has been
lifted enough for its upper reaches to attain its LCL – it does not
tell you anything about the airmass and its stability above this
level, it could be capped by a further EML (Saharan overlying Iberian)
or subsidence.

Steve.www.rockbeareweather.co.uk

Sorry for any typos found in the above comments - a quick brain dump
with no proof reading!

Steve.
www.rockbeareweather.co.uk