"Martin Rowley" wrote in message
...
Will Hand wrote:
snip-lots
I said in another thread that I thought that the 564 DAM line was just
a bit further north than it should be. snip
but the persistent closeness of that warm and moist 564 DAM air is
bothering me

"Keith(Southend)" wrote ...


Admittedly, I can't say where the 564 DAM line normally is this time of
year, but looking at the charts I don't see it being particularly far
north.
http://www.wzkarten.de/pics/brack0a.gif
http://www.wzkarten.de/pics/brack1a.gif
http://www.wzkarten.de/pics/brack3.gif
and in the middle of the Atlantic it seem quite far south. Certainly it's
close to us on the near Continent, but am I missing something here?

... I've posted the mean 500-1000 hPa thickness (ex. NOAA/ESRL) he-

http://freespace.virgin.net/martin.r...HK_JULmean.gif

I've also looked at the last 8 days of analysis using the wetter3.de
archive. You can see from the mean chart for July that there is a thermal
trough with an axis roughly along 20degW at latitude ~ 40/45degN, which I
assume is the area that we are talking about?

For the last 8 days, the mean value of thickness at a point 41N, 20W
(where the mean for 1968-1996 is given as 564 dam), was 565dam: +1dam over
the long-term mean. This doesn't seem to me to show anything excessively
warm. Of the last 8 days, 5 had 00Z values at or below 564dam, with a
temporary peak (passing wave) of 573 dam.

I also looked at a couple of other warm Julys (warm in BI that is: July
1995 is one example), and the mean 564 dam contour was comfortably further
north than this mean - typically lying from Nova Scotia to Greater Paris,
with a dip in the 15-25degW area. Given that these are *mean* values, the
day-to-day position must have been even further north than that for
extended periods.

I can't see from any data that I have available that there is anything
particularly unusual about the latitudinal position of the 564dam contour.

Martin.


Thanks ever so much for that Martin. It is sort of re-assuring I guess,
although 1 DAM over the long term mean may not sound a lot it takes on a
significance if we are looking for signs of warming. Take your point about
1995 et al though. I suppose if we see the mean over southern England
creeping up then that would be more definite evidence as historical mean
thickness values over the ocean will be calculated on limited sonde data
with satellite retrievals in their infancy and subject to large errors due
cloud contamination etc.

Will
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