On Nov 24, 7:02*pm, "Col" wrote:
Tudor Hughes wrote:
On Nov 24, 1:53 pm, Richard Dixon wrote:
On Nov 24, 1:04 pm, Stephen Davenport wrote:

Highs and lows are only defined relative to surrounding pressure and
are a human construct. There are no absolutes. The notation in this
instance merely marks the point of highest pressure in that locale.
Plus, yes, enthusiastic labelling.

Stephen.

I would guess though that (at least in the N Hemisphere winter) that
lower central pressure "highs" are more likely further north over
Greenland/Iceland/Norway given the low mean surface pressure in this
region? ( e.g.http://badc.nerc.ac.uk/data/ecmwf-era/pics/mslp.gif)

Richard

* * * * * * * The High is not real anyway because MSL pressures over
the Greenland icecap are meaningless. *The place only has "mean sea
level" at the edges and the interior is far too high for any
extrapolation to sea level to be useful. *It's probably best to regard
Greenland as a discontinuity in the MSL pattern and to regard the
isobars as formalities which do not necessarily bear any relation to
the circulation at the land surface. *The same is true of Antarctica,
even more so because of the greater extent and altitude of the
continent.

Are isobars over other extensive highland areas such as the
Tibetan plateau regarded in the same fashion?
--
Col

Bolton, Lancashire
160m asl- Hide quoted text -

- Show quoted text -

In South Africa, AFAIK, the standard method is to use the 850 mb
height (in effect the pressure at about 5000 ft) as the surface
isobars. This wouldn't work for Cape Town or Durban, for example and
the MSL isobars are then used, there being a discontinuity on the
charts. This is not as bad as it might seem since the drop from the
tableland to sea level is fairly abrupt.
As Freddie mentions, all this has an effect on 1000-500 thickness
values and an interesting example is seen in comparing Ojmjakon with
Jakutsk in winter. Both are extremely cold (-45 and -50 routinely)
and lie under spectacular inversions of 20 deg and more. Since
Ojmjakon is so much higher than Jakutsk (740 m vs 100 m) the surface
temperature at Ojmjakon is much lower than that in the free air at the
same height over Jakutsk and this low figure is extrapolated to sea
level thus in effect creating a virtual inversion 700 m deep and at
-50°C. There is simply no way of getting a meaningful 1000-500 mb
thickness from Ojmjakon under these conditions. The high altitude of
the surface simply pushes the whole inversion upwards. In summer
(briefly) the opposite applies, Ojmjakon being a high-level heat
source like the Tibetan plateau and many other areas.

Tudor Hughes, Warlingham, Surrey.