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or subdaily 'snapshot' of the principal pressure
systems over a very large area, ignoring local
circulations. The subpolar lows over Iceland and
the Aleutians (see
Figure 7.9
) shown on mean
monthly pressure maps are reflected in synoptic
maps in the passage of deep depressions across
these areas downstream of the upper longwave
troughs. The mean high pressure areas, however,
represent more or less permanent highs. The
intermediate zones located about 50-55°N and
40-60
45°
S. Hemisphere
N. Hemisphere
40°
35°
30°
26°
15°
20° 25° 30°
Meridional temperature difference (°C) in the
300-700mb layer (previous month)
35°
40°
45°
S are affected by traveling depressions and
ridges of high pressure; they appear on the mean
maps as being of neither markedly high nor
markedly low pressure. The movement of
depressions is considered in Chapter 9F.
On comparing the mean surface and tropos-
pheric pressure distributions for January (see
Figures 7.4, 7.5, 7.9
and
7.10
), it is apparent that
only the subtropical high pressure cells extend to
high levels. The reasons for this are evident from
Figures 7.1B
and D. In summer, the equatorial low
pressure belt is also present aloft over South Asia.
The subtropical cells are still discernible at 300mb,
showing them to be a fundamental feature of
the global circulation and not merely a response
to surface conditions.
°
Figure 7.11
A plot of the meridional temperature
difference at the 300-700mb level in the previous
month against the latitude of the center of the sub-
tropical high pressure belt, assuming a constant
vertical tropospheric lapse rate.
Source: After Flohn, in Proceedings of the World Climate
Conference, WMO N0.537 (1979, p. 257, Fig.2).
Tibetan massif and the Himalayas, and in part
due to the presence of low-level cold air pools
associated with the extensive snow cover. There
are genesis centers over northeastern Russia
extending eastward towards Chukotka, over
Kazakhstan, and eastern China. Over the high-
elevation Greenland and Antarctic ice sheets, it
is meaningless to speak of sea-level pressure
(adjustment of surface pressure to sea level is
fraught with difficulty) but, on average, there
is high pressure over the 3-4km-high eastern
Antarctic plateau.
Building on earlier discussion, the mean
circulation in the Southern Hemisphere is much
more zonal at both 700mb and sea level than in the
Northern Hemisphere, due to the more limited
area and effect of land masses. There is also
little difference between summer and winter
circulation intensity (see
Figures 7.3
and
7.10
). It
is important at this point to differentiate between
mean pressure patterns and the highs and lows
shown on daily or subdaily synoptic weather
maps. In the Southern Hemisphere, the zonality
of the mean circulation conceals a high degree of
day-to-day variability. The
synoptic map
is a daily
B THE GLOBAL WIND BELTS
The importance of the subtropical high pressure
cells is evident from the preceding discussion.
Dynamic, rather than immediately thermal in
origin, and situated between 20
latitude,
they seem to provide the key to the world's major
wind belts, shown by the maps in
Figure 7.12
. In
the Northern Hemisphere, the pressure gradients
surrounding these cells are strongest between
October and April. In terms of actual pressure,
however, oceanic cells experience their highest
pressure in summer, the belt being counter-
balanced at low levels by thermal low pressure
conditions over the continents. Their strength and
persistence clearly mark them as the dominating
factor controlling the position and activities of
both the trades and the westerlies.
°
and 30
°
