Geoscience Reference
In-Depth Information
velocity air termed jet streams. There are seasonal
variations in wind speeds aloft, with winds being
much greater in the Northern Hemisphere during
winter months, when the meridional tempera-
ture gradients are at a maximum. Such seasonal
variation is less pronounced in the Southern
Hemisphere. In addition, the greater persistence
of these gradients tends to cause the Southern
Hemisphere upper winds to be more constant in
direction. A history of upper air observations is
given in
Box 7.1
.
(
A)
(B)
6
20
L
L
4
10
2
L
H
0
0
WARM
COLD
WARM
WARM
COLD
WARM
(
C)
(D)
6
20
H
H
4
10
2
L
H
0
0
COLD
WARM
COLD
COLD
WARM
COLD
1 The vertical variation of pressure
systems
The air pressure at the surface, or at any level in
the atmosphere, depends on the weight of the
overlying air column. In Chapter 2B, we noted
that air pressure is proportional to air density and
that density varies inversely with air temperature.
Accordingly, increasing the temperature of an air
column between the surface and, say, 3km will
reduce the air density in the column and therefore
lower the air pressure at the surface without
affecting the pressure at 3km altitude (the weight
of the atmospheric column above 3km remains
the same). Correspondingly, if we compare the
heights of the 1000 and 700mb pressure surfaces,
warming of the air column will lower the height
of the 1000mb surface but will not affect the
height of the 700mb surface (i.e., the thickness of
the 1000-700mb layer increases).
The models shown in
Figure 7.1
illustrate the
relationships between surface and tropospheric
pressure conditions. A low pressure cell at sea level
with a cold core will intensify with elevation
(
Figure 7.1A
), whereas one with a warm core will
tend to weaken and may be replaced by high
pressure. A warm air column of relatively low
density causes the pressure surfaces to bulge
upward, and conversely a cold, more dense air
column leads to downward contraction of the
pressure surfaces. Thus, a surface high pressure
cell with a cold core (a
cold anticyclone
), such as
the Siberian winter anticyclone, weakens with
Figure 7.1
Models of the vertical pressure dis-
tribution in cold and warm air columns. A: A surface
low pressure intensifies aloft in a cold air column.
B: A surface high pressure weakens aloft and
may become a low pressure in a cold air column.
C: A surface low pressure weakens aloft and may
become a high pressure in a warm air column. D: A
surface high pressure intensifies aloft in a warm air
column.
increasing elevation and is replaced by low
pressure aloft (
Figure 7.1B
). Cold anticyclones are
shallow and rarely extend their influence above
about 2500m. By contrast, a surface high with a
warm core (a
warm anticyclone
) intensifies with
height (
Figure 7.1D
). This is characteristic of the
large subtropical cells, which maintain their
warmth through dynamic subsidence. The warm
low (
Figure 7.1C
) and cold high (
Figure 7.1B
) are
consistent with the vertical motion schemes
illustrated in
Figure 6.7
, whereas the other two
types are primarily produced by dynamic
processes. The high surface pressure in a warm
anticyclone is linked hydrostatically with cold,
relatively dense air in the lower stratosphere.
Conversely, a cold depression (
Figure 7.1A
) is
associated with a warm lower stratosphere.
Mid-latitude low pressure cells have cold air in
the rear, and hence the axis of low pressure slopes
with height towards colder air to the northwest.
High pressure cells slope towards the warmest
air (
Figure 7.2
). Thus, Northern Hemisphere
subtropical high pressure cells are shifted 10-15
°
