Geoscience Reference
In-Depth Information
Air masses in low latitudes present consider-
able problems of interpretation. The temperature
contrasts found in middle and high latitudes are
virtually absent, and the differences that do exist
are due principally to moisture content and to
the presence or absence of subsidence.
Equatorial
air
is usually cooler than that subsiding in the
subtropical anticyclones, for example. On the
equatorward sides of the subtropical anticyclones
in summer, the air moves westward from areas
with cool sea surfaces (e.g., off northwest Africa
and California) towards higher sea-surface tem-
peratures. Moreover, the southwestern parts of
the high pressure cells are affected only by weak
subsidence due to the vertical structure of the
cells. As a result, the mT air moving westward on
the equatorward sides of the subtropical highs
becomes much less stable than that on their
northeastern margin. Eventually, such air forms
the very warm, moist, unstable 'equatorial air' of
the Intertropical Convergence Zone (see
Figures
9.2
and
9.4
).
Monsoon air
is indicated separately
in these figures, although there is no basic
difference between it and mT air. Modern
approaches to tropical climatology are discussed
in Chapter 11.
stagnate over it. Such air is termed
Mediterranean
.
In winter, it is convectively unstable (see
Figure
4.6
) as a result of the moisture picked up over the
Mediterranean Sea.
The length of time during which an air mass
retains its original characteristics depends very
much on the extent of the source area and the type
of pressure pattern affecting the area. In general,
the lower air is changed much more rapidly
than that at higher levels, although dynamic
modifications aloft are no less significant in terms
of weather processes. Modern air-mass concepts
must therefore be flexible from the point of view
of both synoptic and climatological studies.
D
FRONTOGENESIS
The first real advance in our understanding of
mid-latitude weather variations was made with
the discovery that many of the day-to-day changes
are associated with the formation and movement
of boundaries, or
fronts
, between different
air masses. Observations of the temperature,
wind direction, humidity and other physical
phenomena during unsettled periods showed that
discontinuities often persist between impinging
air masses of differing characteristics. The term
'front' for these surfaces of air-mass conflict was
a logical one, proposed during the First World
War by a group of meteorologists led by Vilhelm
Bjerknes working in Norway (see
Box 9.1
). Their
ideas are still an integral part of weather analysis
and forecasting in middle and high latitudes.
3 The age of the air mass
Eventually, the mixing and modification that
accompanies the movement of an air mass away
from its source causes the rate of energy exchange
with its surroundings to diminish, and the various
associated weather phenomena tend to dissipate.
This process leads to the loss of its original identity
until, finally, its features merge with those of
surrounding airstreams and the air may come
under the influence of a new source region.
Northwest Europe is shown as an area of
'mixed' air masses in
Figures 9.2
and
9.4
. This
refers to the variety of sources and directions
from which air may invade the region. The same
is also true of the Mediterranean Sea in winter,
although the area does impart its own particular
characteristics to polar and other air masses that
1 Frontal waves
The typical geometry of an air-mass interface, or
front, resembles a wave form (
Figure 9.7
). Similar
wave patterns are, in fact, found to occur on the
interfaces between many different media, for
example, waves on the sea surface, ripples on
beach sand, eolian sand dunes and so on. Unlike
these wave-forms, however, the frontal waves in
the atmosphere are usually unstable; that is, they
suddenly originate, increase in size, and then
