Geoscience Reference
In-Depth Information
In the case of mT air with high moisture content, surface
cooling may produce advection fog, which is partic-
ularly common, for example, in the southwestern
approaches to the English Channel during spring and
early summer, when the sea is still cool. Similar devel-
opment of advection fog in mT air occurs along the
South China coast in February to April, and also off
Newfoundland and over the coast of northern California
in spring and summer. If the wind velocity is sufficient
for vertical mixing, low-stratus cloud forms in the place
of fog, and drizzle may result. In addition, forced ascent
of the air by high ground, or by overriding of an adjacent
airmass, can produce heavy rainfall.
The cT air originating in those parts of the sub-
tropical anticyclones situated over the arid subtropics
in summer is extremely hot and dry. It is typically
unstable at low levels and dust storms may occur, but the
dryness and the subsidence of the upper air limit cloud
development. In the case of North Africa, cT air may
move out over the Mediterranean, rapidly acquiring
moisture, with the consequent release of potential
instability triggering off showers and thunderstorm
activity.
Airmasses in low latitudes present considerable
problems of interpretation. The temperature contrasts
found in middle and high latitudes are virtually absent,
and what differences 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 temperatures. 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.
3 The age of the airmass
Eventually, the mixing and modification that accom-
panies the movement of an airmass 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'
airmasses in Figures 9.2 and 9.4. This is intended to refer
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 airmasses that stagnate over it. Such air is termed
mediterranean
. In winter, it is convectively unstable
(see Figure 5.6) as a result of the moisture picked up
over the Mediterranean Sea.
The length of time during which an airmass 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 airmass 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 airmasses. Observations of the tem-
perature, wind direction, humidity and other physical
phenomena during unsettled periods showed that dis-
continuities often persist between impinging airmasses
of differing characteristics. The term 'front' for these
surfaces of airmass 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.
