Geoscience Reference
In-Depth Information
illustrated in Figures 3.29 and 3.30, show where these
effects are important.
Pacific (see Figure 7.9). Over middle latitudes of the
southern hemisphere, the circumpolar ocean gives rise
to a continuous zone of mP air that, in summer, extends
to the margin of Antarctica. In this season, however, a
considerable gradient of ocean temperatures associated
with the Antarctic convergence makes the zone far from
uniform in its physical properties. Bright periods and
squally showers, with a variable cloud cover of cumulus
and cumulonimbus, typify the weather in cP airstreams.
As mP air moves eastward towards Europe, the cooler
sea surface may produce a neutral or even stable
stratification near the surface, especially in summer, but
subsequent heating over land will again regenerate
unstable conditions. Similar conditions, but with lower
temperatures, arise when cA air crosses high-latitude
oceans, producing maritime Arctic (mA) air.
When cP air moves southward in winter, over central
North America, for example, it becomes more unstable,
but there is little gain in moisture content. The cloud
type is scattered shallow cumulus, which gives showers
only rarely. Exceptions occur in early winter around
the eastern and southern shores of Hudson Bay and the
Great Lakes. Until these water bodies freeze over,
cold airstreams that cross them are warmed rapidly
and supplied with moisture, leading to locally heavy
snowfall (p. 233). Over Eurasia and North America, cP
air may move southward and later recurve northward.
Some schemes of airmass classification cater for such
possibilities by specifying whether the air is colder (k),
or warmer (w), than the surface over which it is passing.
In some parts of the world, the surface conditions
and air circulation produce airmasses with intermediate
characteristics. Northern Asia and northern Canada fall
into this category in summer. In a general sense, the air
has affinities with continental polar airmasses but these
land areas have extensive bog and water surfaces, so the
air is moist and cloud amounts quite high. In a similar
manner, melt-water pools and openings in the Arctic
pack-ice make the area a source of maritime Arctic
(mA) air in summer (see Figure 9.4A). This designation
is also applied to air over the Antarctic pack-ice in
winter that is much less cold in its lower levels than the
air over the continent itself.
b Dynamic changes
Dynamic (or mechanical) changes are superficially
different from thermodynamic changes because they
involve mixing or pressure changes associated with the
actual movement of the airmass. The physical properties
of airmasses are considerably modified, for example, by
a prolonged period of turbulent mixing (see Figure 5.7).
This process is particularly important at low levels,
where surface friction intensifies natural turbulence,
providing a ready mechanism for the upward transfer
of heat and moisture.
The radiative and advective exchanges discussed
earlier are
diabatic
, but the ascent or descent of air
causes adiabatic changes of temperature. Large-scale
lifting may result from forced ascent by a mountain
barrier or from airstream convergence. Conversely,
sinking may occur when high-level convergence sets up
subsidence or when stable air, that has been forced
up over high ground by the pressure gradient, descends
in its lee. Dynamic processes in the middle and upper
troposphere are in fact a major cause of airmass modi-
fication. The decrease in stability aloft, as air moves
away from areas of subsidence, is a common example
of this type of mechanism.
2 The results of modification: secondary
airmasses
Study of the ways in which airmasses change in
character tells us a great deal about many common
meteorological phenomena.
a Cold air
Continental polar air frequently streams out from
Canada over the western Atlantic in winter, where it
undergoes rapid transformation. Heating over the Gulf
Stream drift makes the lower layers unstable, and
evaporation into the air leads to sharp increases of
moisture content (see Figure 4.6) and cloud formation
(see Plate 9). The turbulence associated with the
convective instability is marked by gusty conditions.
When the air reaches the central Atlantic, it becomes a
cool, moist, maritime polar (mP) airmass. Analogous
processes occur with outflow from Asia over the north
b Warm air
The modification of warm airmasses is usually a gradual
process. Air moving poleward over progressively cooler
surfaces becomes increasingly stable in the lower layers.
