Geoscience Reference
In-Depth Information
over the Gulf Stream Drift makes the lower
layers unstable, and evaporation into the air
leads to sharp increases of moisture content (see
Figure 3.7
) and cloud formation. 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) air mass. Analogous
processes occur with outflow from Asia over the
North Pacific (see
Figure 7.2
). Over mid-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 oceanic 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 cumu-
lonimbus, 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, pro-
ducing 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 only rarely gives
showers. 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 snowfalls (p. 292). Over
Eurasia and North America, cP air may move
southward and later recurve northward. Some
schemes of air-mass 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 air masses
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 air masses but these land
areas have extensive bog and water surfaces, so the
air is moist and cloud amounts are quite high. In
a similar manner, melt-water ponds and openings
in the arctic pack ice make the area a source of 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.
Warm air
The modification of warm air masses is usually
a gradual process. Air moving poleward over
progressively cooler surfaces becomes increasingly
stable in the lower layers. In the case of mT air
with high moisture content, surface cooling
may produce advection fog, which is particularly
common, for example, in the southwestern
approaches to the English Channel during spring
and early summer, when the sea is still cool.
Similar development 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 air mass, can produce
heavy rainfall.
The cT air originating in those parts of the
subtropical 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.
