Geoscience Reference
In-Depth Information
Figure 9.6
Airmass source regions. (A) Northern hemisphere. (B) Southern hemisphere. Numbers show the areas affected by each
airmass in months per year.
Sources
: After Wendland and Bryson (1981), and Wendland and McDonald (1986), by permission of the American Meteorological Society.
1 Mechanisms of modification
the airstream climatology is much simpler (Figure
9.6B). Source areas are the oceanic subtropical anti-
cyclones. Antarctica is the major continental source,
with another mainly in winter over Australia.
The mechanisms by which airmasses are modified are,
for convenience, treated separately, although in practice
they may operate together.
C AIRMASS MODIFICATION
a Thermodynamic changes
As an airmass moves away from its source region it is
affected by different heat and moisture exchanges with
the ground surface and by dynamic processes in the
atmosphere. Thus a barotropic airmass is gradually
changed into a moderately
baroclinic
airstream in which
isosteric and isobaric surfaces intersect one another.
The presence of horizontal temperature gradients means
that air cannot travel as a solid block maintaining an
unchanging internal structure. The trajectory (i.e. actual
path) followed by an air parcel in the middle or upper
troposphere will normally be quite different from that
of a parcel near the surface, due to the increase of
westerly wind velocity with height in the troposphere.
The structure of an airstream at a given instant is largely
determined by the past history of airmass modification
processes. In spite of these qualifications, the airmass
concept retains practical value and is now used in air
chemistry research.
An airmass may be heated from below either by passing
from a cold to a warm surface or by solar heating of the
ground over which the air is located. Similarly, but in
reverse, air can be cooled from below. Heating from
below acts to increase airmass instability, so the effect
may be spread rapidly through a considerable thickness
of air, whereas surface cooling produces a temperature
inversion, which limits the vertical extent of the cooling.
Thus, cooling tends to occur gradually through radiative
heat loss by the air.
Changes may also occur through increased evap-
oration, the moisture being supplied either from the
underlying surface or by precipitation from an overlying
airmass layer. In reverse, the abstraction of moisture by
condensation or precipitation can also cause changes.
An associated, and most important, change is the
respective addition or loss of latent heat accompanying
this condensation or evaporation. Annual values of
latent and sensible heat transfers to the atmosphere,
