Geoscience Reference
In-Depth Information
exchanges with the ground surface and by
dynamic processes in the atmosphere. Thus, a
barotropic air mass 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 and
changes in the wind direction aloft. The structure
of an airstream at a given instant is largely deter-
mined by the past history of air-mass modification
processes. In spite of these qualifications, the air-
mass concept retains practical value and is now
used in air chemistry research.
addition or loss of latent heat accompanying this
condensation or evaporation. Annual values of
latent and sensible heat transfers to the atmos-
phere, illustrated in
Figures 3.30
and
3.31
, show
where these effects are important.
Dynamic changes
Dynamic (or mechanical) changes are super-
ficially different from thermodynamic ones
because they involve mixing or pressure changes
associated with the actual movement of the air
mass. The physical properties of air masses
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 air-mass modification. The decrease
in stability aloft, as air moves away from areas of
subsidence, is a common example of this type
of mechanism.
1 Mechanisms of modification
The mechanisms by which air masses are modified
are, for convenience, treated separately, although
in practice they may operate together.
Thermodynamic changes
An air mass 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
air-mass 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 can also occur through increased
evaporation, the moisture being supplied either
from the underlying surface or by precipitation
from an overlying air-mass layer. In reverse,
the abstraction of moisture by condensation or
precipitation can also cause changes. An associ-
ated, and most important, change is the respective
2 The results of modification:
secondary air masses
Study of the ways in which air masses change in
character tells us a great deal about many common
meteorological phenomena.
Cold air
Continental polar air frequently streams out from
Canada over the western North Atlantic in winter,
where it undergoes rapid transformation. Heating
