Geoscience Reference
In-Depth Information
Fig. 2.3
Stable atmosphere in which
the temperature decrease
with elevation (solid line) is
smaller than the dry
adiabatic lapse rate (dashed
line), so that
Γ
<
Γ
d
.
zz
+
δ
AIR
PARTICLE
z
zz
−
δ
Temperature,T
Unstable conditions typically occur whenever the atmosphere is being heated by the
surface below, for example, as a result of solar radiation on days with clear sky, or as
relatively cold air moves over a relatively warmer surface, such as a lake or the ocean
in fall and early winter. An unstable atmosphere is subject to more intense mixing and
turbulence than a neutral one; this also results in larger turbulent transport. Under certain
conditions an unstable atmosphere will even give rise to various types of organized
motions, with scales ranging from mere local updrafts and dust devils to large tropical
storms. The atmosphere is often stable when the air is being cooled from below. This
typically occurs at night under clear skies, when the surface is cooled by outgoing long-
wave radiation or when relatively warm air flows over a relatively colder lake or ocean
surface in spring. In addition, stable conditions, also called inversions, may result from
larger-scale weather patterns, when relatively warmer air masses move over colder layers.
Not surprisingly, stable conditions have the opposite effect of unstable conditions. Thus
mixing and turbulence are suppressed, and atmospheric transport is normally smaller.
Under extreme conditions, the turbulence may be eliminated altogether and the flow
of the air may be laminar. Such conditions are sometimes visible in the evening of a
calm sunny day, around sunset, when the air near the ground becomes chilly and the
smoke from a chimney can be seen moving slowly through the tree tops of a forest. On
a somewhat larger scale, as a result of the reduced turbulence and dispersion, inversion
conditions also tend to aggravate pollution problems in populated areas.
Potential temperature
To repeat briefly, during small displacements, parcels of air undergo adiabatic tempera-
ture changes in accordance with Equation (2.22). In a perfectly neutral atmosphere the
lapse rate of the atmosphere is also
c
p
; therefore under such conditions a displaced
parcel will, on average at least, always be surrounded by air at the same temperature and,
as a result, there is no net exchange of heat. This means that even though there is a verti-
cal temperature difference under neutral conditions, the heat flux is zero; consequently,
−
g
/
