Geoscience Reference
In-Depth Information
Fig. 2.2
Unstable atmosphere in
which the temperature
decrease with elevation
(solid line) is larger than
the dry adiabatic lapse
rate (dashed line), so that
Γ
>
Γ
d
.
zz
+
δ
z
AIR
PARTICLE
zz
−
δ
Temperature
,
T
Equation (2.21) this yields for the temperature change of the parcel, as it moves up or
down,
dT
1
/
dz
=−
g
/
c
p
(2.22)
which is of the order of 9.8 K km
−
1
. The vertical rate of decrease in temperature of the
atmosphere,
−
dT
/
dz
, is the lapse rate of the air, denoted here by
. A lapse rate of the
atmosphere, that is equal to
g
/
c
p
, is called a dry adiabatic lapse rate,
d
.
Whenever the actual lapse rate in the atmosphere
is larger than
d
, a particle
undergoing a small upward displacement
z
and changing its temperature according
to Equation (2.22), will be warmer and therefore lighter than its surroundings; this
means that it will have a tendency to continue its upward motion (see Figure 2.2). By
the same token, a particle undergoing a small downward displacement
δ
z
under the
same lapse rate conditions, will be colder and thus heavier than the surrounding air;
again, it will have a tendency to continue its downward movement. In both situations,
once displaced however slightly, air parcels tend to continue their motion and amplify
their displacements both upward and downward: under such lapse rate conditions the
atmosphere is unstable. Conversely, in an atmosphere with
δ
<
d
, the upward-moving
parcel, whose temperature change is given by Equation (2.22), becomes surrounded by
air that is relatively warmer (see Figure 2.3); thus it is heavier and it will tend to return to
its original position, where it was in equilibrium with its surroundings. In this case, the
parcel resists being moved away from its original position and vertical displacements are
suppressed: the atmosphere is said to be stable. Under conditions when the actual lapse
rate in the air is dry adiabatic, the atmospheric stability is neutral. In summary, one has
for unsaturated air the following criteria:
>
d
unstable
=
d
neutral
<
d
stable
