Geoscience Reference
In-Depth Information
downward it will warm at the dry adiabatic rate; the
parcel will always be warmer and less dense than the
surrounding air, and tend to return to its former position
(unless prevented from doing so). However, if local
surface heating causes the environmental lapse rate
near the surface to exceed the dry adiabatic lapse
rate (b), then the adiabatic cooling of a convective air
parcel allows it to remain warmer and less dense than
the surrounding air, so it continues to rise through buoy-
ancy. The characteristic of unstable air is a tendency
to continue moving away from its original level when
set in motion. The transition between the stable and
unstable states is termed
neutral
.
We can summarize the five basic states of static
stability which determine the ability of air at rest to
remain laminar or become turbulent through buoyancy.
The key is the temperature of a displaced air parcel
relative to that in the surrounding air.
•
Conditionally unstable: SALR < ELR < DALR
•
Dry neutral: ELR = DALR
•
Absolutely unstable: ELR > DALR
Air that is colder than its surroundings tends to sink.
Cooling in the atmosphere usually results from radiative
processes, but subsidence also results from horizontal
convergence of upper tropospheric air (see Chapter
6B.2). Subsiding air has a typical vertical velocity of
only 1-10 cm s
-1
, unless convective downdraft con-
ditions prevail (see below). Subsidence can produce
substantial changes in the atmosphere; for instance, if
a typical airmass sinks about 300 m, all average-size
cloud droplets will usually be evaporated through the
adiabatic warming.
Figure 5.5 illustrates a common situation where
the air is stable in the lower layers. If the air is forced
upward by a mountain range, or through local surface
heating, the path curve may eventually cross to the right
of the environment curve (the level of free convection).
The air, now warmer than its surroundings, is buoyant
•
Absolutely stable: ELR < SALR
•
Saturated neutral: ELR = SALR
Figure 5.5
Schematic tephigram
illustrating the conditions associated
with the conditional instability of
an airmass that is forced to rise. The
saturation mixing ratio is a broken
line and the lifting condensation level
(cloud base) is below the level of free
convection.
