Geoscience Reference
In-Depth Information
Experimentation with a tephigram shows that
both the convective and the lifting condensation
levels rise as the surface temperature increases,
with little change of dew-point. This is commonly
observed in the early afternoon, when the base of
cumulus clouds tends to be at higher levels.
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 1-10cm s
-1
,
unless convective downdraft conditions prevail
(see below). Subsidence can produce substantial
changes in the atmosphere; for instance, if a
typical air mass sinks about 300m, 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 and free
to rise. This is termed
conditional instability
; the
development of instability is dependent on the air
mass becoming saturated. Since the environ-
mental lapse rate is frequently between the dry and
saturated adiabatic rates, a state of conditional
instability is common. The path curve intersects
the environment curve at 650mb. Above this level
the atmosphere is stable, but the buoyant energy
gained by the rising parcel enables it to move some
distance into this region. The theoretical upper
limit of cloud development can be estimated from
the tephigram by determining an area (B) above
the intersection of the environment and path
curves equal to that between the two curves from
the level of free convection to the intersection (A)
in
Figure 5.5
. The tephigram is so constructed
that equal areas represent equal energy.
These examples assume that a small air parcel
is being displaced without any compensating air
motion or mixing of the parcel with its surround-
ings. These assumptions are rather unrealistic.
Dilution of an ascending air parcel by mixing of
C AIR STABILITY AND
INSTABILITY
If stable (unstable) air is forced up or down it has
a tendency to return to (continue to move away
from) its former position once the motivating
force ceases.
Figure 5.3
shows the reason for
this important characteristic. The environment
temperature curve (A) lies to the right of any
path
curve
representing the lapse rate of an unsaturated
air parcel cooling dry adiabatically when forced to
rise. At any level, the rising parcel is cooler and
more dense than its surroundings and therefore
tends to revert to its former level. Similarly, if air
is forced 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 buoyancy. The characteristic of unstable
air is a tendency to continue to move 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.
Absolutely stable: ELR < SALR
Saturated neutral: ELR = SALR
