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displaced element will return to its original position.
An unstable dynamic system is one in which a displaced
element will accelerate further from its original
position. In a neutrally stable system, the displaced element
neither returns to nor accelerates from its original
position. In the atmosphere, it is easiest to use a parcel
of air as the displaced element. The behavior of a stable
or unstable system is analogous to aircraft stability discussed
in Chapter 3–Aerodynamics of Flight.
For simplicity, assume first that the air is completely
dry. Effects of moisture in atmospheric stability are
considered later. Aparcel of dry air that is forced to rise
expands due to decreasing pressure and cools in the
process. By contrast, a parcel of dry air that is forced
to descend is compressed due to increasing pressure
and warms. If there is no transfer of heat between the
surrounding, ambient air, and the displaced parcel, the
process is called adiabatic. Assuming adiabatic
motion, a rising parcel cools at a lapse rate of 3°C
(5.4°F) per 1,000 feet, known as the dry adiabatic
lapse rate (DALR). As discussed below, on a thermodynamic
chart, parcels cooling at the DALR are
said to follow a dry adiabat. A parcel warms at the
DALR as it descends. In reality, heat transfer often
occurs. For instance, as a thermal rises, the circulation
in the thermal itself (recall the bubble model) mixes in
surrounding air. Nonetheless, the DALR is a good
approximation.
The DALR represents the lapse rate of the atmosphere
when it is neutrally stable. If the ambient lapse rate in
some layer of air is less than the DALR (for instance,
1°C per 1,000 feet), then that layer is stable. If the lapse
rate is greater than the DALR, it is unstable. An unstable
lapse rate usually only occurs within a few hundred
feet of the heated ground. When an unstable layer
develops aloft, the air quickly mixes and reduces the
lapse rate back to DALR. It is important to note that
the DALR is not the same as the standard atmospheric
lapse rate of 2°C per 1,000 feet. The standard atmosphere
is a stable one.
Another way to understand stability is to imagine two
scenarios, each with a different temperature at 3,000
feet above ground level (AGL), but the same temperature
at the surface, nominally 20°C. In both scenarios,
a parcel of air that started at 20°C at the surface has
cooled to 11°C by the time it has risen to 3,000 feet at
the DALR. In the first scenario, the parcel is still
warmer than the surrounding air, so it is unstable and
the parcel keeps rising—a good thermal day. In the
second scenario, the parcel is cooler than the surrounding
air, so it is stable and will sink. The parcel in
the second scenario would need to be forced to 3,000
feet AGL by a mechanism other than convection,
Figure 9-7. Cross-section through a thermal. Darker green
is stronger lift, red is sink.
9-7
being lifted up a mountainside or a front for instance.
[Figure 9-9]
Figure 9-9 also illustrates factors leading to instability.
A stable atmosphere can turn unstable in one of two
ways. First, if the surface parcel warmed by more than
2°C (greater than 22°C), the layer to 3,000 feet would
then become unstable in the second scenario. Thus, if
the temperature of the air aloft remains the same,
warming the lower layers causes instability and better
thermal soaring. Second, if the air at 3,000 feet is
cooler, as in the first scenario, the layer becomes
unstable. Thus, if the temperature on the ground
remains the same, cooling aloft causes instability and
better thermal soaring. If the temperature aloft and at
the surface warm or cool by the same amount, then the
stability of the layer remains unchanged. Finally, if the
air aloft remains the same, but the surface air-cools
(for instance due to a very shallow front) then the layer
becomes even more stable.
An inversion is a layer in which the temperature warms
as altitude increases. Inversions can occur at any altitude
and vary in strength. In strong inversions, the temperature
can rise as much as 10°C over just a few hundred feet of
altitude gain. The most notable effect of an inversion is to
cap any unstable layer below. Along with trapping haze or
pollution below, they also effectively provide a cap to any
thermal activity.
So far, only completely dry air parcels have been considered.
However, moisture in the form of water vapor
is always present in the atmosphere. As a moist parcel
of air rises, it cools at the DALR until it reaches its dew
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