Introduction to Glider Flying > Soaring Weather > The Atmosphere > Atmospheric
Stability in the atmosphere tends to hinder
vertical motion, while instability tends to promote vertical
motion. A certain amount of instability is desirable for glider
pilots, since without it, thermals would not develop. If the
air is moist enough, and the atmospheric instability deep enough,
thunderstorms and associated hazards can form. Thus, an understanding
of atmos-pheric stability and its determination from available
weather data is important for soaring flight and safety. As
a note, the following discussion is concerned with vertical
stability of the atmosphere. Other horizontal atmospheric instabilities,
for instance, in the evolution of large-scale cyclones, are
not covered here.
Generally, a stable dynamic system is one in
which a displaced element will return to its original position.
An unstable dynamic system is one in which a dis-placed element
will accelerate further from its original position. In a neutrally
stable system, the displaced ele-ment 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 dis-cussed in Chapter 3–Aerodynamics
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 ther-modynamic 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 unsta-ble 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 atmos-phere
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 tempera-ture 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 sur-rounding
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,being lifted up a mountainside or a front
for instance. [Figure 9-9]
Figure 9-8. Life cycle of a typical
thermal with cumulus cloud.
Figure 9-9. Stable and unstable parcels
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
warmsas 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 con-sidered. 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 point,
at which time the air in the parcel begins to con-dense. During
the process of condensation, heat (referred to as latent heat)
is released to the surround-ing air. Once saturated, the parcel
continues to cool, but since heat is now added, it cools at
a rate slower than the DALR. The rate at which saturated air-cools
with height is known as the saturated adiabatic lapse rate (SALR).
Unlike the DALR, the SALR varies substan-tially with altitude.
At lower altitudes, it is on the orderof 1.2°C per 1,000
feet, whereas in mid levels it increases to 2.2°C per 1,000
feet. Very high up, above about 30,000 feet, little water vapor
exists to condense, and the SALR approaches the DALR.