Chapter 13, Atmospheric Stability 13-1
13 Atmospheric Stability
13.1 Introduction
Convective clouds and precipitation pose a distinctly different flying environment than stratiform clouds
and precipitation. These sharply contrasting conditions result from the atmosphere either resisting or
accelerating the vertical motion of air parcels. Atmospheric stability is the property of the ambient air that
either enhances or suppresses vertical motion of air parcels and determines which type of clouds and
precipitation a pilot will encounter.
Chapter 13, Atmospheric Stability 13-2
13.2 Using a Parcel as a Tool to Evaluate Stability
An air parcel can be used as a tool to evaluate atmospheric stability within a specified vertical column of
air in the atmosphere. A parcel is selected from a specified altitude (usually the surface) and hypothetically
lifted upward to a specified test altitude. As the parcel is lifted, its tem perature decreases due to the
expansion and latent heat effects discussed in Chapter 12, Vertical Motion and Clouds.
The parcel and the surrounding environmental air temperatures are then compared. If the lifted parcel is
colder than the surrounding air, it will be denser (heavier) and sink back to its original level. In this case,
the parcel is stable because it resists upward displacement. If the lifted parcel is the same temperature as
the surrounding air, it will be the same density and remain at the same level. In this case, the parcel is
neutrally stable. If the lifted parcel is warmer and, therefore, less dense (lighter) than the surrounding air, it
will continue to rise on its own until it reaches the same temperature as its environment. This final case is
an example of an unstable parcel. Greater temperature differences result in greater rates of vertical motion.
13.3 Stability Types
The stability of a column of air in the atmosphere is classified by the distribution of parcel stabilities within
the column. The Earth ’s surface is typically selected as the base while the top determines the column ’s
depth. Four unique types of atmospheric stability can be identified and are discussed in the following
sections.
13.3.1 Absolute Stability
Absolute stability (see Figure 13-1) is the state of a column of air in the atmosphere when its lapse rate of
temperature is less than the moist adiabatic lapse rate. This includes both isothermal and inversion
temperature profiles. An ai r parcel lifted upward would be colder (denser) than the surrounding
environmental air and would tend to sink back to its level of origin.
Chapter 13, Atmospheric Stability 13-3
Figure 13-1. Absolute Stability Example
13.3.2 Neutral Stability
Neutral stability (see Figure 13-2) is the state of a column of air in the atmosphere in which an ascending
(or descending) air parcel always has the same temperature (density) as the surrounding environmental air.
If the column of air is unsaturated, then neutral stability exists when its lapse rate of temperature equals the
dry adiabatic lapse rate. If the column of air is saturated, then neutral stability exists when its lapse rate of
temperature equals the moist adiabatic lapse rate.
Chapter 13, Atmospheric Stability 13-4
Figure 13-2. Neutral Stability Example
13.3.3 Absolute Instability
Absolute instability (see Figure 13-3) is the state of a column of air in the atmosphere when it has a
superadiabatic lapse rate of temperature (i.e., greater than the dry adiabatic lapse rate). An air parcel
displaced vertically would be accelerated in the direction of the displacement. The kinetic energy of the
parcel would consequently increase with increasing distance from its level of origin.
Chapter 13, Atmospheric Stability 13-5
Figure 13-3. Absolute Instability Example
13.3.4 Conditional Instability
Conditional instability (see Figure 13-4) is the state of a column of unsaturated air in the atmosphere when
its lapse rate of temperature is less than the dry adiabatic lapse rate but greater than the moist adiabatic
lapse rate. An air parcel lifted upward would be initially stable, but at some point, above its LCL, it would
become unstable. The term “conditional” means the parcel must be lifted to a particular level (altitude)
before it becomes unstable and rises because of its own buoyancy. The Level of Free Convection (LFC) is
the level at which a parcel of air lifted dry adiabatically until saturated and moist adiabatically thereafter
would first become warmer than the surr ounding environmental air (i.e., unstable) in a conditionally
unstable column of air in the atmosphere. The LFC is a defining feature of a conditionally unstable column
of air.
Chapter 13, Atmospheric Stability 13-6
Figure 13-4. Conditional Instability Example
13.3.5 Summary of Stability Types
Figure 13-5 summarizes the possible atmospheric stability types.