If a ball on a flat table is displaced, it will tend to remain in its new position and is said to be
neutrally stable (Figure 2-20). It will not have a tendency to return to its original position or
move away from its final position.
EQUILIBRIUM ENCOUNTERED AT ANY
POINT OF DISPLACEMENT
Figure 2-20 Neutrally Stable
Now, consider an inverted bowl with a ball balanced on top. Once the ball is displaced, it will
tend to move away from its original position, never to return, and the ball is said to be in an
unstable condition (Figure 2-21).
Figure 2-21 Unstable
In weather, parts or parcels of an air mass become displaced through one of four lifting methods.
The temperature of the surrounding air determines the stability of a quantity of air after it is
lifted. Lifted air that is colder than the surrounding air settles when the lifting action is removed,
since it is denser. This indicates a stable condition. Lifted air that is warmer than the
surrounding air continues to rise when the lifting action is removed because it is less dense. This
indicates an unstable condition. This lifted air that continues to rise has reached the point of free
convection, which occurs when the lifted air rises with no external lifting force, due only to the
parcel's warmer temperature. Lifted air having the same temperature as the surrounding air after
it is lifted will simply remain at the point where the lifting action was removed. This is an
example of a neutrally stable atmosphere. If the air behaves in one of these three ways, then we
can say the atmosphere has the same condition of stability (Figure 2-22).
Atmospheric Mechanics of Winds, Clouds and Moisture, and Atmospheric Stability