Figure 2-21 Unstable

AVIATION WEATHER

CHAPTER TWO

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

2-21