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Types of Stability

In figure 2-4-4 a bowl is set on a flat surface with a ball placed inside it. The ball rests in the bottom of the bowl; but, if you push the ball in any direction, it seeks out the bottom of the bowl again. This is referred to as ABSOLUTE STABILITY (A in fig. 2-4-4). Turn the bowl upside down, position the ball anywhere on the bowl’s bottom surface (B in fig. 2-4-4) and the ball starts moving on its own without any other force being applied. This is a condition of AB-SOLUTE INSTABILITY. If you now remove the bowl and place the ball on the flat surface (C in fig. 2-4-4), you have NEUTRAL STABILITY— that is, if a force is applied to the ball, it moves; but if the force is removed, the ball stops. Air in the atmosphere reacts in a similar man-ner when moved up or down. If it is moved up and becomes more dense than the surrounding air, it returns to its original position and is considered STABLE. If it becomes less dense than the sur-rounding air, it continues to rise and is considered UNSTABLE. When density remains the same as the surrounding air after being lifted, it is considered NEUTRALLY STABLE, with no tend-ency to rise or sink.

Equilibrium of Dry Air

The method used for determining the equilibrium of air is the parcel method, wherein a parcel of air is lifted and then compared with the surrounding air to determine its equilibrium. The dry adiabatic lapse rate is always used as a reference to determine the stability or instability of dry air (the parcel).

ABSOLUTE INSTABILITY.— Consider a column of air in which the actual lapse rate is


Figure 2-4-4.—Analogy depiction of stability.

greater than the dry adiabatic lapse rate. The actual lapse rate is to the left of the dry adiabatic lapse rate on the Skew-T diagram (fig. 2-4-5). If the parcel of air at point A is displaced upward to point B, it cools at the dry adiabatic lapse rate. Upon arriving at point B, it is warmer than the surrounding air. The parcel therefore has a tendency to continue to rise, seeking air of its own density. Consequently the column becomes unstable. From this, the rule is established that if the lapse rate of a column of air is greater than the dry adiabatic lapse rate, the column is in a state of ABSOLUTE INSTABILITY. The term absolute is used because this applies whether the air is dry or saturated, as is evidenced by displac-ing upward a saturated parcel of air from point A along a saturation adiabat to point B. The parcel is more unstable than if displaced along a dry adiabat.

STABILITY.— Consider a column of dry air in which the actual lapse rate is less than the dry adiabatic lapse rate. The actual lapse rate is to the right of the dry adiabatic lapse rate on the Skew-T diagram (fig. 2-4-6). If the parcel at point A were displaced upward to point B, it would cool at the dry adiabatic lapse rate; and upon arriving at point B, it would be colder than the surrounding air. It would, therefore, have a tendency to re-turn to its original level. Consequently, the column of air becomes stable. From this, the rule is established that if the actual lapse rate of a column of DRY AIR is less than the dry adiabatic lapse rate, the column is stable.

NEUTRAL STABILITY.— Consider a col-umn of DRY AIR in which the actual lapse rate is equal to the dry adiabatic lapse rate. The parcel cools at the dry adiabatic lapse rate if displaced upward. It would at all time be at the same temperature and density as the surrounding air. It also has a tendency neither to return to nor to move farther away from its original position. Therefore, the column of dry air is in a state of NEUTRAL STABILITY.

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