Types of Stability

Types of Stability In figure 2-9 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. 2-9). Turn the bowl upside down, position the ball anywhere on the bowl’s bottom surface (B in fig. 2-9) and the ball starts moving on its own without any other force being applied. This is a condition of ABSOLUTE INSTABILITY. If you now remove the bowl and place the ball on the flat surface (C in fig. 2-9), 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 manner when moved up or down. If it is moved up and becomes denser than the surrounding air, it returns to its original position and is considered STABLE. If it becomes less dense than the surrounding 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 tendency 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 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. 2-10). 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 2-16 AG5f0209 BALL IN BOWL FORCE MOVES BALL DISPLACED BALL OSCILLATES (A) ABSOLUTE STABILITY (C) NEUTRAL STABILITY (B) ABSOLUTE INSTABILITY BALL BALANCED ON BOWL RELEASE OF FORCE PERMITS BALL TO MOVE BALL CONTINUES TO MOVE BALL WILL CONTINUE TO MOVE BALL RESTING ON TABLE FORCE MOVES BALL FORCE REMOVED, BALL STOPS BALL REMAINS IN NEW POSITION BALL EVENTUALLY RETURNS TO ORIGINAL POSITION Figure 2-9.—Analogy depiction of stability.