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Convection Stability and Instability

In the discussion so far of convection stability and instability, PARCELS of air have been con-sidered. Let us now examine LAYERS of air. A layer of air that is originally stable may become unstable due to moisture distribution if the entire layer is lifted.

Convective stability is the condition that occurs when the equilibrium of a layer of air, because of the temperature and humidity distribu-tion, is such that when the entire layer is lifted, its stability is increased (becomes more stable). Convective instability is the condition of equilibrium of a layer of air occurring when the temperature and humidity distribution is such that when the entire layer of air is lifted, its instability is increased (becomes more unstable).

CONVECTIVE STABILITY. Consider a layer of air whose humidity distribution is dry at the bottom and moist at the top. If the layer of air is lifted, the top and the bottom cool at the same rate until the top reaches saturation. Thereafter, the top cools at a slower rate of speed than the bottom. The top cools saturation adiabatically (.55c/100 meters), while the bottom continues to cool dry adiabatically (1c/100 meters). The lapse rate of the layer then decreases; hence, the stability increases. The layer must be initially unstable and may become stable when lifting takes place.

CONVECTIVE INSTABILITY. Consider a layer of air in which the air at the bottom is moist and the air at the top of the layer is dry. If this layer of air is lifted, the bottom and the top cool dry adiabatically until the lower portion is saturated. The lower part then cools saturation adiabatically while the top of the layer is still cooling dry adiabatically. The lapse rate then begins to increase and instability in-creases. To determine the convective stability or instability of a layer of air, you should first know why you expect the lifting of a whole layer. The obvious answer is an oro-graphic barrier or a frontal surface. Next, determine how much lifting is to be expected and at what level it commences. Lifting of a layer of air close to the surface of the Earth is not necessary. The amount of lifting, of course, depends on the situation at hand. Figure 2-4-10 illustrates the varying degrees of air stability that are directly related to the rate at which the temperature changes with height.

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