In the remainder of this session we will be evaluating the stability of a layer of air in the atmosphere. First, we will look at the criteria of stability in reference to a single isolated parcel being lifted through a layer of the atmosphere. Later, we will evaluate the atmosphere in reference to an entire layer being lifted. To simplify matters, we will assume that the environmental air, or the air surrounding the rising air, is in hydrostatic equilibrium. Lets take a few minutes to briefly review the basics of an atmosphere that is in hydrostatic equilibrium.

You were introduced to the hydrostatic equation in Session 4's discussion on Hydrostatic Equilibrium. It expresses the relationship between the pressure gradient force in the vertical, which forces air molecules upward, and gravity, which forces them downward. By this equation the pressure gradient force and gravity are equal to each other, and there is no motion either upward or downward, that is, if there are no other forces acting on the air molecules. So, why do we need to assume the environment is in hydrostatic equilibrium for our discussions on stability? Well, by assuming a hydrostatic environment, we can greatly simplify the motion in the atmosphere in order to see the basic responses of the parcel air and the environmental air while a parcel or a layer is being lifted. On the scales that we are discussing, the meso- and microscales, the atmosphere probably is not in hydrostatic equilibrium. There very well could be other forces acting on the environmental air surrounding a rising parcel. Friction and surface heating are two factors which could cause vertical motion in the environmental air. When the environment is not in hydrostatic equilibrium, issues can get very complicated very quickly because you must take into account these forces and responses when determining stability. We just want to understand the basics here, so we will save non-hydrostatic complications for another discussion.

On to Part 4: Stable, Neutral, and Unstable Atmospheres

Back to A Review of Adiabatic Processes

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