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ATOC 184 Lecture17_Mar12_2012

ATOC 184 Lecture17_Mar12_2012 - Atmospheric...

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Atmospheric Stability (Chapter 6)
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Stable, neutral, and unstable positions of a ball on a smooth surface
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Stability in the atmosphere So in general the idea of stability essentially revolves around the ability of an object to return to it’s original position after it is disturbed. In the atmosphere this usually refers to the upwards displacement of a parcel of air. If an air parcel that is pushed upwards continues to rise on its own, the atmosphere is considered to be unstable. Whether an air parcel continues to rise on it’s own, is really dependent on whether it is buoyant or not.
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So what does it mean for a parcel to be buoyant The idea of buoyancy was summed up by Archimedes, a Greek mathematician, in what is known as Archimedes Principle: Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
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So what does it mean for an air parcel to be buoyant The amount of lift (or buoyancy ) depends primarily upon the difference between the temperature the air parcel and the temperature of the air in the surrounding environment. The most typical example is that of a hot air balloon.
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So how do we get there? In order to understand how an air parcel can become unstable, we have to understand a couple of basic principles. The first idea is that of adiabatic expansion, or adiabatic cooling. The second idea is that of condensation and latent heat release .
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Adiabatic Expansion Adiabatic Process - a physical change of the state of the air parcel that does not involve exchange of energy with the air surrounding the air parcel. As air parcel rises - expands & cools
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Adiabatic Expansion Rising air experiences a drop in temperature, even though no heat is lost to the outside . If the pressure of the surrounding air is reduced, then the rising air parcel will expand. The molecules of air are doing work as they expand. The energy can either be used to do the work of expansion, or to maintain the temperature of the parcel, but it can't be used for both. If the total amount of heat in a parcel of air is held constant (no heat is added or released), then when the parcel expands, its temperature drops. http://daphne.palomar.edu/jthorngren/adiabatic_processes.htm
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Condensation Condensation is the process whereby water vapour in the atmosphere is returned to a liquid state. Condensation of water vapour occurs when the temperature of air is lowered to its dew point .
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