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Buoyancy and vertical motion in a fluid



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Buoyancy and vertical motion in a fluid In the atmosphere local density variation is usually a result of differential heating Hot less dense and more buoyant The key is to compare an object s density to that of its surrounding environment If buoyancy is stronger than weight the object will float If weight is greater the object will sink What is a parcel How is buoyancy defined A parcel of air is like a bubble of air that rises upward from near the surface through the atmosphere Much of atmospheric convection can be understood by analyzing the behavior of a rising parcel Buoyancy is the vertical force caused by density difference between parcel and surrounding environment that causes it to rise F m a Newton s 2nd law abuoyancy environment environment parcel Tp Te Te 3 different lapse rates to keep track of Environmental Lapse Rate 6 5 C km Caused by radiation balances between surface and troposphere and atmospheric conditions Dramatic variance from day to day results in much of our weather The reason we launch weather balloons Dry Adiabatic Lapse Rate 9 8 C km Combination of 1st law of Thermodynamics ideal gas law and pressure change with height As air expands molecules are spread apart and slow down conservation of energy Rising Air Cools Moist Adiabatic Lapse Rate 6 C km Condensation of cloud drops releases latent heat reducing cooling Value changes slightly w height and T because of variable condensation rate Positive feedback for instability Rising clouds cool but not as much 3 different types of stability to remember Absolutely Stable env 6 C km When lapse rate of the environment is low causing air to cool with height more slowly than any kind of parcel As a parcel rises it will become colder faster than the environment and quickly become more dense than surroundings causing it to sink Cumulus clouds can t form spontaneously when absolutely stable conditions exist Absolutely Unstable env 9 8 C km When lapse rate of the environment is high rising air cools but



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