Lecture9 - EAS 1600 Introduction to Environmental Sciences...

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EAS 1600 Introduction to Environmental Sciences Class 9 - Circulation of the Atmosphere In this class we discuss the general circulation of the atmosphere and its role in redistributing the energy that was unevenly deposited on the Earth by the absorption of solar energy. It is sometimes said that the atmosphere is one massive heat engine driven by solar radiation and that weather is simply the manifestation of the heat engine’s work. Today we find out why . .. But first a few definitions: General circulation: The long-term average winds (directions and speed) in the atmosphere as a function of location. By long- term,we usually mean on decadal time scales. Climate: The long-term average state of the atmosphere. Weather: The instantaneous state of the atmosphere and its short-term variability. Sensible heat: The energy contained in molecules as a result of their random kinetic energy (or motion). In other words the energy you can feel as heat. Thermal conduction: The transfer of sensible heat through materials or gases that are in contact as a result of the collision of hot molecules with cool molecules. Note no transfer of mass is required. Thermal convection and advection: The transfer of sensible heat by the movement (or exchange) of mass. Convection usually refers to vertical movement and advection to horizontal movement. Our motivation: Recall: The energy budget of solar and planetary radiation is in balance in total, but not as a function of latitude.
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The imbalance requires energy flow from tropics to poles… One possible way for the atmosphere to accomplish this is through its General Circulation. If winds carry warm air pole-ward and cool air to the tropics (on average), there will be a net flow of energy (as sensible heat) like that illustrated above. Let’s see how. .. Air, like all bodies, are subject to the Laws of Motion. Things at rest will tend to remain at rest. To move an object at rest or change the speed or direction of an object, a force must be applied. F = ma So the movement of air requires a force. In general there are two forces available: 1. Gravity 2. Pressure (Remember! This is actually a force per unit area.) If the sum of the gravitational and pressure forces are not in balance there will be a net force and the air will move . The movement of air is wind . Winds averaged over long-time scales is part of the General Circulation. Horizontal Motion is caused by pressure forces alone. Since gravity acts in the vertical motion, it can not influence horizontal movement For pressure to move air, it must supply a net force, and . .. Pressure provides a net force if there is a change in pressure over some distance – we call this the pressure gradient force. Air tends to moves from
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This note was uploaded on 01/16/2012 for the course EAS 1601 taught by Professor Lynch during the Summer '08 term at Georgia Tech.

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Lecture9 - EAS 1600 Introduction to Environmental Sciences...

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