Chapter 12— THE GOVERNING FUNDAMENTALS OF METEOROLOGY 



inertia, he stated that every body continues in 

 its state of rest and uniform motion in a straight 

 line unless it is compelled to change by applied 

 forces. Although the atmosphere is a mixture 

 of gases and has the properties peculiar to 

 gases, it still behaves in many respects as a 

 body when considered in the terms of Newton's 

 law. There would be no movement of great 

 quantities of air unless there was a force to 

 cause it. For instance, air moves from one 

 area to another because there is a force (or 

 forces) great enough to change its direction or 

 to overcome its tendency to remain at rest. 



The second of Newton's laws of motion, 

 force and acceleration, states that change of 

 motion of a body is proportional to the applied 

 force and takes place in the direction of the 

 straight line in which that force is applied. In 

 respect to the atmosphere, this means that the 

 change of the motion of the atmosphere is 

 determined by the force acting upon it, and 

 takes place in the direction of that force. 



Newton's third law of motion, reacting forces, 

 states that to every action there is always an 

 equal and opposite reaction, or the mutual 

 actions of two bodies are always equal and 

 oppositely directed. Consequently, there is never 

 a force acting in nature unless there are two 

 bodies, one impressing, or exerting, the force 

 and the other being impressed by force. Still 

 another aspect of the law is that a force cannot 

 exist by itself; it must exist along with another 

 force. It is clear, then, that there must be at 

 least two bodies and two forces. In the atmos- 

 phere there are many masses, or bodies, of 

 air, each exerting a force and having a force 

 exerted against it. This association of mass 

 and force results in work, and work is energy. 



Energy is defined as the ability to do work. 

 Energy is also something that produces changes 

 in matter. Heat can change water from a liquid 

 to a gas, for example. There are many different 

 kinds of energy, but we are mainly concerned 

 with those kinds which affect the processes in 

 the atmosphere. The energy principle simply 

 stated is that energy is measured by the amount 

 of work a body can do. Work is done only when 

 a force succeeds in moving the body it acts 

 upon. The quantity of work done is equal to the 

 product of the force times the distance moved. 

 Therefore, we derive the formula 



Work is measured in the English system by 

 the foot-pound; that is, if 1 pound of force acts 

 through a distance of 1 foot, it performs 1 

 foot-pound of work. In the metric CGS system, 

 if the force is measured in dynes, the distance 

 is measured in centimeters, and the work is 

 denoted in ergs (an erg is the work done by a 

 force of one dyne exerted for a distance of one 

 centimeter). Another unit used to measure 

 work is the joule. It is simply 10,000,000 ergs, 

 and is equivalent to just under three-fourths of 

 a foot-pound. 



Two kinds of energy are important in our 

 study of atmospheric physics. They are 

 potential energy (or energy at rest or of 

 position) and kinetic energy (or the energy 

 of motion). 



BALANCE OF FORCES— WIND 



Newton's first two laws of motion indicate 

 that motion tends to be in straight lines and 

 only deviates from such lines when acted upon 

 by another force, or by a combination of forces. 

 The air tends, for instance, to move in a 

 straight line from a high-pressure area to a 

 low-pressure area. However, there are forces 

 which prevent the air from moving in a straight 

 line. 



Pressure Gradient Force 



The variation of heating (and consequently 

 the variations of pressure) from one locality 

 to another is the initial factor that produces 

 movement of air, or wind. The most direct 

 path from high to low pressure is one along 

 which the pressure is changing most rapidly; 

 the rate of change is called the pressure 

 gradient. Pressure gradient force is the force 

 that moves air from an area of high pressure 

 to an area of low pressure. The velocity of 

 the wind depends upon the pressure gradient. 

 If the pressure gradient is steep, the wind 

 speed is strong; if the pressure gradient is 

 weak, the wind speed is light. 



W = Fd 



Coriolis Effect 



where W is the amount of work done, F is the 

 force, and d is the distance. 



If pressure gradient force were the only 

 force affecting windflow, the wind would blow 



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