book01seeing - Weather Observation and Analysis John...

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ATMO 251 Chapter 1 page 1 of 17 Weather Observation and Analysis John Nielsen-Gammon Course Notes These course notes are copyrighted. If you are presently registered for ATMO 251 at Texas A&M University, permission is hereby granted to download and print these course notes for your personal use. If you are not registered for ATMO 251, you may view these course notes, but you may not download or print them without the permission of the author. Redistribution of these course notes, whether done freely or for profit, is explicitly prohibited without the written permission of the author. Chapter 1. WAYS OF SEEING 1.1 Introduction First-year college students in a physics course spend lots of time studying objects such as sliding blocks, point masses, and perfect springs. The physics concepts are sometimes difficult to grasp, but the underlying principles apply to the real world too. The idealized treatment in physics class helps to isolate individual concepts and separate them out from other complicating factors. The physics principles are easy to visualize. For example, here is a diagram illustrating an elastic rebound. The object hits the wall and bounces off of it. If there’s no energy loss in the collision, we know the speed of the object after the bounce (it’s the same as it was before the
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ATMO 251 Chapter 1 page 2 of 17 bounce) and the direction of motion of the object after the bounce (its angle away from the wall is equal to its previous angle toward the wall). The complexity increases somewhat if we imagine a molecule hitting another molecule and bouncing off of it. Now both objects are moving, but the motions are still possible to figure out if we make some idealized assumptions about the shapes of the molecules and the nature of the collision. Already, though, the true nature of the real-life interaction between two molecules, which involves electromagnetic forces, the motion of atoms within the molecules, and various ways of exchanging energy, is ignored when the collision is assumed to be ideal. Now take that more complicated situation, one real molecule colliding with another, and multiply it by a thousand. Imagine how complicated the interactions are among a thousand molecules. Now multiply that by a million. Multiply the result by a million. Multiply the result by a million again. We’re up to 10 21 molecules now, not even a mole, but still incomprehensibly large. Now consider a billion billion billion of those assemblages of 10 21 (1,000,000,000,000,000,000,000) molecules. After all that, your’re still not even close to the total number of molecules in the Earth’s atmosphere. It gets worse. The atmosphere is not composed of just one type of molecule. Indeed, it has things in it besides gas molecules such as oxygen, nitrogen, and water vapor. The impurities take the form of cloud droplets, rain and snow, and aerosol particles, with typical sizes of microns (10 -6 m) to millimeters (10 -3 m). Gas-phase chemical reactions that take place in the atmosphere do so when individual molecules interact. Meanwhile, the
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ATMO 251 Chapter 1 page 3 of 17
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