book12structure - Weather Observation and Analysis John...

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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 12. ATMOSPHERIC STRUCTURE 12.1 Structure of the Troposphere and Stratosphere The very names “troposphere” and “stratosphere” refer to the contrast between turbulence and vertical mixing in the troposphere and the stable, layered stratosphere. This chapter examines the basic structure of the troposphere and stratosphere, the two lowest layers of the atmosphere, and considers the importance of the interface between them, the tropopause, in the structure and behavior of weather systems. Without the Sun, the Earth would be a cold, boring ball. Sunlight drives the temperatures and winds of the atmosphere. How much of the typical state of the atmosphere is due directly to the effect of Sun, and how much is due to adjustments and reactions taking place in the atmosphere? In the stratosphere, the basic picture given by the direct effects of the sun is rather close to reality. Quite simply, the top of the stratosphere is warm because a relatively large amount of solar radiation gets absorbed there. One might think that if solar radiation is absorbed at the top of the stratosphere, there ought to be a whole lot more solar radiation absorbed at the bottom of the stratosphere or in the troposphere, simply because the air is a lot denser there. Well, sure, if the same amount of radiation that reaches the top of the stratosphere made it down into the troposphere, there’d be a lot of absorption down low. But only the most energetic wavelengths of radiation are subject to absorption by the gases in the atmosphere, and once you get past the top of the stratosphere, most of those wavelengths have already been absorbed. Because of this ATMO 251 Chapter 12 page 1 of 23
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absorption, the amount of energy still to be absorbed farther down in the stratosphere or troposphere gets smaller and smaller as the radiation penetrates lower and lower into the atmosphere. If absorption of solar radiation by the atmosphere were the only important process heating the air, the temperature would be quite cold at ground level, because there would be essentially no energy left that the air could absorb. As it is, the warmest part of the stratosphere is at its top, and the temperature decreases downward toward the troposphere, becoming quite cold as low as 15 km above ground level. But lower down, there’s something else that absorbs solar radiation: the ground. Remember that the air at low levels is heated by coming in contact
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book12structure - Weather Observation and Analysis John...

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