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Compostition adn Structure of the Atmosphere

Compostition adn Structure of the Atmosphere - Composition...

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Composition & Structure of Atmosphere:1 Composition and Structure of the Atmosphere Thickness Composition Structure Ozone Depletion Let’s continue looking at Earth’s climate and what controls it. We have already seen what causes the seasons and we will bring that up over the next few weeks. As a bit of background, weather refers to the short term conditions of the atmosphere, at time scales of hours, days and weeks. Climate refers to the long term conditions, where averages are more important. Today’s weather may be sunny and hot in Lubbock, but our climate is semi arid. Today’ we may get rain, that’s weather, but average rain this month is X mm and that is a climatic statement. Today we are concentrating on the atmosphere itself: what’s there and how is it arranged. Thickness of the Atmosphere There is no definitive answer to how high the atmosphere is. The gases are densest near the surface and get less dense with altitude. At 16 km, the air is 10% as dense as at the surface and at 50 km, it is 1%. There is almost no gas found above 100 km, but some exists. Composition of the Atmosphere The gases that make up the atmosphere (some liquid and solid material is also found, as we will see) are thought of as either permanent or variable. Permanent gases are in the air at relatively constant proportions in both time and space, while the amounts of the variable gases change. Oxygen, a permanent gas, is roughly the same the world over and has been pretty much the same for millions of years. Water vapor on the other hand can vary considerably from place to place and day to day. The permanent gases are mostly nitrogen (78%), oxygen (just less than 21%) and argon (just less than 1%). While these percentages stay pretty constant, many of these substances are being cycled with the solid Earth. Some plants, for example, take nitrogen out of the air and leave it in the soil, where it is an important nutrient for other plants. As plants die, the nitrogen is returned to the air. Oxygen is taken out by breathing animals and other processes and it, too, gets returned to the atmosphere. The cycling of oxygen is much faster than the cycling of nitrogen. It is estimated that a typical oxygen molecule is in the air for 5000 years before being taken out, while nitrogen lasts for 42 million years.
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Composition & Structure of Atmosphere:2 These are estimates of the residence time for these gases. Because the amounts remain relatively constant over time, we think of a true equilibrium or, because of small variations, a steady state equilibrium. The variable gases are a small fraction of the mass of the air but some are crucial to the operation of the atmosphere and to the existence of life itself. Water vapor averages about a quarter of one percent of the mass of the atmosphere, but that varies from a small fraction of one percent in many deserts to several percent in some tropical locations. The hydrologic cycle describes the movement of water, in gas, liquid and solid forms, throughout the environment.
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