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Module 7: Atmosphere As inhabitants of the planet Earth, we live at the bottom of an ocean. This ocean is the atmosphere, the layers of the air that surround the earth. The atmosphere affects almost everything we do. It directly influences everything in our environment. Changes in the atmosphere cause changes in our weather and climate. This, in turn, influences such important things as: 1. the development of soils; 2. the growth of vegetation (and thus animal life that feeds on plants); 3. the growth and development of minerals and fuel deposits. The atmosphere is a chemical mixture. It contains gases, liquids, and solids. Like all gases, air possesses the property of mass. That is, it occupies space and is influenced by the force of gravity. Scientists who study the atmosphere are called meteorologists. They have found that the atmosphere is striated, that is, it has several different layers. Each layer differs from the others in: 1. temperature, 2. chemical composition, 3. physical properties. A layer of air called the atmosphere surrounds the earth. It extends a few hundred miles above the surface of the planet and is held in place by gravity. Air contains several gases, which we cannot see. However, we need to breathe one of these gases oxygen in order to live. Air also protects us from the strongest rays of the sun called ultraviolet light. The atmosphere is composed of many gases. It is 78 percent nitrogen, 21 percent oxygen, and 0.033 percent carbon dioxide. There are also tiny amounts of argon, helium, krypton, xenon, and hydrogen. Hydrogen and oxygen combine to form water vapor. Each of these gases is important. People need carbon dioxide and nitrogen so that they can grow. Water vapor becomes rain or snow to keep the ground wet for plants to grow. The atmosphere is also filled with tiny particles of dust and smoke. Many people who have allergies sneeze just from breathing air with too much dust in it. We will see later in this lesson that the dust helps with the formation of clouds. First, though, we will see how the gases that make up the air help people, animals, and plants live. Troposphere The lower atmosphere is called the troposphere. This is the layer nearest the earth. Its depth varies from 8 kilometers (4.8 miles) over the poles to about 17.6 kilometers (11 miles) over the equator. At low altitudes, up to about two miles, there is enough air for people and animals to breathe and live. The troposphere extends upward from the ground (sea level) for about ten miles (16 kilometers). The average depth of the troposphere is 11.2 kilometers (7 miles). Most of our weather phenomena are located in this layer. This includes such things as clouds and warm and cold air masses. Almost all of the water in the entire atmosphere is found here. All of the clouds that we see in the sky are in the troposphere. This is the warmest part of the atmosphere. The troposphere is heated by radiation from the sun, both directly and by heat radiating from the earth. It is warmer nearer the earth’s surface. As altitude increases (the higher up you go), temperature decreases. For every kilometer in altitude, the temperature drops 6.5oC (or 3.5oF for every 10,000 feet in altitude). We live in the troposphere, at the bottom of the earth’s atmosphere. We are under the pressure of all the air above us. An instrument called a barometer measures atmospheric pressure. If you listen to the weather report on television, you may hear the weather mention the barometric pressure. When air pressure drops, it often means that a storm is coming. The Stratosphere The next layer is colder, the average temperature is -40oC. There is very little water and this air is less dense in this layer. The stratosphere extends above the troposphere to an average of 50 kilometers (30 miles). Two important features associated with the stratosphere are: 1. The jet stream is an enormous current of air. It’s between the troposphere and the stratosphere. These winds blow from west to east at 200 miles per hour. 2. The ozone layer is found within the upper region of the stratosphere. It is made up of a form of oxygen called ozone (O3). The ozone protects life on earth from the sun’s harmful ultraviolet rays. Ozone is slightly different from the oxygen we breathe and would burn our throats if we tried to breath very much of it. Ozone absorbs invisible ultraviolet light that would other wise be very dangerous to life on the planet. It does this by reacting with the radiation to form a stable form of oxygen (O2). This keeps the radiation from reaching the earth. The reaction gives off heat. Thus the air at the top of the stratosphere is warmer than at the lower levels. The Mesosphere The mesosphere is above the stratosphere and extends up to fifty miles (80 kilometers) above the earth’s surface. The temperature here falls to around -100oC as you reach the top of the mesosphere. Near the top, the temperature stops falling. The Thermosphere The thermosphere extends from 80 to 960 kilometers (50 miles to 600 miles) above the earth. It contains most of the ionosphere, which is not a layer of the atmosphere but is very important to us. Ultraviolet and X-ray energy from the sun electrify the molecules in the air. The atomic particles thus become electrically charged particles called ions. These ions form layers at different layers in the ionosphere. The layers reflect radio waves back to earth. This makes it possible to bounce waves off the ionosphere and back anywhere on the globe. The Exosphere Here the atmosphere is so thin that it barely exists. There are very few air molecules in this region. The composition is mostly hydrogen and helium gases. Beyond the exosphere is outer space---vast emptiness. The Composition of Air When meteorologists describe air, they’re referring to air in the lowest layer, the troposphere. From here we can go on to say that the atmosphere (or air in the atmosphere) is mostly a mixture of gases. What is called pure air is a mixture of four major gases, plus small traces of other gases. It’s colorless, odorless, and tasteless. The Four Major Gases: Nitrogen (N2): Nitrogen is the most abundant gas in the air. But it doesn’t combine easily with other substances. It is important for the development of plant and animal tissues. Nitrogen compounds are also used in fertilizers. Oxygen (O2): Oxygen is used by animal life in the respiration process. It does combine easily with other elements. In fact, many of our life processes involve oxygen compounds. Two of the compounds are water (H2O) and carbon dioxide (CO2). Argon (Ar): Argon is chemically inactive, but is of lesser value to us. Its most common use is in electric light bulbs. The bulbs are filled with argon because it won’t combine chemically with the metal filaments in the light bulbs. Carbon dioxide (CO2): There is not much carbon dioxide in the air. However, it is one of the most important gases in the atmosphere. It’s used by green plants during photosynthesis to produce simple sugars. Most forms of life would cease to exist on our planet if carbon dioxide were not present. This is because the green plants that use carbon dioxide are a source of food for other life forms. The plants give off oxygen into the atmosphere as a waste product of photosynthesis. Water Vapor (H2O): This is the gaseous form of water and can be found in the atmosphere. It helps to trap heat and keep the earth warm. It is found in clouds, fog, snow, hail, and rain. The amount of matter (solids and liquids) in the air varies from place to place on the earth. The percentage of each of the four major gases will remain in the same ratio throughout the atmosphere. Scientists believe the proportion has been essentially the same for tens of thousands of years. That is because plants and animals recycle the O2 and CO2. The O2 and CO2 cycle: CO2 is given off as a waste product from animals during respiration. Plants then use the CO2 for photosynthesis. This use of CO2 by the plants removes CO2 from the atmosphere and fixes it in simple sugars. The simple sugars are used for energy in animal cells. Plants give off O2 in the photosynthesis process as a waste product. Animals use the O2 to live and complete the energy transfer in their cells during respiration. The animal cells give off CO2 as a waste product of respiration. The c returns to the atmosphere for another trip around the cycle. Nitrogen is recycled in the same manner. Plants and animals use nitrogen to build cells. When the plants and animals die, small bacteria break down the cells and the nitrogen is returned into the soil or air. SUMMARY Air Pressure: Physical Properties The atmosphere has matter in it; therefore it has mass. Remember matter is defined as anything that has mass and occupies space. This mass pushes down on the surface of the earth (actually the atmosphere is pulled toward the center of the earth by gravity). The force of this mass is called air pressure. An average pressure of 14.7 ponds is exerted on each square inch of the earth. Air Pressure is defines as: 14.7 lbs/in2 760 mm Hg 760 torr 101325 Pascals 1 atmosphere As you go up into the atmosphere, say going up into the mountains or on an airplane, the air pressure decreases. The opposite is true for decreasing in altitude; air pressure increases. Remember the lower you are in the atmosphere; the more of the atmosphere is pushing down on you. Air pressure depends on how dense the air is. Density is the quantity of mass per unit of volume. Thus, the more molecules of air per given volume, the greater the density. Density is a ratio of mass per volume. Therefore 14 grams of air in 1 liter of space would have a density of 14 g/liter. Three things influence air density. Gravity: The Law of Gravity states that objects closer to each other attract each other more. That is, air particles in the troposphere will be pulled more by gravity than particles farther in the mesosphere. More air molecules will collect in a certain volume near the earth’s surface. Higher up, fewer molecules will collect in a given volume. Density decreases with altitude. Temperature: Heat energy causes molecules to move faster and spread out from each other. This means a cubic foot of warm air has fewer molecules than a cubic foot of cold air. If fewer molecules are present, the density is less. Therefore warmer air is less dense and has less air pressure and will rise over cooler air. Water Vapor: Water vapor lowers the density of air. A cubic foot of water vapor weighs less than a cubic foot of nitrogen or oxygen gas. Thus moist air is less dense than dry air. The average air pressure at sea level and 0oC is 14.7 pounds per square inch. The air doesn’t always exert this pressure everywhere. The pressure changes daily. Meteorologists calculate the air pressure from using with a barometer. They can calculate the change in pressure by using the differences in two readings. High pressure areas will “flow” to low pressure areas. The movement of air caused by the flow from high pressure areas to low pressure areas is called wind.