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Atmosphere Basics So what is the sky? What is the atmosphere? What is air? It's all round you and you need it to live. Sometimes it's a cool breeze. Sometimes it's a really hot and humid day that seems to push on you from all sides. It's really just a thin layer of gases that surrounds the Earth. Just like there are layers inside the Earth, there are layers in the atmosphere. The layers interact, get heated up and mix with the top layer of the Earth's crust. The atmosphere is made up of five layers of air: the Troposphere, the Stratosphere, the Mesosphere, the Thermosphere, and the Exosphere (along with the Ionosphere). In the next few minutes you will be reading certain things about each layer. You will be starting at sea level and reaching up to the height of 20,000 miles. Over 99% of the mass of the Earth's atmosphere is contained in two layers: the troposphere and the stratosphere. Most of the Earth's atmosphere (80 to 90%) is found in the troposphere, the atmospheric layer where we live. The thin envelope of air that surrounds our planet is a mixture of gases, each with its own physical properties. The mixture is far from evenly divided. Two elements, nitrogen and oxygen, make up 99% of the volume of air. Nitrogen (N2), comprises 78% of the atmosphere, and oxygen (O2), accounts for 21% (see chart above). The other 1% is composed of "trace" gases, the most prevalent of which is the inert gaseous element argon (Ar, 0.9%). The rest of the trace gases, although present in only minute amounts, are very important to life on Earth. Two in particular, carbon dioxide (CO2, 0.04%) and ozone (O3), can have a large impact on atmospheric processes. Other trace gases include: neon, helium, krypton and xenon. The Troposphere The first and the lowest of these layers is called the TROPOSPHERE. It is the most crowded with gas molecules (highest density) and is where all our weather, clouds, wind and precipitation are found. It is also the only layer of the atmosphere that can support life. 80% of the atmosphere’s mass and 75% of the gases in the atmosphere are found here. Over the United States the troposphere reaches up to 40,000 feet, almost 8 miles (11 km). The troposphere is also the only layer of the atmosphere that has water vapor in it. This is why all of the weather is in this layer. There is a large range of temperatures in the troposphere. One hot summer day at the bottom of this gas layer the temperature may be 95° F. (35C) But at the top of the troposphere the temperature goes down to a freezing -70° F (-56C). The temperature generally drops 6C for every 1 km you rise. We also know the troposphere for its winds and air currents. At the top of this gas layer it generally blows at 150 to 200 miles per hour in a westerly direction. The troposphere also has many air currents that move up and down. Soaring pilots search for these currents and ride them aloft as though they were in a elevator. These wind currents are called thermal currents. The top of the troposphere is called the TROPOPAUSE. Water vapor is found in only small amounts in the tropopause. The Stratosphere The second layer of air is called the STRATOSPHERE. It begins where the tropopause ends. The stratosphere reaches the height of 150,000 feet or about 30 miles. Lower parts of the stratosphere are filled with winds and are extremely cold. But at higher altitudes in the stratosphere the wind begins to weaken and eventually die. At about 20 miles up the temperature gradually rises from -70° F (-56C) to about 30° F (-1C). The cause of this sudden warming is a layer of gas called the OZONE. The top of the stratosphere is called the STRATOPAUSE. In the stratopause the atmospheric pressure is only 1/1000 the pressure at sea level. The Mesosphere Above the stratosphere is the third major layer, called the MESOSPHERE. It's a layer 20 miles high. The temperature falls lower and lower with a higher altitude. It is the coldest of all the layers in the entire atmosphere. At the bottom of this layer it is 30° F (-1C). But at the highest part of the layer it is -130° F (-90C). Near the top of the mesosphere it is almost black. At these heights the thin air does not scatter as much of the sun's light as the lower, dense atmosphere does. This scattering of light makes the sky appear blue to us on the ground. Even during the day at this altitude the stars shine steadily. Air making up the mesosphere is almost the same air making up the troposphere, except that there is no water vapor and far less carbon dioxide. The top of the mesosphere is called the MESOPAUSE. Above the mesopause the temperature does not fall but rise. The Thermosphere (Ionosphere & Exosphere included) The fourth major layer and the highest is called the THERMOSPHERE. 99.99% of the matter in the atmosphere is below the thermosphere. This is why the thermosphere is the borderline of space from our atmosphere. At the lower level of the thermosphere the temperature begins to rise. From a freezing -130° F (-90C) at the bottom, it turns to a burning 2000°F (1008 C) at the top at an altitude of about 20,000 miles high. In the thermosphere any living creature exposed to the air would perish by being broiled to death on the side facing the sun, or frozen to death on the side in the Earth's shadow. The IONOPSHERE is the layer of air that makes up the lower part of the thermosphere. The ionosphere is the layer of the atmosphere where radio waves can be bounced off of so that the signal can be detected thousands of miles away. These signals are usually not able to be detected because of the curve of the Earth, but since the radio signals are bounced off the ionosphere, it actually acts as a satellite. The Northern Lights (Aurora Borealis) and the Southern Lights (Aurora Australis) also occur in the ionosphere section of the thermosphere. The EXOSPHERE is the upper part of the thermosphere. The exosphere is the very highest part of the thermosphere and is SOMETIMES CONSIDERED ITS OWN LAYER. Here we’ll just consider it a part of the thermosphere. The atmosphere in the exosphere merges into space in the extremely thin air. Air atoms and molecules are constantly escaping to space from the exosphere. In this region of the atmosphere, hydrogen and helium are the prime components and are only present at extremely low densities. This is the area where many satellites orbit the Earth. Mankind’s Influence During the last 200 years, mankind has begun to significantly alter the composition of the atmosphere through pollution. Although air is still made up mostly of oxygen and nitrogen, some of the levels of trace gases have been increasing, in particular the concentrations of greenhouse gases (carbon dioxide, methane, ozone, etc.), which may be causing global warming. Some air pollutants now present in the atmosphere are completely new, such as the CFCs (chlorofluorocarbons), which are solely man-made. Despite their relative scarcity, the so-called greenhouse gases play an important role in the regulation of the Earth's climate. The natural greenhouse gases include carbon dioxide, methane, nitrous oxide and water vapor. Although ozone is also a greenhouse gas it is more commonly associated with the ozone hole and ozone depletion. By trapping heat trying to escape from the surface of the Earth to space, the greenhouse gases warm the atmosphere. Consequently the Earth surface is 33°C warmer than it would be without an atmosphere. This heating process is called the natural greenhouse effect. With an increase in the amount of greenhouse gases, the Earth’s average temperature could rise several degrees, which could be cataclysmic to life on this planet. Why?