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The atmosphere i s an envelope of gases surrounding the solid earth. This mixture of gases provides the oxygen we need to live and is responsible for the global diversity of weather phenomena. In addition, the atmosphere protects life on the earth’s surface from the harmful effects of high-energy solar radiation and keeps the average temperature of the earth at a life-sustaining level through the natural greenhouse effect.
Although the atmosphere is a fluid, it is prevented from escaping out to space by the force of gravity. This downward gravitational pull compresses the lower atmosphere, meaning that atmospheric density is highest near sea level and decreases as one ascends. As a result, most of the atmosphere is held very close to the earth’s surface; approximately 90 percent of the atmosphere is within a mere 16 kilometers (10 miles) of sea level. The atmosphere doesn’t have a real top it fades away gradually as one moves away from the planet.
Atmospheric pressure is due to the weight of the overlying atmosphere pulled down by gravity. At sea level, with the entire atmosphere above us, atmospheric pressure is approximately 1,013 millibars, or 14.7 pounds per square inch (psi). As we move higher, there is less atmosphere above us, and so atmospheric pressure decreases with height. At 35,000 feet, the altitude of a typical cross-country airline flight, the air pressure will be only about 20 percent of the value at sea level.
Composition of the Atmosphere
The composition of the atmosphere is dominated by three gases: nitrogen, oxygen, and argon. These gases are called “permanent gases” because their concentrations are nearly constant over time and space. Nitrogen (78 percent of the atmosphere) and argon (1 percent) are largely inert, meaning that they are used in very few geophysical or biological processes. Oxygen, which is crucial for nearly all life on earth, accounts for about 20 percent of dry air.
In addition to the permanent gases, there are several other gases known as “variable gases,” which show up in changing amounts over time and space. Two of the most important variable gases are carbon dioxide and methane, both of which are potent greenhouse gases. The third, extremely important variable gas is water vapor, which is a strong greenhouse gas and is necessary for the formation of clouds and precipitation.
While air pressure shows a simple pattern of decreasing with height, another important atmospheric variable-temperature-exhibits a more complex vertical pattern. Changes in temperature above sea level allow us to divide the atmosphere into four layers: the troposphere, stratosphere, mesosphere, and thermosphere.
The source of the energy that warms the earth, drives the circulation of the atmosphere and ocean, and makes life possible is the sun. Energy travels from the sun to the earth in the form of electromagnetic (EM) radiation. Solar radiation is often referred to as “shortwave radiation,” because the sun mainly emits radiation like visible light and ultraviolet (UV) radiation, which are characterized as having relatively short wavelengths. The atmosphere is largely transparent to these types of radiation, so most solar energy passes through the atmosphere and is absorbed by the earth’s surface, which causes the surface to warm up.
As the earth’s surface warms, it transfers energy to the overlying atmosphere through conduction and radiation. Because the main source of heat for the lower atmosphere is actually the ground, atmospheric temperature decreases with ascention, up to an elevation of about 12 kilometers (7.5 miles). This lower layer of the atmosphere, which contains nearly all weather phenomena, is called the troposphere.
Above the troposphere, atmospheric temperature begins to increase with height, in a layer known as the stratosphere. This layer contains most of the ozone (tri-atomic oxygen) in the atmosphere. Ozone is very good at absorbing the most dangerous forms of UV radiation. Because ozone in the stratosphere absorbs a significant amount of UV energy, the stratosphere gets warmer as you ascend through the ozone layer. The absorption of harmful UV radiation in the stratosphere provides protection to life at the surface.
The stratosphere continues up to a height of about 50 kilometers (31 miles). Above the stratosphere is the mesosphere (50-80 kilometers), which, like the troposphere, is characterized by decreasing temperature with height. Above the mesosphere is the thermosphere, which shows increasing temperature with height until the atmosphere eventually fades away. The mesosphere and the thermosphere together contain only about 0.1 percent of the atmosphere.
Compared to the sun, the earth emits what is referred to as “longwave radiation,” which refers to forms of EM radiation like thermal infrared that have longer wavelengths than UV and visible light. The atmosphere is much less transparent to longwave radiation than it is to shortwave radiation, and so much of the energy emitted by the earth is absorbed by the atmosphere. The gases that are responsible for this absorption of longwave radiation are called greenhouse gases, and water vapor, carbon dioxide, and methane are the most prominent. The atmosphere also radiates longwave energy, and some of this radiation travels downward to the surface. This exchange of energy between the lower atmosphere and the earth’s surface is the greenhouse effect, and it results in the earth’s surface being about 30 degrees Celsius (54 degrees Fahrenheit) warmer than it would be without an atmosphere.
Over the past century, human activities have had several important impacts on the atmosphere. The first of these is global warming, or the enhanced greenhouse effect. Various human activities such as the burning of fossil fuels, forest clearing, and agriculture have increased the atmospheric concentrations of greenhouse gases like carbon dioxide and methane. With more of these gases in the atmosphere, more longwave energy is absorbed, which causes the temperature of the lower atmosphere to rise. The average global temperature has increased by approximately 0.6 of a degree Celsius (1.1 degrees Fahrenheit) over the past century, and much of this warming is due to human-caused greenhouse gas increases. There have been many attempts to reach an international consensus on reducing greenhouse gases, including the Kyoto Protocol (1997). However, the United States, the world’s largest emitter of greenhouse gases, has not signed on to the treaty.
The second major impact that humans have had on the atmosphere is the depletion of the ozone layer. Synthetic chemicals known as chlorofluorocarbons (CFCs) have been widely used as coolants, solvents, and propellants in aerosol sprays. Unfortunately, some of the CFCs released at the surface mix upward to the ozone layer. There, CFCs combine with and break apart ozone molecules, causing a thinning of the ozone layer. Ozone depletion was first noticed over Antarctica but has been observed over the northern hemisphere as well. Without the protection of the ozone layer, more damaging UV radiation reaches the surface, leading to increased occurrences of skin cancer and cataracts in humans, as well as having detrimental effects on animal and plant life. The dangers of ozone depletion were recognized in the 1970s, and an international agreement to phase out CFCs known as the Montreal Protocol was reached in 1987. Although the Montreal Protocol has successfully reduced CFC emissions, the concentrations of these gases in the stratosphere will remain high for decades, and the damage to the ozone layer will be slow to heal.
- Edward Aguado and James E. Burt, Understanding Weather and Climate, 3rd edition (Prentice-Hall, 2004);
- Frederick K. Lutgens and Edward J. Tarbuck, The Atmosphere, 8th edition (Prentice-Hall, 2001);
- Robin McIlveen, Fundamentals of Weather and Climate (Chapman and Hall, 1992);
- Greg O’Hare, John Sweeney, and Rob Wilby, Weather, Climate, and Climate Change: Human Perspectives (Prentice-Hall, 2005);
- Alan Strahler and Arthur Strahler, Physical Geography: Science and System of the Human Environment, 3rd edition (John Wiley & Sons, 2005).