The Earth’s Atmosphere Michael Beattie

Overview The Earth is surrounded by a blanket of air, which we call the atmosphere. It reaches over 640 km from the surface of the Earth. Almost 99% of the atmosphere lies within 64-80 km of the earth’s surface. This thin blanket of air shields the surface and its inhabitants from the sun’s UV radiant energy, as well as from material from interplanetary space. There is no definite upper limit to the atmosphere; rather, it becomes thinner and thinner, eventually merging with the empty space that surrounds all the planets.

Composition of the Atmosphere The atmosphere is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases. At the surface there is a balance between usage and production of these gases. Nitrogen is removed from the atmosphere primarily by biological processes that involve soil bacteria. It is returned mainly through the decaying of plant and animal matter. Oxygen is removed from the atmosphere during breathing, as the lungs take in oxygen and release carbon dioxide. It is returned during photosynthesis, as plants, in the presence of sunlight, combine carbon dioxide and water to produce sugar and oxygen.

Composition of the Atmosphere Water vapor is an extremely important gas in our atmosphere. Its concentration in the atmosphere varies greatly from place to place, and from time to time. Close to the surface in warm, tropical locations, water vapor may account for up to 4% of atmospheric gases, whereas in colder arctic areas, its concentration may dwindle to a mere fraction of a percent. It forms into cloud particles that grow in size and fall to the earth as precipitation. It is also provides an important source of atmospheric energy in latent heat when it changes from vapor into liquid water or ice. Latent heat is an important source of energy for storms, such as thunderstorms and hurricanes.

Composition of the Atmosphere Carbon dioxide is present as a small but important percentage of air (about 0.037%). It enters the atmosphere mainly from the decay of vegetation, but it also comes from volcanic eruptions, from the burning of fossil fuels, and from deforestation. The removal of carbon dioxide from the atmosphere takes place during photosynthesis. Recent measurements of carbon dioxide show a 15% increase in the atmospheric concentration since 1958. This increase means carbon dioxide is entering the atmosphere at a greater rate than it is being removed. The increase appears to be due mainly to the burning of fossil fuels; however, deforestation also plays a role.

Composition of the Atmosphere Carbon dioxide like water vapor is a greenhouse gas, meaning it traps a portion of the earth’s outgoing energy. As the atmospheric concentration of carbon dioxide increases, so should the average global surface air temperature. Most mathematical model experiments that predict future atmospheric conditions estimate that increasing carbon dioxide levels, along with other greenhouse gases, will result in a global warming of surface air between 1 and 3.5 degrees Celsius by the year 2100.

The Early Atmosphere The atmosphere that originally surrounded the earth was probably much different from the air we breathe today. The earth’s first atmosphere, about 4.6 billion years ago, most likely consisted of hydrogen and helium, the two most abundant gases found in the universe. Most scientists feel that this early atmosphere escaped into space from the earth’s hot surface. A second, more dense atmosphere gradually developed as gases from molten rock within the earth’s hot interior escaped through volcanoes and steam vents. The gases were mostly water vapor (about 80%), carbon dioxide (about 10%) and up to a few percent nitrogen.

The Early Atmosphere Oxygen began an extremely slow increase in concentration as rays from the sun split water vapor into hydrogen and oxygen. The hydrogen, being lighter, probably rose and escaped into space, while the oxygen remained in the atmosphere. This slow increase in oxygen may have provided enough of this gas for primitive plants to evolve, and as plant growth increased our atmosphere was greatly enriched with oxygen.

Air Density and Pressure Air density is greatest at the surface and decreases as we move up into the atmosphere; rapidly at first, then more slowly as we move farther from the surface. The weight of all the air around the earth is 5600 trillion tons. Atmospheric pressure always decreases with increasing height. Normal atmospheric pressure near sea level is 1013.25 mb or 14.7 pounds per square inch.

Layers of the Atmosphere Troposphere: The troposphere is the lowest region in the earth's atmosphere. On the earth, it goes from ground level up to about 17 kilometers high. The weather and clouds occur in the troposphere. In the troposphere, the temperature generally decreases as altitude increases. Tropopause: The tropopause is the boundary zone (or transition layer) between the troposphere and the stratosphere. The tropopause is characterized by little or no change in temperature as altitude increases. Stratosphere: The stratosphere is characterized by a slight temperature increase with altitude and the absence of clouds. The stratosphere extends between 17 to 50 kilometers above the earth's surface. The earth's ozone layer is located in the stratosphere. Ozone, a form of oxygen, is crucial to our survival; this layer absorbs a lot of ultraviolet solar energy.

Layers of the Atmosphere Mesosphere: The mesosphere is characterized by temperatures that quickly decrease as height increases. The mesosphere extends from between 50 and 80 kilometers above the earth's surface. Ionosphere: The ionosphere starts at about 70-80 km high and continues for hundreds of kilometers (about 640 km). It contains many ions and free electrons (plasma). The ions are created when sunlight hits atoms and tears off some electrons. Auroras occur in the ionosphere. Thermosphere: The hot layer above the mesosphere. In the thermosphere there are relatively few atoms and molecules. Consequently, the absorption of a small amount of solar energy can cause a large increase in air temperature that my exceed 500 degrees Celsius. Exosphere: The exosphere is the outermost layer of the Earth's atmosphere. The exosphere goes from about 640 km high to about 1,280 km. The lower boundary of the exosphere is called the critical level of escape, where atmospheric pressure is very low (the gas atoms are very widely spaced) and the temperature is very low.