Atmosphere
The atmosphere is a complex and dynamic layer of gases surrounding Earth, primarily composed of approximately 78% nitrogen and 21% oxygen, with trace amounts of other gases such as argon and carbon dioxide. This mixture, commonly known as air, plays a critical role in sustaining life, regulating temperature, and facilitating weather patterns. The atmosphere is structured into several layers, each with distinct characteristics; the troposphere, where most weather occurs, lies closest to Earth's surface, followed by the stratosphere, mesosphere, and thermosphere, which extend outward.
The atmosphere is also integral to the hydrological cycle, which involves the evaporation of water from oceans, its transport as vapor, and eventual precipitation on land, contributing to freshwater resources. Human activities have increased the presence of certain gases, impacting processes like ozone formation and climate. Additionally, atmospheric conditions affect human health; extreme weather conditions, such as heat waves, can lead to health crises and agricultural losses. Overall, the atmosphere is vital for both ecological balance and human well-being, underscoring its importance in environmental studies and public health.
Atmosphere
The atmosphere is the envelope of gases that surrounds Earth. Held in place by the attractive force of gravity, Earth’s atmosphere pervades all facets of the environment. Almost every aspect of Earth’s system is dependent upon or markedly influenced by the behavior of weather systems spawned within the atmosphere. The atmosphere provides resources in the form of individual gases, which can be separated industrially; it also directly affects other resources, most notably food resources.
Background
The composition of the atmosphere (excluding water vapor) below 80 kilometers is about 78 percent nitrogen and 21 percent oxygen by volume (76 percent nitrogen, 23 percent oxygen by mass). The remaining 1 percent includes all other dry gases, chiefly argon, carbon dioxide, neon, helium, krypton, hydrogen, and ozone. Water vapor, the most variable constituent of the atmosphere, typically occupies between 0 percent and 4 percent of the atmospheric volume. This mixture of gases is commonly referred to as “air.”
The two principal constituents of air are greatly dissimilar in their chemical properties. While oxygen is an extremely active chemical, reacting with many substances, nitrogen reacts only under limited conditions. The inert nature of nitrogen is believed to be the reason it came to be the atmosphere’s most abundant constituent. Volcanic outgassing in Earth’s early history is the likely source of its present atmosphere. Though nitrogen is a minor component of volcanic emissions, the lack of chemical reactions able to remove it from the atmosphere allowed its concentration to grow dramatically over time. Photosynthesis and, to a lesser degree, photodissociation of water by sunlight are believed to account for atmospheric oxygen.
Carbon dioxide, a principal constituent of volcanic emissions, is also released into the atmosphere by the oceans, respiration, and fossil fuel combustion. Argon, far more abundant in Earth’s atmosphere than any of the other noble (inert) gases, is a by-product of the radioactive decay of an of potassium. Helium is also mainly a by-product of radioactive decay.
Vertical Structure
The atmosphere has a well-defined lower boundary but extends indefinitely away from the earth; at 30,000 kilometers molecules are no longer effectively held in orbit by gravity. The atmosphere can be thought of as a series of layers. However, the layering is far subtler than what may be found in, for example, a geologic formation. The most common method of demarcating layers is to examine the average change of temperature as a function of elevation. Earth’s surface, warmed by the absorption of solar radiation, conducts heat into the lowest portion of the atmosphere. This lowest layer, known as the troposphere, extends to about 10 kilometers above the surface and is characterized by temperatures that decrease with height. Virtually all the phenomena that are commonly referred to as “weather” occur in the troposphere. The average of air at sea level is about 1.225 kilograms per cubic meter. Because air is a compressible fluid, air density decreases logarithmically with height. Half the mass of the atmosphere lies below about 5.5 kilometers. Approximately 80 percent of the atmosphere’s mass is found in the troposphere.
Between 10 and 50 kilometers, temperatures increase with increasing altitude in the layer known as the stratosphere. The warming of air in this layer is accounted for by the heat released as molecules absorb ultraviolet wavelengths of solar radiation. Ozone concentration is at a maximum in this layer. Historically, it was thought that there was little exchange of air between the troposphere and stratosphere, except during volcanic and atomic explosions, because temperature profiles such as that found in the stratosphere typically suppress mixing. However, the occurrence of human-made chlorofluorocarbons (CFCs) in the stratosphere is evidence that exchange does take place. The presence of CFCs in the stratosphere is detrimental to ozone and serves as an ozone sink that has no compensating source.
Temperatures once again decrease with increasing height between 50 and 80 kilometers in the mesosphere. The troposphere and stratosphere together account for about 99.9 percent of the atmosphere’s mass. The mesosphere contains about 99 percent of the remaining mass.
The thermosphere is situated above the mesosphere and extends indefinitely away from the earth. Temperatures once again increase with height in this layer and can reach 500 to 2,000 Kelvin depending upon the amount of solar activity. However, temperatures begin to take on a different meaning at these altitudes owing to the relatively small number of molecules and the relatively large mean free path between collisions.
The tops of these four layers are known as the tropopause, stratopause, mesopause, and thermopause, respectively. Temperatures typically remain constant for a few kilometers at the interface of the layers. A feature of note at the tropopause is the jet stream, an especially swift current of air.
The atmosphere can also be partitioned vertically based on how uniformly mixed its constituents are. Turbulent processes in the atmosphere below about 80 kilometers keep the constituents in the lower atmosphere well mixed. This region is known as the homosphere. Air sampled near both the top and bottom of the homosphere will contain nearly equal percentages of each constituent gas, although the densities of the samples will be markedly different. Above 80 kilometers, the vertical mixing of constituents is controlled by molecular diffusion, allowing them to separate by mass, with the lightest gases (hydrogen and helium) present at the highest levels. This region is known as the heterosphere. Sunlight in the heterosphere is more intense than sunlight that penetrates to the homosphere because little filtering has taken place. As a result, ionization occurs in the heterosphere, and this ionization affects the transmittable range of commercially broadcast radio signals that are redirected by the ionized molecules.
Balance of Energy
The sun is the source of nearly all of the energy the atmosphere receives. Minor amounts of energy are contributed by lightning and Earth’s internal heat sources. There is a global balance between the solar radiation that heats the atmosphere and the terrestrial radiation emitted to space. However, the balance does not hold for individual latitudes. The complex geometry of a spherical planet having its rotational axis tilted with respect to an elliptical orbit about the sun results in an imbalance between absorbed and emitted radiation. Over the course of a typical year, the tropical region of the Earth between about 37° north and 37° south latitude receives more energy from the sun than what is regionally emitted back to space. Poleward of this region, Earth radiates to space more than it receives from the sun.
As a result of the regional imbalance of energy, there is a continuous transport of energy in the atmosphere and the oceans from the tropical latitudes, where there is a surplus of energy, to the polar latitudes, where there is a deficit. If this transport did not occur, the tropics would continually warm while the polar latitudes would grow colder year after year. The transport of energy by winds and systems is most apparent in the middle latitudes of the planet across the interface between the regions of surplus and deficit. In the lower atmosphere, the principal forms of the energy are internal energy (associated with the temperature of the air) and latent energy (associated with the phase of water). In the case of the latter, the evaporation of ocean water in the tropics transforms internal energy into latent energy. Water vapor, being a gas and thus highly mobile, is transported away from the tropics and may subsequently condense to form clouds or dew. Condensation releases an amount of energy equal to that used in evaporation. Evaporation and condensation are first-order processes in Earth’s heat budget. In addition, they play key roles in Earth’s hydrologic cycle. This cycle purifies and redistributes the planet’s single most important compound and the resource without which life would not exist.
The Hydrologic Cycle
Though there are approximately 1.3 billion cubic kilometers of water on Earth, about 97 percent of this is ocean water rather than fresh water. Evaporation of ocean water into the atmosphere, its transport by weather systems, and the subsequent condensation in clouds provide life’s most precious resource, fresh water, to the continents. The evaporation of water from the oceans and evapotranspiration over land, the transport of water in the atmosphere, and its eventual return to the oceans are collectively known as the hydrologic cycle.
Over the continents, precipitation exceeds evaporation, while the reverse is true over the oceans. Some of the water vapor added to the atmosphere by evaporation from the oceans is transported to the continents, where it combines with water vapor from evapotranspiration, condenses, and falls as precipitation. Some of this precipitation percolates into and becomes part of underground aquifers, or groundwater. Some precipitation is returned to the ocean by runoff in rivers. Water vapor is also transported from over the continents to over the oceans in the atmosphere. Generally, water evaporated in one location is not the same water that precipitates on that location. Water vapor is usually transported hundreds or even thousands of kilometers from its source. For example, the majority of water that falls as precipitation on the portion of the United States east of the Rocky Mountains is evaporated off the Gulf of Mexico. Evaporation off the Indian Ocean is the source of the precipitation for the wet Indian monsoon. The is rarely completed on a local scale.
Observations indicate that rain and snowfall on the continents is well in excess of the runoff from these same areas. Only about 20 percent of the precipitation that falls on land is returned to the ocean by runoff. While some of the remaining precipitation is stored underground in permeable rock, the majority of the excess is transported back to the oceans by air masses. Cold, dry air masses moving equatorward over land areas are warmed and moistened by evapotranspiration from the surfaces over which they pass. Studies of the change in moisture content of continental polar air moving equatorward over the Mississippi River drainage basin in the United States indicate that these air masses can remove, by evapotranspiration, a quantity of water equal to nine times the average discharge of the Mississippi River. The hydrologic cycle is subject to great disruptions under conditions of short-term or long-term change. Examples of such disruptions include floods and droughts.
Resources from the Atmosphere
The atmosphere is a ready source of several gases used in industry and other applications. The industrial use of gases obtained from the atmosphere began in the early years of the twentieth century. The separation of the constituents of air is basically a three-step process. First, impurities are removed. Second, the purified air is liquefied by compression and refrigeration. Third, the individual components are separated by distillation, making use of the fact that each component boils at a different temperature.
Air separation plants produce oxygen, nitrogen, and argon for delivery in both the gaseous and liquid phases. The total mass of the atmosphere is about 5.27 × 1018 kilograms. Given the percentage, by mass, of nitrogen (76 percent) and oxygen (23 percent) in the atmosphere, there are about 1.2 × 1018 kilograms of oxygen and 4.0 × 1018 kilograms of nitrogen available for separation and use.
Gases from the atmosphere are used by the steel industry in the cutting and welding of metals. Other user communities include the aerospace industry, chemical companies, and the medical industry. Liquid nitrogen is used in applications requiring extreme cold. The inert nature of gaseous nitrogen makes it ideal for flushing air out of systems when one also needs to prevent chemical reactions from occurring. The atmosphere also provides a source of argon, neon, krypton, and xenon and is the only known source of several of the rare gases.
The Atmosphere and Human Health
In addition to being a resource itself, the atmosphere has direct and indirect effects on many other resources and on human health. Examples of aspects dependent on atmospheric conditions include the resistance of crops to disease and insects; the health and productivity of forests; milk, wool, and egg production; and meat quality. Biometeorology, also known as bioclimatology, is the branch of atmospheric science concerned with the effects of weather and climate on the health and activity of human beings.
Deaths from heart attacks and heart disease increase when the human body experiences great thermal stress, as in extreme heat or cold or when temperature changes abruptly. Deaths tend to peak in winter in colder climates and in summer in warmer climates.
An example of the devastating effect high temperature can have on human health is the European heat wave of 2003. Temperatures varied from country to country, but France reported seven days that exceeded 40° Celsius. More than 50,000 people died throughout Europe as a result of the aberrant climate. In Switzerland, where temperatures reached 41° Celsius, flash floods occurred because of melting glaciers. The European agricultural industry suffered extensive losses because of this heat wave: wheat production fell by 20 percent in France, and grapes ripened prematurely. The heat wave was caused by an anticyclone, which inhibited precipitation.
In 2017 and 2018, as Europe experienced similarly alarming heat waves once more, the state of California, particularly in the north, experienced record heat waves that fueled especially devastating wildfires. After more than forty people died as a result of the fires that raged mainly in October 2017, a heat wave in July 2018 that resulted in the state's hottest temperatures for the month in history contributed to more disastrous fires; the November fire that spread throughout Butte County was responsible for the deaths of more than eighty people. In the late 2010s and early 2020s, increasing numbers of people died due to heat-related causes in the United States. According to the Centers for Disease Control and Prevention, the national heat-related mortality rate doubled between 2019 and 2023.
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