Atmosphere’s Structure and Thermodynamics

An atmosphere is a layer of gases that surrounds a planetary surface. Earth’s current atmosphere is a complex, dynamic system that interacts closely with and controls the surface environment. Atmospheric thermodynamics involves the process by which energy from the sun is absorbed and deposited on Earth, including its oceans and atmosphere. Early life forms substantially altered Earth’s atmosphere, and humans continue to do so.

Atmospheric Content

“Atmosphere” usually refers to the layer of gases that covers Earth. Although most planets have atmospheres of some sort, Earth’s atmosphere is unique among those known in this solar system. It contains a substantial amount of oxygen and supports the equilibrium existence of water in solid, liquid, and vapor phases.

The atmosphere of Earth contains 78 percent nitrogen and 21 percent oxygen; the remainder consists of 0.9 percent argon, 0.03 percent carbon dioxide, and traces of hydrogen, methane, nitrous oxide, and inert gases. Additionally, depending on local and global events, the atmosphere carries varying amounts of water vapor and aerosol particles, such as dust and volcanic ash. As early as 3.5 billion years ago, primitive algae-like life forms emerged and fed on carbon dioxide by using photosynthesis to metabolize carbon dioxide and water molecules to form simple sugars and more complex polysaccharides. Over time, photosynthesis gradually brought about a drastic change in the ratios of gases in the atmosphere, eventually producing the current carbon dioxide concentration of only 0.03 percent and free molecular oxygen concentration of 20 percent.

A Dynamic System

Gases are compressible and absorb or transmit varying amounts of electromagnetic radiation. Because of these characteristics, the atmosphere is not static but a highly dynamic system. Phenomena, such as weather and climate, are short- and long-term events involving the exchange of energy and transport of mass within the atmosphere and between the solid Earth, liquid oceans, and space.

Links between solar activities (especially sunspot cycles), weather, and climate have been sought for decades, but associations between them remain inconclusive. Interactions between the oceans and the air, however, are more substantial and, therefore, more readily discernible. Winds drive waves and affect ocean currents; in return, the oceans act as a heat source or sink for the atmosphere. The most famous interaction is the El Niño/Southern Oscillation event. El Niño (“the child”), which usually happens around Christmas, is an upwelling of cold water off the Pacific coast of South America that occurs every two to seven years. Besides having a disastrous effect on the fishing industry, it is associated with changes in circulation and precipitation patterns in the atmosphere over the Pacific basin.

Temperature

The atmosphere has no clear upper boundary, but it is generally considered to extend to an altitude of about 300 kilometers, at which point it responds more to electromagnetic effects and acts less like a fluid body. It can be described in three major characterizations—temperature, chemistry, and electrical activity.

Temperature changes with altitude and, as there is less overlying gas with increasing altitude, pressure, and radiation exposure also affect temperature. The lowest region of the atmosphere, enclosing virtually all life and weather on Earth, is the troposphere, extending to an altitude of 8 to 18 kilometers. The name is taken from the Greek word tropo, meaning “turn,” and refers to the fact that this region turns with the solid earth. The bottom of the troposphere is the boundary layer, where the atmosphere interacts directly with the planet's surface. The boundary layer is often turbulent, as moving air masses (winds) encounter and flow around or over obstructions and exchange heat with the ground or water. Temperature and pressure in the troposphere decrease at about 2 degrees Celsius per kilometer until the top of the troposphere, the tropopause, is reached. Life becomes increasingly difficult to maintain at altitudes (humans may require additional oxygen above 4 kilometers and must wear pressure suits above 10.6 kilometers). Some 90 percent of the atmosphere's mass is contained below the tropopause.

At the tropopause, temperature reaches a minimum of about -50 degrees Celsius, then rises again as one enters the stratosphere, to peak at about 15 degrees Celsius at the stratopause at an altitude of about 50 kilometers. Above this level, in the mesosphere, temperature declines again to a low of -60 degrees Celsius at the mesopause, at an altitude of 85 kilometers. Only 1 percent of the atmosphere is in the mesosphere and above; 99 percent lies below. Particles from atomic nuclei to meteors generally are destroyed in the mesosphere. Nuclei, or cosmic rays, encountering gas molecules in the mesophase will be shattered into secondary and tertiary particles. Most meteors are heated by friction and vaporize when they encounter the mesosphere. Above the mesosphere, the thermosphere (the hot atmosphere) extends to approximately 300 kilometers in altitude, and temperatures soar to between 500 and 2,000 degrees Celsius, depending on solar activity. Because the atmosphere is so thin, the total heat present is minuscule. Finally, beyond the thermosphere is the exosphere (outer layer), which extends from 300 kilometers to the solar wind (also called the interplanetary medium).

Solar and Infrared Radiation

Atmospheric thermodynamics can be described as the process by which energy from sunlight is absorbed by matter on Earth’s surface, in the oceans, and in the atmosphere. This energy must be returned to space as infrared radiation for the planet to maintain its stable cyclic range of ambient temperatures. Even though solar radiation and infrared radiation must be approximately balanced on a global level, they are frequently out of balance on a local level, accounting for the ambient weather changes seen on a day-by-day basis.

Absorption of solar energy is most concentrated on Earth’s surface, particularly in tropical regions, whereas most infrared radiation going out into space originates in the middle troposphere and is more evenly distributed between the equatorial and polar regions. The time-averaged temperature distribution is maintained by the system by transporting heat from regions where solar heating dominates over infrared cooling to regions where radiative cooling can dominate. In this way, the atmosphere transports heat from the ground to the upper troposphere, and the atmosphere and the oceans work together to transport heat toward the Arctic and Antarctic poles from the equatorial belt.

Net Radiative Heating

Net radiative heating, the difference between heat gained by absorption of solar energy and heat lost due to infrared reradiation, is the driving force for atmospheric thermodynamics. Solar radiation reaches the top of the atmosphere, where about 30 percent is reflected back into space. The remaining radiation is absorbed. About 69 percent of solar absorption happens at the surface, and about 40 percent leaves the surface as net infrared radiation. This leaves a net surface radiative heating of about 150 watts per square meter. On a global and annual average, the net radiative heating of Earth’s surface is equivalent to the net radiative cooling of the atmosphere; normal thermodynamics within the atmosphere thus maintains an appropriate balance between the two. A prolonged imbalance between these two systems would result in global warming, which could melt the polar ice caps and raise the levels of the oceans.

Over the oceans, most of the energy from net radiative heating of the surface is used in water evaporation. Energy is removed from the surface as latent heat; when water vapor condenses, it deposits almost all of its latent heat into the air rather than the condensed water, which returns to the surface as precipitation. The result is that heat is transported from the surface to the air, where the potential for condensation takes place. On a global average, about 70 percent of the net radiative heating of the surface is removed by latent heat flux, and the remaining 30 percent leaves the surface by conduction of the sensible heat to the overlying air. The atmosphere thus experiences diabatic heating or cooling from four ongoing processes: latent heating, sensible heating at the surface, solar absorption, and infrared heating or cooling.

Temperature Inversion

The general temperature decrease within the troposphere is called the “ environmental lapse rate.” The occurrence of shallow layers where temperatures actually reverse this normal pattern and get warmer with increasing height is known as a “ temperature inversion.” Temperature inversions are often seen in cities located at high altitudes with a low partial pressure of oxygen and in basins surrounded by mountains that block winds and trap industrial pollutants. A “ greenhouse effect” inversion causes a reversal of an ecosystem’s normal atmospheric temperature gradient, thermally altering harmful air-borne chemical compounds and enhancing their negative effects on organisms living below. A notable example is the strong eastern wind that blows toward Denver, Colorado, and traps a brown cloud of pollutants against the Rocky Mountains. This requires the daily broadcast of air-quality reports on radio and television and leads to frequent calls for weak and older adult residents to stay indoors during hotter parts of the day. Trapped carbon monoxide, coming mainly from automobile tailpipes via the incomplete combustion of gasoline, combines quickly with hemoglobin, the oxygen-carrying compound in the blood of humans and animals. By taking up binding sites on the hemoglobin molecule, carbon monoxide impairs oxygen delivery to the tissues. Older persons with heart disease are at special risk through any restrictions in oxygen delivery.

The thermosphere, which does not have well-defined limits, exhibits another temperature increase due to the absorption of very short-wavelength solar energy by atoms of oxygen and nitrogen. Although temperatures rise to values over 1,000 degrees Celsius in this outermost layer, it is difficult to compare the temperature in this layer with that seen on Earth’s surface. Temperature is directly proportional to the average speed at which molecules move, and because gases within the thermosphere are moving at very high speeds, ambient temperature remains very high. However, the gases in this region are so sparse that only a tiny number of these fast-moving air molecules collide with foreign bodies, thus causing only a very small amount of energy to be transferred. For this reason, the temperature of a satellite orbiting Earth in the thermosphere is determined by the amount of solar radiation it absorbs and not directly by the temperature of the surrounding environment. Thus, if an astronaut in the space shuttle were to expose their hand to ambient space, it would not feel hot.

Ground inversions occur frequently and commonly extend upward for 100 meters or more. They can develop from several different causes. Their primary cause is radiative cooling, which occurs under clear skies at night. During the day, the ground stores thermal energy from solar radiation. After sunset, the ground surface radiates the stored heat energy, thereby cooling the ground surface. Energy from the relatively warm air is physically conducted into the radiationally cooled surface of the ground as the two are in contact, thereby cooling the air immediately above it. Only about the first 100 meters experience a temperature decrease due to radiative cooling. Such induced temperature inversions are more likely to occur on nights with clear skies than on nights with cloudy skies. Calm wind conditions or light breezes are more conducive to developing a ground inversion. Ground inversions are more likely to develop and to last longer in cold climates because snow and ice reflect sunlight, and the small amount of heat absorbed is utilized in the melting process, thus cooling the surface rapidly and producing an inversion.

A second mechanism for the formation of an inversion is the result of a phenomenon called air drainage. On cold nights over rolling topography, denser cold air responds to gravity and moves downslope to collect in local depressions. Continued cooling can cause inversions to extend over larger areas vertically and horizontally if the vertical cooling extends above the summits of the rolling terrain. Evidence of a ground inversion's initial development is often heavy dew or frost. Ground fogs frequently occur in association with inversions because of air cooling. This is particularly true in an air drainage situation where fog first appears in depressions at the surface.

A third way in which ground inversions are formed is through the movement of a warm air mass into a region. A warm current of air may move over a cool ground surface or a cooler layer of surface air. The lower portions of the air mass are cooled, and stable or nonturbulent conditions result, producing an inversion.

The frequency of ground inversions varies across the United States (frequency expressed as a percentage of the total time a region has inversions). Most ground inversions occur at night in winter. Summer inversions are less frequent than winter inversions, but they do occur.

Chemistry

From a chemical standpoint, the atmosphere is divided into two major realms: the homosphere and the heterosphere. The homosphere—which overlaps the troposphere, stratosphere, and mesosphere—has a chemical makeup essentially identical to sea-level proportions of nitrogen, oxygen, and trace gases, even though the absolute numbers of atoms and molecules drop sharply. With increasing altitude, some important differences start to appear. Ozone becomes an important constituent of the atmosphere in this realm. Ozone is formed in the stratosphere by short-wavelength ultraviolet sunlight splitting apart oxygen molecules by photodissociation. These free oxygen atoms then form ozone with oxygen molecules. Molecular nitrogen is also dissociated.

Although the stratosphere and mesosphere (sometimes treated together as the middle atmosphere) are quite tenuous compared to the troposphere, gases in them form an optically dense layer that absorbs or reflects short-wavelength ultraviolet and X-ray radiation that would be damaging to life on the surface. Ozone is especially important with regard to the absorption of ultraviolet radiation. Nevertheless, ozone is quite fragile and can be destroyed by chlorofluorocarbons (Freons and related compounds) used for several years as spray can propellants and refrigeration system coolants. Studies indicate that these gases migrate upward in the atmosphere and chemically remove thousands of times their own mass in ozone molecules before they are broken down after several decades or even centuries. This is believed to have led to the formation of an ozone hole over the South Polar region and the appearance of a similar, but less pronounced, effect over the Arctic. Atomic oxygen becomes more common in the mesosphere. In the heterosphere, the gas mixture changes drastically, and hydrogen and helium become dominant.

Electrical Activity

From an electrical standpoint, the regions are designated as the neutral atmosphere and the ionosphere. The neutral atmosphere, below 50 kilometers in altitude, is largely devoid of electrical activity other than lightning, which might be regarded as localized “noise.” Above 50 kilometers, atoms and molecules are ionized largely by sunlight (ultraviolet radiation in particular) and, to a lesser extent, by celestial X-ray sources, collisions with other atoms, and geomagnetic fields and currents. Although the ionosphere as a whole is electrically neutral, it comprises positive (ion) and negative (electron) elements that conduct currents and respond to magnetic disturbances. The ionosphere starts at about 50 to 100 kilometers in altitude and extends outward to more than 900 kilometers as it gradually merges with the magnetosphere and its components. It is sometimes called the Heaviside layer after Oliver Heaviside (1850-1925), who predicted a layer of radio-reflecting ionized gases. The ionosphere is divided into C, D, E, and F layers, which in turn are subdivided (F1, for example).

The ionosphere is one of the most active regions of the atmosphere and one of the most responsive to changes in solar activity. Ions and electrons in the ionosphere form a mirrorlike layer that reflects radio waves. Radio waves are absorbed by the lower (D) region of the ionosphere (which also reaches down into the mesosphere). The D-layer dissipates at night in the absence of solar radiation, allowing radio waves to be reflected by the F-layer at higher altitudes, thus causing radio “skip.” These effects vary at different wavelengths. Intense solar activity can alter the characteristics of the ionosphere and make it unreliable as a radio reflector, either through the input of high-energy radiation or by the injection of particles carried by the solar wind.

Atmospheric Light Displays

Such particle injections would go unnoticed but appear as the aurora borealis and aurora australis (the Northern and Southern Lights, respectively). Earth’s magnetic field shields the planet from most charged radiation particles. In the polar regions, however, where the magnetic field lines are vertical (rising from the surface), the environment is magnetically open to space. Many charged particles from space or from the solar wind are “funneled” into the polar regions along the lines of the force of the planetary magnetic field. When the particles strike the atmosphere, they surrender their energy as light, with spectral lines unique to the electrochemical interactions taking place. These auroral displays generally take place at 120 to 300 kilometers in altitude, with some occurring as high as 700 kilometers.

The aurora is the best-known atmospheric light display. Other “dayglow” and “nightglow” categories are caused by lithium, sodium, potassium, magnesium, and calcium at altitudes from about 60 to 200 kilometers. These metals may be introduced by meteors as they are vaporized upon entering the atmosphere. A layer of hydroxyl radicals causes an infrared glow at about 100 kilometers, and dull airglows are caused by poorly understood effects at 100 to 300 kilometers in altitude.

Jet Streams and Waves

Although the principal division of the atmosphere is vertical, there are horizontal differences related to latitude and to weather. Two major phenomena that affect the atmosphere are the jet streams and waves. The jet stream is a high-speed river of air moving at about 10 to 20 kilometers in altitude and at 100 to 650 kilometers per hour. Its location plays a major role in the movements of larger air masses that make up weather fronts in the troposphere.

More than twenty wave phenomena take place in the atmosphere in response to different events. The three principal categories are gravity, Rossby, and acoustic. Gravity waves are not associated with relativity but with vertical oscillations of large air masses causing ripples, like a bottle bobbing in a pond. Rossby (or planetary) waves are associated with the wavelike distribution of weather systems. Acoustic waves are related to sound.

Atmospheric Study Tools

The earliest types of instruments used for atmospheric study remain among the most important. Barometers, thermometers, anemometers, and hygrometers provide the most immediate records of atmospheric change and warnings of impending events. Vertical profiles of atmospheric conditions are obtained by transporting such instruments up into the air using balloons and suborbital rockets. The term “sounding rocket” comes from the earliest days of atmospheric study, when scientists were “sounding” the ocean of air just as they would the ocean of water: Small charges were attached to balloons, and the time sound took to reach the ground was a crude measure of atmospheric density. Balloon-borne instrument packages continued to be called radiosondes (“radio sounders”).

Instrumentation carried aboard spacecraft is of a different nature. Many of the most revealing devices have been spectrometers of various types that analyze light reflected, emitted, or adsorbed by the atmosphere. Absorption of ultraviolet light by the atmosphere led to the discovery of the hole in the Antarctic ozone cover; drops in absorption meant that ultraviolet light was passing through rather than being returned to space (typically, such measurements also require observation of the solar ultraviolet output). Optical instruments are usually most effective when they view the atmosphere “edge-on” so as to increase the brightness of the signal (somewhat like viewing a soap bubble at the edges). Atmospheric studies can be difficult when viewing straight down because the weak signals from airglow and other effects are washed out by the brighter glow of Earth or the stellar background. Special techniques can be employed. The U.S. space shuttle has twice carried sensors designed to monitor carbon monoxide pollution in the atmosphere. Gas cells containing carbon monoxide at different pressures acted as filters that blocked all signals, but the wavelengths corresponding to carbon monoxide at the same pressure (that is, altitude) as that in the cell.

The most powerful tools used in studying the atmosphere have been the weather satellites deployed to observe the atmosphere from geostationary orbit (affording continuous views of half a hemisphere) and from lower polar orbits. Images from these satellites reveal the circulation of the atmosphere by the motion of clouds. Other sensors (called sounders) provide temperature profiles of the atmosphere at various altitudes.

The most extensive analyses of the atmosphere have been carried out by the Atmosphere Explorers, the Orbiting Geophysical Observatories, the Atmosphere Density Explorers, and the Dynamics Explorers. These spacecraft have enabled the determination of the structure and composition of the atmosphere and the changes it experiences with seasons and solar activity operating in the upper reaches of the atmosphere. The more sensitive chemical assays, however, have been conducted by instruments carried aboard the manned Spacelab 1 and 3 missions of the NASA space shuttle program. An Imaging Spectrometric Observatory carried out on Spacelab 1 produced highly detailed emission spectra of the atmosphere between 80 and 100 kilometers in altitude. Atmospheric Trace Molecules Observed by Spectroscopy (ATMOS) on Spacelab 3 measured the altitude ranges of some thirty chemicals and identified five, such as methyl chloride and nitric acid, in the stratosphere, where previously they were only suspected.

Comparative planetology analyzes the differences and similarities between and among the planets. Earth, Venus, and Mars are used most often in comparative atmospheric studies. These three “terrestrial” planets are similar in size and in general chemistry but totally different in environment, largely because of their different atmospheres. Venus has a dense atmosphere composed largely of carbon dioxide and topped by clouds of sulfuric acid, which has led to surface temperatures of 900 degrees Celsius and to normal atmospheric pressures ninety times greater than those of Earth. The circulation pattern, though, is unaltered by precipitation and oceans and thus can be used as a model in studying Earth. Efforts to understand how Venus became a “runaway greenhouse” have suggested a similar scenario for Earth. Mars, in contrast, has a tenuous atmosphere composed of carbon dioxide and traces of water vapor and oxygen. Studies of Mars focus on how its climate and atmosphere evolved and whether it was once Earthlike.

Atmospheric Alteration by Life Forms

The atmosphere as it currently exists is a relatively recent phenomenon brought about by the gradual alteration of the environment by life forms. Awareness of this global alteration is helping humans understand the effects they are having on the environment over a relatively short time span—essentially since the onset of the Industrial Revolution. The widespread use of fossil fuels and the burning of forests to clear land for agriculture have converted the carbon that plants spent billions of years converting into solid carbon compounds back into gaseous carbon dioxide. Furthermore, the plants that were “sinks,” or absorbers, of carbon dioxide are available in lesser quantities to liberate oxygen. Sulfur compounds are naturally introduced by volcanoes, biological decay, and oceanic processes, but large quantities have been added by industrial processes, including coal burning. One product, sulfur dioxide, combines with water vapor at low altitudes to form sulfuric acid. At high altitudes, it can also alter the ozone layer and the terrestrial radiation balance. In addition, the ratios of nitrogen compounds are altered by combustion and by the widespread use of nitrogen-based fertilizers; these products, too, have an adverse effect on ozone.

The immediate concern is not that the oxygen supply will be depleted, although that is a credible, long-term possibility, but that the increased amounts of carbon dioxide in the atmosphere will cause a greenhouse effect. In the greenhouse effect, long-wavelength (infrared) radiation emanating from the soil or ground is absorbed by the atmosphere and retained. Glass serves this purpose for a greenhouse by reflecting the radiation back into the interior of the structure and so increasing the interior temperature. Carbon dioxide has the same effect in Earth’s atmosphere, but it functions by absorbing and retaining infrared energy rather than by reflecting it back toward the ground. Other human-made gases that enhance the greenhouse effect are nitrous oxide, methane (which is also produced naturally), and chlorofluorocarbons (which also deplete ozone, thus allowing more radiation to enter). Because little is known about causes and effects in this field, there are uncertainties in predicting what will happen. It is expected, however, that increases in the carbon dioxide content of the atmosphere will raise global temperatures and that such a rise in temperature will shift weather patterns and cause large portions of the polar ice caps to melt, thus flooding coastal regions.

Agricultural Applications

There are many practical applications of the knowledge of thermodynamic data, one of which is regularly utilized in agriculture to estimate the approximate date when crops will be ready for harvest. The growing degree-day index estimates the number of growing degree-days for a particular crop on any given day as the difference between the daily mean temperature and the minimum temperature required for the growth of a particular crop. For example, the minimum growing temperature for corn is 10 degrees Celsius (50 degrees Fahrenheit), which means that on a day when the mean temperature is 24 degrees Celsius (75 degrees Fahrenheit), the number of growing degree-days for sweet corn is estimated at fourteen. The daily growing degree-day values are added starting with the onset of the growth season. If 1,111 growing degree-days are needed for corn to mature in a particular region, the corn should be ready to harvest when the total number of growing degree-days reaches that figure.

Principal Terms

adiabatic: characterizing a process in which no heat is exchanged between a system and its surroundings

air drainage: the flow of cold, dense air downslope in response to gravity

chlorofluorocarbons (CFCs): chemicals in which chlorine and fluorine replace one or more of the hydrogen atoms in the molecular structure of the corresponding hydrocarbon

cosmic rays: high-energy atomic nuclei and subatomic particles, as distinct from electromagnetic radiation

environmental lapse rate: the general temperature decrease within the troposphere; the rate is variable but averages approximately 6.5 degrees Celsius per kilometer

exosphere: the outermost layer of Earth’s atmosphere

greenhouse effect: a planetary phenomenon in which the atmosphere absorbs and retains more heat radiation than it passes back out into space

growing degree-day index: a measurement system that uses thermal principles to estimate the approximate date when crops will be ready for harvest

heterosphere: a zone of the atmosphere at an altitude of eighty kilometers, including the ionosphere, made up of rarefied layers of oxygen atoms and nitrogen molecules

homosphere: a major zone of the atmosphere below the heterosphere whose chemical makeup is consistent with the proportions of nitrogen, oxygen, argon, carbon dioxide, and trace gases at sea level; includes the troposphere, stratosphere, and mesosphere

infrared radiation: electromagnetic radiation with frequency in the range of 10¹³ to 10¹⁴ Hertz (Hz)

ionosphere: the layer of ionized gases in Earth’s atmosphere, starting about 50 to 100 kilometers above the surface of the planet (between the thermosphere and the exosphere)

latent heat: the energy absorbed or released during a change of physical state

mesosphere: the extremely rarefied atmospheric layer at altitudes from fifty to eighty kilometers above the surface, characterized by rapid decreases in temperature

net radiative heating: the driving force for atmospheric thermodynamics, essentially the difference between heat entering the atmosphere due to solar heating and heat leaving the atmosphere as infrared radiation

photodissociation: the condition in which light energy absorbed by a molecule is sufficient to dissociate the bonds between atoms in the molecule, typically caused by light in the ultraviolet range

radiational cooling: the cooling of Earth’s surface and the layer of air immediately above it by a process of radiation and conduction

stratosphere: the atmospheric zone twenty to fifty kilometers above the surface that contains the functional ozone layer

temperature inversion: a condition in which a region of warmer occupies a position above its normal location, causing air temperature to increase with increasing elevation from the Earth’s surface

thermosphere: the atmospheric zone extending from 80 to 480 kilometers in altitude and containing the ionosphere

troposphere: the lowest atmospheric layer, extending from Earth’s surface to an altitude of about eighteen kilometers, containing 90 percent of the total mass of the atmosphere, marked by considerable turbulence and a decrease in temperature with increasing altitude

ultraviolet light: electromagnetic radiation having a frequency in the range of 10¹⁵ to 10¹⁷ Hz

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