Atmospheric physics

Fields of Study:Aeronomy; acoustics; applied physics; atmospheric dynamics; calculus; chemistry; climatology; cloud physics; computer modeling; engineering; environmental physics; fluid dynamics; fluid mechanics; mathematics; meteorology; physics; radiation; remote sensing; statistics; statistical mechanics; spatial statistics; thermodynamics.

Definition:Atmospheric physics is a subfield of atmospheric science that studies the physics at work in atmospheric phenomena. Atmospheric physicists study the flow of energy as cold and warm air masses collide and as fast and slow rivers of air interact. Mathematics is an integral part of studying these processes, as physics itself depends on mathematics. In atmospheric physics, the goal is to mathematically model, and therefore predict, what occurs in the atmospheric layers that surround the earth and other planets. Statistics and computers play an important role in studying the atmosphere, as does chemistry; the chemical makeup of the atmosphere affects its activity. To gather data, the field depends on the advanced design and manufacture of sensing devices.

Basic Principles

Historically, atmospheric research has been two-pronged. Physical meteorology studies what is seen and heard in the atmosphere, such as cloud formation, rainfall, lightning, tornadoes, and other tangible phenomena. The dynamics side of atmospheric research studies large-scale atmospheric motions, such as those that are hundreds of miles or many days long. This includes frontal systems, tropical storms, jet streams, and related effects. Also central to atmospheric physics are air pressure, density, and the capture of water, especially from oceans.

These two historical approaches are best merged in an interdisciplinary approach. The study of climate change, for instance, involves advanced dynamics, chemistry, and radiation research. The mathematics of physics, especially fluid flow equations, remains essential to this study. For example, scattering theory uses mathematics to understand the behavior of atmospheric matter scattering in particles and/or waves. The study of atmospheric physics also uses mathematics to model wave propagation, or the ways that waves travel.

The application of physics to atmospheric studies has grown in recent decades. The late twentieth and early twenty-first centuries brought advances in atmospheric physics thanks to satellites and computers. The gaseous envelope of atmosphere that makes life on earth possible is never fully at rest, so much of scientists’ knowledge of it still relies on Newton’s laws of motion, formulated in the seventeenth century. The constant motion of air makes it difficult to capture facts without the application of continuum mechanics, which studies the behavior of solids and fluids as entire masses rather than separate particles. Joseph-Louis Lagrange (1736–1813) was the next scientist after Isaac Newton (1643–1727) to make an in-depth study of mechanics. Lagrange’s work enabled the development of mathematical physics, and his approach has become widespread ever since. In the Lagrangian description of fluid motion, a physicist examines a parcel or several parcels of air to learn how properties transform and interact as a single system within those bounds and with the environment beyond the parcels.

Core Concepts

The atmosphere is a complicated mixture of chemicals, water, and solid matter. A relatively thin layer compared to the mass of the planet, it has far more horizontal than vertical movement. The atmosphere, like the ocean, has tides. For the most part, they are created through daily heating by the sun’s radiation, solar gravitation, and molecular resonance, in which a molecule vibrates among several alternate structures. Though solar radiation has an effect on atmospheric physics, gravity, compressing most of the air to within ten miles of the surface, has more effect. The pressure exerted by gravity causes air to become denser at sea level than in the mountains or on airplanes. Density decreases exponentially with altitude, and pressure changes in the troposphere create flows of air mass. These air masses are also affected by the rotation of the planet. Planetary rotation impacts the upper atmosphere too, by creating waves that affect the movement of heat, chemicals, and aerosols.

Atmospheric Composition. The earth’s atmosphere contains mostly nitrogen and oxygen. The remaining gases, including carbon dioxide (CO2) and ozone (O3), make up only 1 percent of the air. One of the most important components of the atmosphere is water, occurring both in molecular form and in fine aerosols. Aerosols are tiny solid or liquid particles suspended in gas. The air carries natural aerosols, coming from volcanic ash, sea spray, pollen, and other sources; the air also carries human-produced aerosols.

Ozone is made in the stratosphere when solar rays collide with O2 molecules and break them into two O1 atoms. These then combine with O2, resulting in O3—ozone. Stratospheric ozone is good, as it protects the earth from harmful ultraviolet rays coming from the sun. Harmful ozone is a type of surface pollution, which happens in the troposphere when sunlight and heat trigger chemical reactions involving nitrogen oxides—often created by human activity, such as cars—and volatile organic chemicals. Harmful ozone can make breathing difficult for people with respiratory ailments.

Water Movement.Water movement in the air is a central concern for meteorologists. Water vapor, considered only a trace of what makes up the atmosphere, is found almost exclusively in the troposphere. It is produced at the surface and in the tropics, drying out increasingly toward the upper troposphere and the poles. Convection, or the movement of molecules within fluids, builds cells (clouds) vertically near the tropics, but most cloud motion is horizontal. As trained physicists, weather forecasters need to understand the basic elements of motion. For example, speed differs from velocity. Speed describes how fast in time an object moves (e.g., miles per hour), whereas velocity describes both the speed and particular direction of an object’s movement. Acceleration in units per second per second, and force, including the force of gravity, are part of understanding cloud physics and other parts of the atmosphere. The earth’s rotation adds centripetal (inward-pulling) and centrifugal (outward-pushing) forces. In addition, the Coriolis effect causes atmospheric masses to sway with relation to the ground, as viewed from above. Northern Hemisphere air masses swerve right, and Southern appear to go left; thus, cyclones spin clockwise north of the equator, and counterclockwise south of it.

Thermodynamics.The study of thermodynamics focuses on the relationship between energy and work and the transfer of energy between systems. One aspect of thermodynamics in atmospheric physics is thermal equilibrium, in which a transfer of heat has occurred between two systems to the point at which there is no longer an exchange of energy; the systems’ temperatures are now the same. Another thermal effect of the atmosphere is that it makes water change states. Water can be liquid, gas, solid, or plasma. The discipline of thermodynamics supplies the necessary formulae and conceptual mathematics to study the phase changes of water, such as how vapor transforms into water or ice. An understanding of thermodynamics also helps physicists to calculate the condensation of vapor, which must occur faster than evaporation in order to form liquid water. These are just two basic examples of the applications of thermodynamics to atmospheric physics.

Atmospheric Zones.The earth’s atmosphere is considered to be about three hundred miles thick, though gravity concentrates most of it to within ten miles of the surface of the earth. The atmosphere also does not end at a particular height. Rather, it continuously thins until it has merged with space. It is helpful to understand the earth’s atmosphere by dividing it into different zones.

• The thermosphere is the highest layer of the earth’s atmosphere. Temperature within it increases with altitude, because a smaller amount of gas is absorbing a large amount of solar radiation. The top portion of the thermosphere, right before space begins, is called the exosphere. The exosphere extends from about four hundred miles high up to a thousand miles or more, depending on solar activity. The second part of the thermosphere is the ionosphere, which stretches up from approximately fifty miles high from the earth’s surface. The ionosphere contains a large amount of ions—the sun’s radiation is so powerful here that it breaks electrons free, producing ions out of molecules—which, when impacted by solar wind, cause aurora light displays. The ionosphere is also where NASA’s space shuttle orbits; radio waves are reflected here, making radio communication possible.

• Below the thermosphere is the mesosphere, the coldest layer of earth’s atmosphere, stretching from about thirty miles to fifty miles high. The air here, while thin, is thick enough that it can burn up meteoroids and cause meteor showers to be visible. The mesosphere is too high for airplanes and weather balloons, but too low for orbital satellites.

• The next layer, the stratosphere, stretches between approximately ten and thirty miles high. The stratosphere’s temperature increases the higher it goes, due to heating ozone. This is where “good” ozone acts as a protective shield, keeping away ultraviolet rays from the sun.

• The zone closest to the earth’s surface and where most of the earth’s atmosphere is concentrated is called the troposphere. This is where weather happens. The sun warms the earth’s surface, then the surface warms air masses, which rise as cool air masses fall. The troposphere cools with gains in altitude; its vertical instability is one major cause of weather. Between the troposphere and the stratosphere is the tropopause, which acts as a soft boundary. High-altitude jet streams race along this level. The height of the tropopause is affected by vertical instability from convection, meaning that it lies closer to the cold of the poles than the warmth of the equator. The tropopause is very dry and caps the weather zone; just above it is the best place for commercial airplanes to fly.

Applications Past and Present

Remote Sensing.On October 4, 1957, the Soviet Union launched Sputnik I, the first earth-orbiting, human-built satellite; this started the space race of the 1950s and 1960s. Since the launch of Sputnik I, satellites have come a long way. Remote sensor satellites, basically platforms carrying sensors that orbit the earth and send back data, have become so common that debris associated with those no longer functioning—space junk—is a navigational hazard. Even so, remote sensing has revolutionized communications, intelligence gathering, weather forecasting, and atmospheric research. NOAA, the National Oceanic and Aeronautic Administration, operates satellites that track storms, gather ocean temperatures, take innumerable data readings, and make these readings available around the clock to scientists in the field. Every satellite reports to an earth base, where scientists receive the data, process it, and make it available to others. The US Office of Satellite Products and Operations, OSPO, can provide data on such topics as atmospheric profiles, rain, clouds, wind, ozone, aerosols, and the radiation budget.

The satellites that remote sensing technology rides on are platforms that exist in the harsh conditions of space. Their data gathering cameras or other sensors are dependent on power systems and antennae, and none of it can be maintained once the unit is in orbit. Even when a satellite is performing well, it can be thrown off course by gravitational forces and will lose altitude over time. Correcting for position is often done with gas canisters; the usable life of the satellite may depend less on its advanced technology than on how many gas canisters can be packed onto the platform. Despite this difficulty, satellites provide information that cannot be obtained in other ways. Scientists continually work toward the improvement of satellite technology so that more accurate and detailed data, about the weather and other atmospheric concerns, can be provided.

Meteorology.Meteorology, the study of the phenomena in the atmosphere collectively known as weather—changes in air pressure, moisture, temperature, and wind direction—has been part of atmospheric physics since long before the satellite age. Since antiquity, for example, people with rheumatism have been said to feel pressure changes in their joints. “Weather” in the atmospheric physics community is a fluid term, but an important focus. Weather can be defined as air flow patterns miles long, occurring in the troposphere over the course of days rather than weeks. Meteorologists can track weather patterns, given that they show some regularity over months and in annual cycles. However, weather can still be random on any given day and generally remains unpredictable. Beyond meteorology, other fields of study and their corresponding data sets, such as ocean temperature change, impinge on atmospheric research. Atmospheric physics is part of the overall study of climate change. The field therefore must keep advancing so that civilization-impacting effects—such as droughts, hurricanes, and the effects of climate change—can be predicted and appropriate planning can take place.

Flight.As society demands increasingly fuel-efficient airplanes and effective military aircraft, aerodynamic design depends on increasing knowledge of atmospheric physics. Wings of aircraft “fly” because the speeding up of airflow over the top surface of a certain shape creates a difference in air pressure between the top and bottom of the surface. Orville and Wilbur Wright were the first test this concept when their Flyer rose in the air above Kitty Hawk, North Carolina, in 1903. Pilots know that air pressure differences and motions cause winds and turbulence, requiring skill and experience to negotiate. Aircraft must withstand greater atmospheric temperatures the faster they fly. The space shuttle Columbia, for example, was destroyed in 2003 by hot gases that penetrated its exterior in one spot that was under intense pressure during reentry through the atmosphere. Such challenges concerning altitude and speed are a significant part of atmospheric physics’ importance to aircraft design.

Acoustics.Sound waves are one type of atmospheric wave that physicists study. Acoustics, the effect of sound in a certain space, affects modern living in a number of ways. One is noise control. In an increasingly noisy world, the design and manufacture of materials that can, for example, separate highway noise from homes or appliance noise from one apartment to the next, are important. Acoustics affects architecture in transmitting sound as well. Public arenas, such as concert and lecture halls, represent Lagrangian parcels, each with their own needs. Acoustics is also important in industrial and military diagnostics, as high powered engines can be tested for imperfections by listening for a rattling sound. Finally, the study of acoustics is needed for the ever-better transmission of communications over wireless areas, as these areas steadily grow.

Climate Change.Weather occurs over large spaces and in short amounts of time. Climate, however, occurs across the planet over decades and centuries. The study of climate change is a relatively new field, in the sense that its effects are more pressing than they have been in past centuries. The United States Environmental Protection Agency (EPA) reported that the first decade of the twenty-first century had a planet-wide average temperature rise of 1.4 degrees Fahrenheit. The summer of 2012 saw widespread drought in the United States’ grain-growing regions, resulting in record-high prices for corn and soybeans. These staple crops are used for more than just food; ethanol, certain plastics, and other products are made with corn and soy. Climate change has also caused storms to rage worldwide and with increased ferocity. Atmospheric carbon dioxide and other greenhouse gases are widely held to be causes of global warming. The study of atmospheric physics, therefore, is used in most efforts made by industry or government to address climate change and to secure the well-being of future generations.

Energy Balance and Thermal Equilibrium. The first law of thermodynamics states that in a closed system, such as the atmosphere, energy can never be created nor destroyed, but it does transition from one form to another. Additionally, the amount of work done by a system is equal to the amount of energy available. According to this law, the atmosphere will heat or cool internal to itself to the degree equal to heat supplied by the sun, minus work done by the atmosphere. That work is the radiation of excess heat back out into space. If the earth simply absorbed heat from the sun, it would be like Venus, a boiling ball. Instead, the earth’s atmosphere accepts the sun’s shortwave radiation and emits longwave, or infrared radiation (IR). A portion of shortwave radiation, known as albedo, is reflected off the ground and off clouds. Clouds play an important role in the earth’s thermal equilibrium because they are highly reflective. Heat bouncing off the ground can sometimes hit the bottoms of clouds. When this happens, the clouds will still reflect albedo, but this time back to the earth’s surface. In addition to this complexity, aerosol particles help form clouds by providing places for water vapor to coagulate. They also absorb IR from anywhere and scatter shortwave solar rays. Aerosols’ effect on thermal equilibrium has been documented throughout history in association with volcanic ash emitted from massive eruptions. However, the full effects of aerosols are believed to be more complex than scientists yet know, since the amounts and kinds of aerosols in a given atmospheric parcel are often impacted by human activity.

To achieve equilibrium, incoming solar energy is distributed throughout the atmosphere as the earth turns. Outgoing longwave radiation (OLR) is complicated by the greenhouse effect, in which the troposphere turns half the OLR back to the earth’s surface to bounce again. The troposphere is a heat sink—meaning that it absorbs heat—that both drives atmospheric circulation and acts as a blanket to trap heat and make life on the earth possible. As atmospheric chemical and water vapor levels change, atmospheric physicists are needed to determine the effects this will have.

Social Context and Future Prospects

Climate Change.The issue of climate change is often approached from two areas: forcings and feedbacks. Forcings are what impact or initiate change, whether anthropogenic (caused by human activity) or natural. Aerosols are considered forcing agents. Some types of aerosols initiate atmospheric cooling, while others initiate warming. Other forcings include anthropogenic changes in the atmosphere’s composition, changes in land use, volcanic eruptions, alterations in solar output, and long-term changes of the earth’s orbital parameters. Though research into these effects has been conducted since the beginning of the twenty-first century, much more needs to be done.

The second area that scientists watch closely is called feedback. Feedbacks are the results of forcings. They are either positive, by increasing warming, or negative, by decreasing warming. Feedbacks, which have been studied more than forcings, come from water vapor, albedo, atmospheric lapse-rate (the rate at which temperature decreases upward), and clouds. For example, an increase in greenhouse gases may increase the amount of bright, low-level clouds. This, in turn, leads to less absorption of the sun’s rays and thus an increase solar radiation hitting the earth. The Intergovernmental Panel on Climate Change (IPCC) has called for research into clouds as a top priority. Studies of precipitation feedback have been in the earliest stages of mathematical modeling; models vary so widely that more atmospheric researchers who focus in cloud physics and precipitation are needed.

Communications.Storms in the ionosphere disrupt radio, electronic navigation, and GPS (Global Positioning Systems). Modern living has become increasingly dependent on electronic navigation and GPS. Satellites that serve cell phone communications depend on electronic navigation to stay balanced, stay in orbit, and keep away from space junk. Large-scale agriculture has become more dependent on tractor-mounted GPS to plant and maintain crops. Better knowledge of the earth’s ionosphere will matter more and more as human reliance on technology, whether large or small, continues to increase into the future.

Air Quality.More people live in densely-populated areas than ever before. As the world’s population grows, concerns about urban air quality also grow. Ground-level ozone is one pollutant associated with industries and automobiles. Another pollutant is the particular matter of urban atmospheric aerosols, such as dust and ash. Particulate matter is one of the most harmful pollutants for people. While scientists have been working to lower ozone-creating emissions, controlling dust is also important. Atmospheric physicists who understand the motion of particles at the very lowest levels of the troposphere are needed for this purpose.

Bibliography

"Atmospheric Physics." ECMWF, www.ecmwf.int/en/research/modelling-and-prediction/atmospheric-physics. Accessed 25 Sept. 2023.

Dessler, Andrew E., and Edward A. Parson. The Science and Politics of Global Climate Change: A Guide to the Debate. Cambridge UP, 2010.

Frederick, John E. Principles of Atmospheric Science. Jones & Bartlett, 2008.

Houghton, John T. The Physics of Atmospheres. 3rd ed., Cambridge UP, 2002.

NASA Goddard Institute for Space Studies. National Aeronautics and Space Administration, 21 Sept. 2023, www.giss.nasa.gov/. Accessed 25 Sept. 2023.

Spencer, Roy. "Global Warming. "Roy Spencer, Accessed 25 Sept. 2023.

About the Author

Amanda R. Jones has an MA from Virginia Tech and a PhD in English from the University of Virginia. She has written several articles for EBSCO and has published in the Children’s Literature Association Quarterly.