Atmospheric sciences

Summary

Atmospheric sciences include the fields of physics and chemistry and the study of the composition and dynamics of the layers of air that constitute the atmosphere. Related topics include climatic processes, circulation patterns, chemical and particulate deposition, greenhouse gases, oceanic temperatures, the interaction between the atmosphere and the ocean, the ozone layer, precipitation patterns and amounts, climate change, air pollution, aerosol composition, atmospheric chemistry, modeling of pollutants both indoors and outdoors, and anthropogenic alteration of land surfaces that in turn affect conditions within the ever-changing atmosphere.

Definition and Basic Principles

Atmospheric sciences is the study of various aspects of the nature of the atmosphere, including its origin, layered structure, density, and temperature variation with height. Also included are natural variations and alterations associated with anthropogenic impacts. It also focuses on similarities or differences from other atmospheres within the solar system. The present-day atmosphere is in all likelihood quite dissimilar from the original atmosphere. The form and composition of the present-day atmosphere is believed to have developed about 400 million years ago in the late Devonian period of the Paleozoic era when plant life developed on land. This vegetative cover allowed plants to take in carbon dioxide and release oxygen as part of the photosynthesis process.

The atmosphere consists of a mixture of gases that remain in place because of the gravitational attraction of the Earth. Although the atmosphere extends about 6,000 miles above the Earth's surface, the vast proportion of its gases (97 percent) are located in the lower 19 miles. The bulk of the atmosphere consists of nitrogen (78 percent) and oxygen (21 percent). The last 1 percent of the atmosphere contains all the remaining gases, including an inert gas (argon), which accounts for 0.93 percent of the 1 percent, and carbon dioxide (CO₂). This makes up a little less than 0.04 percent. Carbon dioxide has the ability to absorb longwave radiation leaving the Earth and shortwave radiation from the Sun; therefore, any increase in carbon dioxide in the atmosphere has profound implications for global warming.

Background and History

Evangelista Torricelli, an Italian physicist, mathematician, and secretary to Galileo, invented the barometer, which measures barometric pressure, in 1643. The first attempt to explain the circulation of the global atmosphere was made in 1686 by Edmond Halley, an English astronomer and mathematician. In 1735, George Hadley, an English optician, described a pattern of air circulation that became known as a Hadley cell. In 1835, Gustave-Gaspard Coriolis, a French engineer and mathematician, analyzed the movement of air on a rotating Earth, a pattern that became known as the Coriolis effect. In 1856, William Ferrel, an American meteorologist, developed a model of hemispheric circulation of the atmosphere that became known as a Ferrel cell. Christophorus Buys Ballot, a Dutch meteorologist, explained the relationship between the distribution of pressure, wind speed, and direction in 1860.

Manned hot-air balloon flights beginning in the mid-nineteenth century facilitated high-level observations of the atmosphere. For example, in 1862, English meteorologist James Glaisher and English pilot Henry Coxwell reached 29,000 feet, at which point Glaisher became unconscious, and Coxwell was partially paralyzed so that he had to move the control valve with his teeth. In 1902, Léon Teisserenc de Bort of France was able to determine that air temperatures begin to level out at 39,000 feet and actually increase at higher elevations. In the twentieth century, additional information about the upper atmosphere became available through radio waves, rocket flights, and satellites.

How It Works

A knowledge of the basic structure and dynamics of the atmosphere is a necessary foundation for understanding applications and practical uses based on atmospheric science.

Layers of the Atmosphere. The heterosphere and the homosphere form the two major subdivisions of the Earth's atmosphere. The uppermost subdivision, the heterosphere, extends from about 50 miles above the Earth's surface to the outer limits of the atmosphere at about 6,000 miles. Nitrogen and oxygen, the heavier elements, are found in the lower layers of the heterosphere, and lighter elements, such as hydrogen and helium, are found at the uppermost layers of the atmosphere. The homosphere, or lowest layer, contains gases that are more uniformly mixed, although their density decreases with height. Some exceptions to this statement occur with the existence of an ozone layer at an altitude of 12 to 31 miles and with variations in concentrations of carbon dioxide, water vapor, and air pollutants closer to the Earth's surface.

The atmosphere can be divided into several zones based on decreasing or increasing temperatures as elevation increases. The lowest zone is the troposphere, where temperatures decrease from sea level up to an altitude of 10 miles in equatorial and tropical regions and up to an altitude of 4 miles at the poles. This lowermost zone holds substantial amounts of water vapor, aerosols that are very small, and light particles that originate from volcanic eruptions, desert surfaces, soot from forest and brush fires, and industrial emissions. Clouds, storms, and weather systems occur in the troposphere.

The tropopause marks the boundary between the troposphere and the next higher layer, the stratosphere, which reaches an altitude of 30 miles above the Earth's surface. Circulation in this layer occurs with strong winds that move from west to east. There is limited circulation between the troposphere and the stratosphere. However, manned balloons, certain types of aircraft (Concorde and the U-2), volcanic eruptions, and nuclear bomb tests are able to fly above the tropopause and enter the stratosphere.

The gases in the stratosphere are generally uniformly mixed, with the major exception of the ozone layer, which is found at an altitude range of 12 to 31 miles above the Earth. This layer is extremely important because it shields life on Earth from the intense and harmful ultraviolet radiation from the Sun. The ozone layer has been diminishing because of the release of chlorofluorocarbons (CFCs), organic compounds containing chlorine, fluorine, and carbon, used as propellants in aerosol sprays and in synthetic chemical compounds used for refrigeration purposes. In 1978, the use of CFCs in aerosol sprays was banned in the United States, but their use continued in some refrigeration systems. Other countries continue to use CFCs, which eventually get into the ozone layer and result in ozone holes of considerable sizes. In 1987, members of the international community took steps to reduce CFC production through the Montreal Protocol. By 2003, the rate of ozone depletion had begun to slow down. This trend continued through the 2020s. At that time, projections were that a near-complete recovery of the ozone layer could happen by approximately 2050.

Although the manufacture and use of CFCs can be controlled, natural events that are detrimental to the ozone layer cannot be prevented. For example, the 1991 eruption of Mount Pinatubo in the Philippines reduced the ozone layer in the midlatitudes by nearly 9 percent.

Temperatures decrease with elevation at the stratopause, where the mesosphere layer begins at about 30 miles and continues to an altitude of about 50 miles. The mesopause at about 50 miles marks the beginning of the thermosphere, where the density of the air is very low and holds minimal amounts of heat. However, even though the atmospheric density is minimal at altitudes above 155 miles, there is enough atmosphere to have a drag effect on spaceships.

Atmospheric Pressure. The gas molecules in the atmosphere exert a pressure due to gravity that amounts to about 15 pounds per square inch on all surfaces at sea level. As the distance from the Earth gets larger, in contrast to the various increases and decreases in atmospheric temperature, atmospheric pressure decreases at an exponential rate. For example, air pressure at sea level varies from about 28.35 to 31.01 inches of mercury, averaging 29.92 inches. The pressure at the top of Mount Everest at 20,029 feet can get as low as 8.86 inches. This means that each inhalation of air at this altitude is about one-third of the pressure at sea level, producing severe shortness of breath.

Earth's Global Energy Balance. The Earth's elliptical orbit about the Sun ranges from 91.5 million miles at perihelion (closest point to the Sun) on January 3 to 94.5 million miles at aphelion (furthest from the Sun) on July 4. Its overall average distance is 93 million miles. The Earth intercepts only a tiny fraction of the total energy output of the Sun. Upon reaching the Earth, part of the incoming radiation is reflected back into space, and part is absorbed by the atmosphere, land, or oceans. Over time, the incoming shortwave solar radiation is balanced by a return to outer space of longwave radiation.

Earth-Moon Differences. Scientists believe that the moon's surface has a large number of craters formed by the impact of meteorites. In contrast, there are relatively few meteorite craters on the Earth, even though, based simply on its size, the Earth is likely to have been hit by as many or even more meteorites than the Moon. This notable difference is attributed to the Earth's atmosphere, which burns up incoming meteorites, particularly small ones (the Moon does not have an atmosphere). Larger meteorites can pass through the Earth's atmosphere, but their impact craters may have been filled in or washed away over millions of years. Only the more recent impacts, such as Meteor Crater in northern Arizona, left a deformity with a diameter of 4,000 feet and a depth of 600 feet.

Air Masses. Different types of air masses within the troposphere, the lowest layer of the atmosphere, can be delineated on the basis of their similarity in temperature, moisture, and, to a certain extent, air pressure. These air masses develop over continental and maritime locations that strongly determine their physical characteristics. For example, an air mass starting in the cold, dry interior portion of a continent develops thermal, moisture, and pressure differences that can be substantially different from an air mass that develops over water. Atmospheric dynamics also allow air masses to modify their characteristics as they move from land to water and vice versa.

Air mass and weather front terminology were developed in Norway during World War I. Norwegian meteorologists were unable to get weather reports from the Atlantic theater of operations. They consequently developed a dense network of weather stations that led to impressive advances in atmospheric modeling that are still being used in the twenty-first century.

The Radiation Budget. The incoming solar energy that reaches the Earth is primarily in the shortwave or visible portion of the electromagnetic spectrum. The Earth's energy balance is attained with about one-third of this incoming energy being reflected back to space. The other two-thirds leave the Earth as outgoing longwave radiation. This balance between incoming and outgoing energy is known as the Earth's radiation budget. Begun at the end of the twentieth century, the National Aeronautics and Space Administration (NASA) program known as Clouds and the Earth's Radiant Energy System (CERES) is designed to measure how much shortwave and longwave radiation leaves the Earth from the top of the atmosphere. This program continued into the 2020s.

Clouds play a very important role in the global radiation balance as they constantly change over time and in type. Some clouds, such as high cirrus clouds found near the top of the troposphere at 40,000 feet, can have a substantial impact on atmospheric warming. Accordingly, the value of CERES is based on its ability to observe if human or natural changes in the atmosphere can be measured, even if they are smaller than large-scale energy variations.

Greenhouse Effect. Selected gases in the lower parts of the atmosphere trap heat and then radiate some of that heat back to Earth. If there was no natural greenhouse effect, the Earth's overall average temperature would be close to 0 degrees Fahrenheit rather than the existing 57 degrees Fahrenheit.

The burning of coal, oil, and gas makes carbon dioxide (CO₂) the major greenhouse gas, accounting for nearly half of the total amount of heat-producing gases in the atmosphere. Before the Industrial Revolution in Great Britain, which began in the mid-eighteenth century, the estimated level of carbon dioxide in the atmosphere was about 280 parts per million by volume (ppmv). Estimates for the natural range of carbon dioxide for the past 650,000 years range from 180 to 300 ppmv. All these values are less than the 391 ppmv recorded in January 2011. In 2013, the Mauna Loa Observatory reported that the global concentration of carbon dioxide in the atmosphere had reached 400 ppmv for the first time in history, and that number was subsequently surpassed by 2016. At that time, carbon dioxide levels had been increasing since 2000 at a rate of 1.9 ppmv each year. The radiative effect of carbon dioxide accounts for about one-half of all the factors that affect global warming. In 2023, carbon dioxide was estimated at 419.3 ppmv with a yearly incremental rise of 2.8 ppm. The oceans of the world are assessed to increased toxicity by 30 percent.

The second most important greenhouse gas is methane (CH₄), which accounts for about 14 percent of all global warming factors. The origin of this gas is attributed to the natural decay of organic matter in wetlands, but anthropogenic activity—rice paddies, manure from farm animals, the decay of bacteria in sewage and landfills, and biomass burning (both natural and human induced)—results in a doubling of the amount of this gas over what would be produced solely by wetland decay.

Chlorofluorocarbons (CFCs) absorb longwave energy (warming effect), but they also have the ability to destroy stratospheric ozone (cooling effect). The warming radiative effect is three times greater than the cooling effect. CFCs account for about 10 percent of all global warming factors. Tropospheric ozone from air pollution, nitrous oxide (N₂O) from motor vehicle exhaust, and bacterial emissions from nitrogen fertilizers account for about 10 percent and 5 percent, respectively, of all global warming factors.

Several kinds of human actions lead to a cooling of the Earth's climate. For example, the burning of fossil fuels results in the release of tropospheric aerosols, which acts to scatter incoming solar radiation back into space, thereby lowering the amount of solar energy that can reach the Earth's surface. These aerosols also lead to the development of low and bright clouds that are quite effective in reflecting solar radiation back into space.

Applications and Products

Atmospheric science is applied in many ways. It is used to help people better understand their global and interplanetary environment and to make it possible for them to live safely and comfortably within that environment. By using the principles of this field, researchers, engineers, and space scientists have developed a vast number of applications. Among the most important are those used to track and predict weather cycles and climate.

Remote Sensing Techniques. Oceans cover about 71 percent of the Earth's surface, which means that large portions of the world do not have weather stations or places where precipitation can be measured with standard rain gauges. To provide more information about precipitation in the equatorial and tropical parts of the world, NASA and the Japan Aerospace Exploration Agency initiated the Tropical Rainfall Monitoring Mission (TRMM) in 1997. The orbit of the TRMM satellite, which lasted until 2015, monitored the Earth between 35 degrees north and 35 degrees south latitude. The goal of the study was to obtain information about the extent of precipitation, along with its intensity and length of occurrence. The major instruments on the satellite were radar to detect rainfall, a passive microwave imager that could acquire data about precipitation intensity and the extent of water vapor, and a scanner that could examine objects in the visible and infrared portions of the electromagnetic spectrum. The goal of data collection was to obtain the necessary climatological information about atmospheric circulation in this portion of the Earth to develop better mathematical models for determining large-scale energy movement and precipitation.

In 2021, a study reviewed the performance of the Dual-frequency Precipitation Radar (DPR), Weather Research and Forecasting (WRF) model, and the Global Precipitation Measurement mission (GPM) regarding retrieval of distribution of raindrops according to their diameters or raindrop size distribution (DSD) under various conditions. The fusion of the GPM and WRF model improved scientists' understanding of rainfall's microphysical traits in different areas.

Geostationary Satellites. Geostationary operational environmental satellites (GOES) enable researchers to view images of the planet from what appears to be a fixed position above the Earth. The satellites are actually circling the globe at a speed that is in step with the Earth's rotation. This means that a satellite at an altitude of 22,200 miles will make one complete revolution in the same twenty-four hours and direction that the Earth is turning above the equator. At this height, the satellite is in a position to view nearly one-half of the planet at any time. On-board instruments can be activated to look for special weather conditions such as hurricanes, flash floods, and tornadoes. On-board instruments are also used to make precipitation estimates during storm events.

Doppler Radar. Doppler radar was first used in England in 1953 to pick up the movement of small storms. The basic principle guiding this type of radar is that back-scattered radiation frequency detected at a certain location changes over time as the target, such as a storm, moves. A transmitter is used to send short but powerful microwave pulses. When a foreign object or target is intercepted, some of the outgoing energy is returned to the transmitter, where a receiver can pick up the signal. An image or echo from the target can then be enlarged and shown on a screen. The target's distance is revealed by the time that elapses between transmission and return. The radar screen cannot only indicate where the precipitation is taking place but also reveal the intensity of the rain by the amount of the echo's brightness. In short, Doppler radar has become a very useful device for determining the location of a storm and the intensity of its precipitation and for obtaining good estimates of the total amount of precipitation.

Responses to Climate Change. Since the 1970s, many scientists have pointed out the possibility that human activity is having more than a short-term impact on the atmosphere and, therefore, on weather and climate. Although much debate continues on the full impact of human activities and greenhouse gas emissions, the atmospheric sciences have led to conferences, United Nations conventions, and agreements among nations on ways that human beings can alter their behavior to halt or at least mitigate the possibility of global climate change. The long-term impact of these agreements remains unknown—as does the overall effect of human activity on weather and climate (the models for which are highly complex). However, the insights contributed by the atmospheric sciences to the overall debate on whether climate change is primarily anthropogenic (human-caused)—and whether global warming is actually taking place—have caused many nations and individuals to modify their attitudes toward human relationships with the global environment, resulting in national and intergovernmental changes in policies concerning carbon emissions, as well as personal decisions ranging from the consumption of “green” building materials to the purchase of vehicles fueled by noncarbon sources of energy.

Careers and Course Work

The study of the physical characteristics of the atmosphere falls within the purview of atmospheric scientists. Aspirants can get jobs as weather analysts in airlines, government sectors, and the reinsurance industry. Those interested in a career in this technical area should recognize that there are several categories of specialization. The major group of specialists are operational meteorologists, who are responsible for weather forecasts. They have to carefully study the temperature, humidity, wind speed, and barometric pressure from a number of weather stations to make daily and long-range forecasts. They use data from weather satellites, radar, special sensors, and observation stations in other locations to make forecasts.

In contrast to meteorologists, who focus on short-term weather forecasts, the study of changes in weather over longer periods of time, such as months, years, and, in some cases, centuries, is handled by climatologists. Other atmospheric scientists concentrate on research. For example, physical meteorologists are concerned with various aspects of the atmosphere, such as its chemical and physical properties, energy transfer, severe storm mechanics, and the spread of air pollutants over urbanized areas. The growing interest in air pollution and water shortages has led to another group of research scientists known as environmental meteorologists.

Given the importance of weather forecasting on a daily basis, operational meteorologists who work in weather stations may work on evenings, weekends, and holidays. Research scientists who are not engaged in weather forecasts may work regular hours.

In 2016, the American Meteorological Society estimated that there were more than one hundred undergraduate and graduate atmospheric science programs in the United States that offered courses in such departments as physics, earth science, environmental science, geography, and geophysics. Entry-level positions usually require a bachelor's degree with at least twenty-four semester hours in courses covering atmospheric science and meteorology. The acquisition of a master's degree enhances the chances of employment and usually means a higher salary and more opportunities for advancement. A doctorate is required only for those who want a research position at a university.

Faster-than-average employment growth for atmospheric scientists was projected into the 2030s. Various universities, such as the University of Miami in Florida, Stony Brook University in New York, and Iowa State University, offered bachelor’s degrees in atmospheric sciences. Various online platforms offered courses in weather and meteorological observation encoding.

Social Context and Future Prospects

Climate change may be caused by both natural internal/external processes in the Earth-Sun system. This can also be a result of human-induced changes in land use and the atmosphere. Article 1 of the United Nations Framework Convention on Climate Change (UNFCCC), which entered into force in March 1994, stated that the term “climate change” should refer to anthropogenic changes that affect the composition of the atmosphere rather than natural causes. These should be referred to as “climate variability.” An example of natural climate variability is the global cooling of about 0.5 degrees Fahrenheit in 1992-1993, which was caused by the 1991 eruption of Mount Pinatubo in the Philippines. The 15 million to 20 million tons of sulfuric acid aerosols ejected into the stratosphere reflected incoming radiation from the sun, thereby creating a cooling effect. Many suggest that the above-normal temperatures experienced in the first decades of the twenty-first century provide evidence of climate change caused by human activity. Based on a variety of techniques that allow scientists to estimate the temperature in previous centuries, the ten warmest years on record have all occurred since 2005. A 2009 article published by the American Geophysical Union suggested that human intervention in Earth systems had reached a point where the Holocene epoch of the past 12,000 years was becoming an Anthropocene epoch in which human systems had become primary Earth systems rather than simply influencing natural systems. In 2019, a report attributed 4.2 million deaths annually worldwide due to exposure to fine particle pollution and around 250,000 deaths due to pollution by tropospheric ozone. Moreover, weather changes could increase the frequency and intensity of extreme weather conditions like heat waves, droughts, and floods, leading to adverse impacts on society.

Numerous observations strongly suggest a continuing warming trend. Snow and ice have retreated from areas such as Mount Kilimanjaro in Tanzania, which at 19,340 feet is the highest mountain in Africa. Glaciated areas in Switzerland also provide evidence of this warming trend. The Intergovernmental Panel on Climate Change (IPCC) computer climate models project a global temperature increase of 35.2 to 39.2 degrees Fahrenheit by the year 2100.

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