Sometimes the term air is used as if it were a specific gas, but it is not. Rather, air is a mixture of many discrete gases, each with its own physical properties, in which varying quantities of tiny solid and liquid particles are suspended.

Major Components

The composition of air is not constant; it varies from time to time and from place to place. However, if the water vapor, dust, and other variable components were removed from the atmosphere, we would find that its makeup is very stable worldwide up to an altitude of about 80 kilometers (50 miles).

As you can see in the figure below ▼, two gases—nitrogen and oxygen—make up  percent of the volume of clean, dry air. Although these gases are the most plentiful components of air and of great significance to life on Earth, they are of minor importance in affecting weather phenomena. The remaining 1 percent of dry air is mostly the inert gas argon (0.93 percent) plus very small concentrations of a number of other gases.

Carbon Dioxide (CO2)

Carbon dioxide, although present in only minute amounts (0.0420 percent, or 420 parts per million [ppm]), is nevertheless an important constituent of air. Carbon dioxide is of great interest to meteorologists because it is an efficient absorber of energy emitted by Earth and thus influences the heating of the atmosphere. Although the proportion of carbon dioxide in the atmosphere is relatively uniform, its percentage has been rising steadily for over  years. The figure below ▼ is a graph called the Keeling Curve that shows the growth in atmospheric CO2 over time. This record represents the longest-running measurement of atmospheric CO2 in the world. In 1958, American chemist Charles Keeling established infrared gas analyzers on the summit of Mauna Loa to measure atmospheric CO2 concentrations. On its first day of operation, the analyzers recorded an atmospheric concentration of 313 ppm CO2. Measurements for 2023 indicated a record-high concentration of 419.3 ppm CO2. Other sources of data, such as those collected at the South Pole and Alaska, radiosonde measurements, and orbiting satellite measurements, independently confirm the global increase in atmospheric CO2 as demonstrated in the Keeling Curve. 

Much of this rise is attributed to the burning of large quantities of fossil fuels, such as coal, oil, and natural gas. Some of this additional carbon dioxide is absorbed by the ocean or is used by plants, but about 45 percent remains in the atmosphere. Estimates project that by sometime in the second half of the twenty-first century, levels will be twice as high as the pre-industrial level.

Atmospheric scientists agree that increased carbon dioxide concentrations have contributed to a warming of Earth’s atmosphere over the past several decades and will continue to do so in the decades to come. The magnitude of such temperature changes depends on the quantities of CO2 contributed by human activities in the years ahead. The role of carbon dioxide and other “greenhouse gases” in the atmosphere and their current and projected effects on climate are examined later this semester.

Variable Components

Air also includes many gases and particles whose quantities vary significantly at different times and places. Important examples include water vapor, dust particles, and ozone. Although usually present in small percentages, they can have significant effects on weather and climate.

Water Vapor

You are probably familiar with the term humidity from reading or watching weather reports. Humidity is a reference to water vapor in the air. As you will learn in later this semester, there are several ways to express humidity. The amount of water vapor in the air varies considerably, from practically none at all up to about 4 percent by volume. Why is such a small fraction of the atmosphere so significant? The fact that water vapor is the source of all clouds and precipitation would be enough to explain its importance. However, water vapor has other roles. Like carbon dioxide, water vapor absorbs heat given off by Earth as well as some solar energy. It is, therefore, important when we examine the heating of the atmosphere.

When water changes from one state to another, it absorbs or releases heat. This energy is termed latent heat, which means “hidden heat.” As we shall see in later chapters, water vapor in the atmosphere transports this latent heat from one region to another, and it is the energy source that helps drive many storms.

Aerosols

The movements of the atmosphere are sufficient to keep a large quantity of solid and liquid particles suspended within it. Although visible dust sometimes clouds the sky, these relatively large particles are too heavy to stay in the air very long. Many other particles are microscopic and remain suspended for considerable periods of time. They may originate from many sources, both natural and human-made, and include sea salts from breaking waves, fine soil blown into the air, smoke and soot from fires, pollen and microorganisms lifted by the wind, ash and dust from volcanic eruptions, and more ▼. Collectively, these tiny solid and liquid particles are called aerosols.

From a meteorological standpoint, aerosols, often invisible particles, can be significant. First, many act as surfaces on which water vapor can condense, an important function in the formation of clouds and fog. Second, aerosols can absorb, reflect, and scatter incoming solar radiation. Thus, when an air-pollution episode is occurring or when ash fills the sky following a volcanic eruption, the amount of sunlight reaching Earth’s surface can be measurably reduced. Finally, aerosols contribute to an optical phenomenon we have all observed—the varied hues of red and orange at sunrise and sunset.

Ozone

Another important component of the atmosphere is ozone, which is a form of oxygen that combines three oxygen atoms into each molecule (O3). Ozone is not the same as oxygen we breathe, which has two atoms per molecule (O2). There is very little ozone in the atmosphere, and its distribution is not uniform. It is concentrated between 10 and 50 kilometers (6 and 31 miles) above the surface, in a layer called the stratosphere.

In this altitude range, oxygen molecules (O2) are split into single atoms of oxygen (O) when they absorb ultraviolet radiation emitted by the Sun. Ozone is then created when a single atom of oxygen (O) and a molecule of oxygen (O2) collide. This must happen in the presence of a third, neutral molecule that acts as a catalyst by allowing the reaction to take place without itself being consumed in the process. Ozone is concentrated in the 10- to  50-kilometer (6- to 31-mile) height range because a crucial balance exists there: The ultraviolet radiation from the Sun is sufficient to produce single atoms of oxygen, and there are enough gas molecules to bring about the required collisions.

The presence of the ozone layer in our atmosphere is crucial to those of us who dwell on Earth. The reason is that ozone absorbs much of the potentially harmful ultraviolet (UV) radiation from the Sun. If ozone did not filter a great deal of the ultraviolet radiation, and if the Sun’s UV rays reached the surface of Earth undiminished, our planet would be uninhabitable for most life as we know it. Thus, anything that reduces the atmosphere’s naturally occurring ozone could affect the well-being of life on Earth. Just such a problem exists and is described in the next section.

Although naturally occurring ozone in the stratosphere is critical to life on Earth, ozone is regarded as a pollutant when produced at ground level because it can damage vegetation and can be harmful to human health. Ozone is a major component in a noxious mixture of gases and particles called photochemical smog, which forms when strong sunlight triggers reactions among pollutants from car exhaust and industrial sources.

Ozone Depletion: A Global Issue

Although stratospheric ozone is 10 to 50 kilometers (6 to 31 miles) above Earth’s surface, it is vulnerable to human activities. Chemicals produced by people are breaking up ozone molecules in the stratosphere, weakening our shield against UV rays. This loss of ozone is a serious global-scale environmental problem. Measurements over the past three decades confirm that ozone depletion is occurring worldwide and is especially pronounced above Earth’s poles. You can see this effect over the South Pole in the figure below ▼.

Over the past 70 years, people have unintentionally placed the ozone layer in jeopardy by polluting the atmosphere. The most significant of the offending chemicals are known as chlorofluorocarbons (CFCs for short). Over the decades, many uses were developed for CFCs: They were used as coolants for air-conditioning and refrigeration equipment, cleaning solvents for electronic components, and propellants for aerosol sprays, and they were used in the production of certain plastic foams.

Because CFCs are practically inert (that is, not chemically active) in the lower atmosphere, some of these gases gradually make their way to the ozone layer, where sunlight separates the chemicals into their constituent atoms. The chlorine atoms released this way break up some of the ozone molecules.

Because ozone filters out most of the UV radiation from the Sun, a decrease in its concentration permits more of these harmful wavelengths to reach Earth’s surface. The most serious threat to human health is an increased risk of skin cancer. An increase in damaging UV radiation can also impair the human immune system as well as promote cataracts, a clouding of the eye lens that reduces vision and may cause blindness if not treated.

In response to this problem, an international agreement known as the Montreal Protocol was developed under the sponsorship of the United Nations to eliminate the production and use of CFCs. More than 190 nations eventually ratified the treaty. Although strong action was taken, CFC levels in the atmosphere will not drop rapidly. Once in the atmosphere, CFC molecules can take many years to reach the ozone layer, and once there, they can remain active for decades. Not until between 2060 and 2075 is the abundance of ozone-depleting gases projected to fall to values that existed before the ozone hole began to form in the 1980s.

• Air is a mixture of many discrete gases, and its composition varies from time to time and from place to place. If water vapor, dust, and other variable components of the atmosphere are removed, clean, dry air is composed almost entirely of nitrogen (N2) and oxygen (O2). Atmospheric carbon dioxide (CO2), although present only in minute amounts, is important because it absorbs heat radiated by Earth and, thus, helps keep the atmosphere warm.

• Among the variable components of air, water vapor is important because it is the source of all clouds and precipitation. Like carbon dioxide, water vapor can absorb heat emitted by Earth. When water changes from one state to another, it absorbs or releases heat. In the atmosphere, water vapor transports this latent (“hidden”) heat from place to place; latent heat provides the energy that helps drive many storms.

• Aerosols are tiny solid and liquid particles that are important because they may act as surfaces on which water vapor can condense and are also absorbers and reflectors of incoming solar radiation.

• Ozone, a form of oxygen that combines three oxygen atoms into each molecule (O3), is concentrated in the 10- to 50-kilometer (6- to 31-mile) height range in the atmosphere. This gas is important to life because of its ability to absorb potentially harmful ultraviolet radiation from the Sun.

• aerosols: Tiny solid and liquid particles suspended in the atmosphere.

• air: A mixture of many discrete gases, of which nitrogen and oxygen are most abundant and in which varying quantities of tiny solid and liquid particles are suspended.

• Keeling curve: A daily record of global atmospheric carbon dioxide concentrations, continuously recorded since 1958 at Mauna Loa Observatory in Hawaii. Named after its creator, Charles David Keeling (1928–2005).

• ozone: A molecule of oxygen that contains three oxygen atoms.