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16.1: Significance and Composition of the Atmosphere

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    Significance of the Atmosphere

    Earth’s atmosphere is a thin blanket of gases and tiny particles called air. We are most aware of air when it moves and creates wind. All living things need some of the gases in air for life support. Without an atmosphere, Earth would likely be just another lifeless rock. Earth’s atmosphere, along with the abundant liquid water at Earth’s surface, are the keys to our planet’s unique place in the solar system. Much of what makes Earth exceptional depends on the atmosphere. Let’s consider some of the reasons we are lucky to have an atmosphere.

    Sunlight filtering through the clouds in the sky over an empty field.
    Figure \(\PageIndex{1}\): The atmosphere makes life possible on Earth.

    Indispensable for Life of Earth

    Without the atmosphere, Earth would look a lot more like the Moon. Atmospheric gases, especially carbon dioxide (CO2) and oxygen (O2), are extremely important for living organisms. How does the atmosphere make life possible? How does life alter the atmosphere?

    In photosynthesis, plants use CO2 and create O2. Photosynthesis is responsible for nearly all of the oxygen currently found in the atmosphere. By creating oxygen and food, plants have made an environment that is favorable for animals. In respiration, animals use oxygen to convert sugar into food energy they can use. Plants also go through respiration and consume some of the sugars they produce.

    Crucial Part of the Water Cycle

    As part of the hydrologic (water) cycle, water spends a lot of time in the atmosphere, mostly as water vapor. All weather takes place in the atmosphere, virtually all of it in the lower atmosphere. Weather describes what the atmosphere is like at a specific time and place, and may include temperature, wind, and precipitation. Weather is the change we experience from day to day. Climate is the long-term average of weather in a particular spot. Although the weather for a particular winter day in Tucson, Arizona, may include snow, the climate of Tucson is generally warm and dry.

    Clouds over a rocky beach and the ocean
    Figure \(\PageIndex{2}\): Atmospheric motion brings water from the ocean to the land.

    Ozone Layer Makes Life Possible

    Ozone is a molecule composed of three oxygen atoms, (O3). Ozone in the upper atmosphere absorbs high-energy ultraviolet (UV) radiation coming from the Sun. This protects living things on Earth’s surface from the Sun’s most harmful rays. Without ozone for protection, only the simplest life forms would be able to live on Earth.

    The hole in the ozone is larger than the continent of Antarctica.
    Figure \(\PageIndex{3}\): The hole in the ozone layer, shown in purple, is larger than the continent of Antarctica.

    Moderates Earth’s Temperature

    Along with the oceans, the atmosphere keeps Earth’s temperatures within an acceptable range. Greenhouse gases trap heat in the atmosphere so they help to moderate global temperatures. Without an atmosphere with greenhouse gases, Earth’s temperatures would be frigid at night and scorching during the day. Important greenhouse gases include carbon dioxide, methane, water vapor, and ozone.

    Composition of the Atmosphere

    Nitrogen and oxygen together make up 99% of the planet’s atmosphere. The rest of the gases are minor components but sometimes are very important. Humidity is the amount of water vapor in the air. Humidity varies from place to place and season to season. This fact is obvious if you compare a summer day in Atlanta, Georgia, where humidity is high, with a winter day in Phoenix, Arizona, where humidity is low. When the air is very humid, it feels heavy or sticky. Dry air usually feels more comfortable. Higher humidity is found around the equatorial regions because air temperatures are higher and warm air can hold more moisture than cooler air. Of course, humidity is lower near the polar regions because air temperature is lower. Some of what is in the atmosphere is not gas. Particles of dust, soil, fecal matter, metals, salt, smoke, ash, and other solids make up a small percentage of the atmosphere. Particles provide starting points (or nuclei) for water vapor to condense on and form raindrops.

    The atmosphere is 79 percent nitrogen, 20 percent oxygen, and 1 percent other gases.
    Figure \(\PageIndex{4}\): Pie chart of the composition of the atmosphere. Table \(\PageIndex{1}\) below lists the percent breakdown of atmospheric gases.
    Table \(\PageIndex{1}\): Atmospheric gases as a percent by volume in the atmosphere
    Gas Symbol Volume (%)
    Nitrogen N2 78.0840
    Oxygen O2 20.9460
    Argon Ar 0.9340
    Carbon Dioxide CO2 0.0417
    Neon Ne 0.0018
    Helium He 0.0005
    Hydrogen H2 < 0.0001

    Atmospheric Pressure and Density

    The atmosphere has different properties at different elevations above sea level or altitudes. The air density (the number of molecules in a given volume) decreases with increasing altitude. This is why people who climb tall mountains, such as Mt. Everest, have to set up camp at different elevations to let their bodies get used to the decreased air. Why does air density decrease with altitude? Gravity pulls the gas molecules towards Earth’s center. The pull of gravity is stronger closer to the center at sea level. Air is denser at sea level where the gravitational pull is greater. Gases at sea level are also compressed by the weight of the atmosphere above them. The force of the air weighing down over a unit of area is known as its atmospheric pressure. Atmospheric pressure can be measured in pascals (abbreviated Pa) where 101,325 Pa (or 101.325 kPa) is about 14.7 pounds per square inch (psi). The reason why we are not crushed by this weight is because the molecules inside our bodies are pushing outward to compensate. Atmospheric pressure is felt from all directions, not just from above.

    Diagram Explaining how Atmospheric Pressure Changes with Altitude . At sea level (0 m), the atmospheric pressure is slightly over 100 kPa. At 2000 m, the atmospheric pressure is around 80 kPa. At 5000 m, the atmospheric pressure is around 55 kPa. At 8000 m, the atmospheric pressure is around 36 kPa. At 10,000 m, the atmospheric pressure is around 27 kPa.
    Figure \(\PageIndex{5}\): Graph illustrating how atmospheric pressure (kPa) changes with altitude (m). As altitude increases, pressure decreases.

    At higher altitudes the atmospheric pressure is lower, and the air is less dense than at higher altitudes. If your ears have ever “popped,” you have experienced a change in air pressure. Gas molecules are found inside and outside your ears. When you change altitude quickly, like when an airplane is descending, your inner ear keeps the density of molecules at the original altitude. Eventually, the air molecules inside your ear suddenly move through a small tube in your ear to equalize the pressure. This sudden rush of air is felt as a popping sensation.

    Although the density of the atmosphere changes with altitude, the composition stays the same with altitude, with one exception. In the ozone layer, at about 20 km to 40 km above the surface, there is a greater concentration of ozone molecules than in other portions of the atmosphere.

    Air density or the number of moleculeA greater number of molecules depicted close to the surface of the Earth, with a decreasing number of molecules at higher elevations.
    Figure \(\PageIndex{6}\): The number of molecules in the atmosphere decreases with height.

    References

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