High School Earth Science/The Atmosphere

Earth's atmosphere is a thin blanket of gases and tiny particles—together called air. Without air, the Earth would just be another lifeless rock orbiting the Sun. Although we are rarely aware of it, air surrounds us. We are most aware of air when it moves, creating wind. Like all gases, air takes up space. These gases that make up our air are packed closer together near the Earth's surface than at higher elevations.

All living things need some of the gases in air for life support. In particular, all organisms rely on oxygen for respiration—even plants. Plants also require carbon dioxide in the air for photosynthesis. All weather happens in the atmosphere. The atmosphere has many other important roles as well. These include moderating Earth's temperatures and protecting living things from the Sun's most harmful rays.

Lesson Objectives

Describe the importance of the atmosphere to our planet and its life.
Outline the role of the atmosphere in the water cycle.
List the major components of the atmosphere and know their functions.
Describe how atmospheric pressure changes with altitude.

Significance of the Atmosphere

Without the atmosphere, planet Earth would be much more like the Moon than like the planet we live on today. The Earth's atmosphere, along with the abundant liquid water on the Earth's surface, are 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 many reasons we are lucky to have an atmosphere.

Atmospheric Gases are Indispensable for Life on Earth

Without the atmosphere, Earth would be lifeless. Carbon dioxide (CO₂) and oxygen (O₂) are the most important gases for living organisms. CO2 is vital for use by plants in photosynthesis, in which plants use CO2 and water to convert the Sun's energy into food energy. This food energy is in the form of the sugar glucose (C₆H₁₂O₆). Plants also produce O₂. Photosynthesis is responsible for nearly all of the oxygen currently found in the atmosphere.

The chemical reaction for photosynthesis is:

6CO₂ + 6H₂O + solar energy → C₆H₁₂O₆ + 6O₂

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.

The chemical reaction for respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + usable energy

Notice that respiration looks like photosynthesis in reverse. In photosynthesis, CO₂ is converted to O₂ and in respiration, O₂ is converted to CO₂.

The Atmosphere is a Crucial Part of the Water Cycle

Water moves from the atmosphere onto the land, into soil, through organisms, to the oceans and back into the atmosphere in any order. This movement of water is called the water cycle or hydrologic cycle (Figure 15.1).

Water changes from a liquid to a gas by evaporation. Water vapor is the name for water when it is a gas. When the Sun's energy evaporates water from the ocean surface or from lakes, streams, or puddles on land, it becomes water vapor. The water vapor remains in the atmosphere until it condenses to become tiny droplets of liquid. The tiny droplets may come together to create precipitation, like rain and snow. Snow may become part of the ice in a glacier, where it may remain for hundreds or thousands of years. Eventually, the snow or ice will melt to form liquid water. A water droplet that falls as rain, could become part of a stream or a lake, or it could sink into the ground and become part of groundwater.

At the surface, the water will eventually undergo evaporation and reenter the atmosphere. If the water is taken up by a plant and then evaporates from the plant, the process is called evapotranspiration.

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. It is the changes 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.

The physical and chemical changes that happen on Earth's surface due to precipitation, wind and reactions with the gases in our atmosphere are called weathering. Weathering alters rocks and minerals and shapes landforms at the Earth's surface. Without weathering, Earth's surface would not change much at all. For example, the Moon has no atmosphere, water or winds, so it does not have weathering. The footprints that astronauts made on the Moon decades ago will remain there until someone (human or alien) smooths them out! You would only need to spend a few minutes at the beach to know that Earth's surface is changing all the time.

Ozone in the Upper Atmosphere Makes Life on Earth Possible

Ozone is a molecule composed of three oxygen atoms, (O₃). Ozone in the upper atmosphere absorbs high energy ultraviolet radiation (UV) 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 Atmosphere Keeps Earth's Temperature Moderate

Our atmosphere keeps Earth's temperatures within an acceptable range; the difference between the very coldest places on Earth and the very hottest is about 150°C (270°F). Without our atmosphere, Earth's temperatures would be frigid at night and scorching during the day. Our daily temperatures would resemble those seen on the Moon, where the temperature range is 310°C (560°F) because there is no atmosphere. Greenhouse gases trap heat in the atmosphere. Important greenhouse gases include carbon dioxide, methane, water vapor and ozone.

Atmospheric Gases Provide the Substance for Waves to Travel Through

The atmosphere is made of gases, mostly nitrogen and oxygen. Even though you can't see them, gases take up space and can transmit energy. Sound waves are among the types of energy that can travel though the atmosphere. Without an atmosphere, we could not hear a single sound. Earth would be as silent as outer space. Of course, no insect, bird or airplane would be able to fly since there would be no atmosphere to hold it up!

Composition of Air

Table 15.1: Concentrations of Atmospheric Gases
Gas	Symbol	Concentration (%)
Nitrogen	N₂	78.08
Oxygen	O₂	20.95
Argon	Ar	0.93
Neon	Ne	0.0018
Helium	He	0.005
Hydrogen	H	0.00006
Xenon	Xe	0.000009
Water vapor	H₂O	0 to 4
Carbon dioxide	CO₂	0.038
Methane	CH₄	0.00017
Krypton	Kr	0.00011
Nitrous oxide	N₂O	0.0005
Ozone	O₃	0.000004
Particles (dust, soot)		0.000001
Chlorofluorocarbons (CFCs)		0.00000002

Air is made almost entirely of two gases. The most common gas is nitrogen, and the second most common gas is oxygen (O₂). Nitrogen and oxygen together make up 99% of the planet's atmosphere. All other gases together make up the remaining 1%. Although each of these trace gases are only found in tiny quantities, many such as ozone, serve important roles for the planet and its life. One very important minor gas is carbon dioxide, CO₂, which is essential for photosynthesis and is also a very important greenhouse gas (Table (15.1).

In nature, air is never completely dry. Up to 4% of the volume of air can be water vapor. Humidity is the amount of water vapor in air. The humidity of the air 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 very high, with a winter day in Phoenix, Arizona where humidity is very low. When the air is very humid, it feels heavy or sticky. Your hair might get really curly or frizzy when it is very humid outside. Most people feel more comfortable when the air is dry. The percentage of water vapor in our atmosphere is listed with a wide range of values in the table above because air can be both very humid or dry.

Argon, neon, helium, xenon, and krypton are noble gases. They are colorless, odorless, tasteless, and they do not become part of ordinary chemical reactions because they are chemically inert. The noble gases simply exist in the atmosphere.

Some of what is in the atmosphere is not a gas. Particles of dust, soil, fecal matter, metals, salt, smoke, ash and other solids make up a small percentage of the atmosphere. This percentage is variable, as anyone who has spent a windy day in the desert knows (Figure 15.2). Particles are important because they provide starting points (or nuclei) for water vapor to condense on, which then forms raindrops. Some particles are pollutants, which are discussed in the chapter on human actions and the atmosphere.

Pressure and Density

The atmosphere has different properties at different elevations above sea level, or altitudes. The density of the atmosphere (the number of molecules in a given volume) decreases the higher you go. This is why explorers who climb tall mountains, like Mt. Everest, have to set up camp at different elevations to let their bodies get used to the changes. What the atmosphere is made of, or the composition of the first 100 kilometers of the atmosphere stays the same with altitude, with one exception: the ozone layer at about 20-40 kilometers above the Earth. In the ozone layer, there is a greater concentration of ozone than in other portions of the atmosphere (Figure 15.3).

The molecules in gases are able to move freely. If no force acted on a gas at all, it would just escape or spread out forever. Gravity pulls gas molecules in towards the Earth's surface, pulling stronger closer to sea level. This means that atmospheric gases are 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 weight of the atmosphere on a person's shoulders is equal to more than one ton. The force of the air weighing down over a unit of area is known as its atmospheric pressure, or air pressure. People and animals are not crushed because molecules inside our bodies are pushing outward to compensate. Air pressure is felt from all directions, not just from above.

The atmosphere has lower atmospheric pressure and is less dense at higher altitudes. There is less pull from gravity and there is less gas to push down from above. Without as much weight above them, the gases expand, so the air is lighter. For each 6 km (3.7 mile) increase in altitude, the air pressure decreases by half. At 5,500 meters (18,000 feet) above sea level, the air pressure is just less than half of what it is at sea level. This means that the weight of the air on a person’s shoulders at that altitude is only one-half ton. At a high enough altitude, there is no gas left. The density of the atmosphere at 30 km (19 miles) above sea level is only 1% that of sea level. By 700 km (435 miles) from the planet's surface, the air pressure is almost the same as that in the vacuum of deep space.

If your ears have ever 'popped', you have experienced a change in air pressure. This occurs when you go up or down in altitude quickly, such as flying in an airplane or riding in a car as it goes up or down a mountain. Gas molecules are found inside and outside your ears. When you change altitude quickly, your inner ear keeps the density of molecules at the original altitude. The popping occurs when the air molecules inside your ear suddenly move through a small tube in your ear equalizing the pressure. This sudden rush of air is felt as a popping sensation.

Colder, drier places on Earth usually have higher air pressure, while warmer, more humid places usually have lower air pressure. This happens because large areas of air move up or down by convection. Air pressure also often changes over time, as low and high pressure systems change locations. These phenomena will be discussed when we learn about weather.

Lesson Summary

Without its atmosphere, Earth would be a very different planet. Gases in the atmosphere allow plants to photosynthesize and animals and plants to engage in respiration.
Water vapor, which is an atmospheric gas, is an essential part of the water cycle.
All weather takes place in the atmosphere.
While the amount of gases do not vary relative to each other in the atmosphere, there is one exception: the ozone layer. Ozone in the upper atmosphere protects life from the Sun's high-energy ultraviolet radiation.
Air pressure varies with altitude, temperature and location.

Review Questions

What gas is used and what gas is created during photosynthesis? What gas is used and what gas is created during respiration?
Describe two reasons why photosynthesis is important.
Briefly describe the movement of water through the water cycle.
What is evapotranspiration?
What will happen if the humidity of the atmosphere increases?
Is weathering more effective in a humid or a dry climate?
On an unusual February day in Portland, Oregon, the temperature is 18°C (65°F) and it is dry and sunny. The winter climate in Portland is usually chilly and rainy. How could you explain a warm, dry day in Portland in winter?
What important role do greenhouse gases play in the atmosphere?
Why do your ears pop when you are in an airplane and the plane descends for a landing?
If air pressure at sea level is one ton and at 5,500 m (18,000 feet) is one-half ton, what is air pressure at 11,000 m (36,000 feet)?

Vocabulary

air pressure: The force of air pressing on a given area.
altitude: Distance above sea level.
climate: The long-term average of weather.
condenses: Changes state from a gas to a liquid; in the case of water, from water vapor to liquid water.
evaporation: The change in state of a substance from a liquid to a gas; in the case of water, from liquid water to water vapor.
evapotranspiration: Water loss by plants to the atmosphere.
greenhouse gases: Gases that trap heat in the atmosphere; these include water vapor, carbon dioxide, methane and ozone.
groundwater: Fresh water found beneath the ground surface.
humidity: The amount of water vapor held in the air; usually refers to relative humidity, meaning the amount of water the air holds relative to the total amount it could hold.
noble gases: Gases that usually do not react chemically and have no color, taste, or odor; these are helium, neon, argon, xenon and krypton.
ozone: A molecule made of three oxygen atoms; ozone in the lower atmosphere is a pollutant, but in the upper atmosphere it filters out the sun’s most harmful ultraviolet radiation.
photosynthesis: The process in which plants produce simple sugars (food energy) from solar energy; the process of photosynthesis changes carbon dioxide to oxygen.
precipitation: Condensed moisture including rain, sleet, hail, snow, frost or dew.
respiration: The process in which animals and plants use oxygen and produce carbon dioxide; in respiration, organisms convert sugar into food energy they can use.
water vapor: The gas form of water.
weather: The temporary state of the atmosphere in a region; the weather in a location depends on the air temperature, humidity, precipitation, wind and other features of the atmosphere.

Points to Consider

How would Earth be different if it did not have an atmosphere?
What are the most important components of the atmosphere?
How does the atmosphere vary with altitude?

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