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.

Vocabulary

• air pressure
• altitude
• atmosphere
• greenhouse gas
• humidity
• ozone
• respiration
• ultraviolet (UV) radiation
• water vapor
• weather

Introduction

Earth’s atmosphere is a thin blanket of gases and tiny particles — together 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.

Significance of the Atmosphere

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.

Atmospheric Gases Are Indispensable for Life on 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. The chemical reaction for photosynthesis is:

6CO₂ + 6H₂O + solar energy → C₆H₁₂O₆ (sugar) + 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 + useable energy

How is respiration similar to and different from photosynthesis? They are approximately the reverse of each other. In photosynthesis, CO₂ is converted to O₂ and in respiration, O₂ is converted to CO₂ (Figure below).

The Atmosphere is a Crucial Part of the Water Cycle

As part of the hydrologic cycle, which was detailed in the Earth’s Fresh Water chapter, 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.

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 (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 Atmosphere Keeps Earth’s Temperature Moderate

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 (Figure below). 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.

Atmospheric Gases Provide the Substance for Waves to Travel Through

The atmosphere is made of gases that take up space and transmit energy. Sound waves are among the types of energy that 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 because there would be no atmosphere to hold it up. Explosions in movies about space should be silent.

Composition of Air

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 (Figure below).

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.

Where around the globe is mean atmospheric water vapor higher and where is it lower and why (Figure below)? 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. Some particles are pollutants, which are discussed in the Human Actions and the Atmosphere chapter.

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 (Figure below).

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, or air pressure. Why are we not crushed? The molecules inside our bodies are pushing outward to compensate. Air pressure is felt from all directions, not just from above.

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

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.
• Although 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 and temperature.

Review Questions

1. What gas is used and what gas is created during photosynthesis? What gas is used and what gas is created during respiration?

2. Describe two reasons why photosynthesis is important.

3. 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?

4. What important role do greenhouse gases play in the atmosphere?

5. Why do your ears pop when you are in an airplane and the plane descends for a landing?

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?