APCC - Characteristics of the Atmosphere (Lesson)

Characteristics of the Atmosphere

Energy from the Sun

Most of the energy that reaches the Earth's surface comes from the Sun (see Figure). About 44% of solar radiation is in the visible light wavelengths, but the Sun also emits infrared, ultraviolet, and other wavelengths.

Ultraviolet

Of the solar energy that reaches the outer atmosphere, ultraviolet (UV) wavelengths have the greatest energy. Only about 7% of solar radiation is in the UV wavelengths. The three types are:

• UVC: the highest energy ultraviolet, does not reach the planet's surface at all.
• UVB: the second-highest energy- is also mostly stopped in the atmosphere.
• UVA: the lowest energy, travels through the atmosphere to the ground.

Infrared

The remaining solar radiation is the longest wavelength, infrared. Most objects radiate infrared energy, which we feel as heat.

Some of the wavelengths of solar radiation traveling through the atmosphere may be lost because they are absorbed by various gases (see Figure ). Ozone completely removes UVC, most UVB, and some UVA from incoming sunlight. O₂, CO₂, and H₂O also filter out some wavelengths.

 

Atmospheric gases filter some wavelengths of incoming solar energy. Yellow shows the energy that reaches the top of the atmosphere. Red shows the wavelengths that reach sea level. Ozone filters out the shortest wavelength UV and oxygen filters out most infrared.

Did life evolve to match the atmosphere or is the fit just a coincidence?

Life as we know it would not survive if there were no ozone layer to protect it from high-energy ultraviolet radiation. Most life needs oxygen to survive. Nitrogen is also needed, albeit in a different form from that found in the atmosphere. Greenhouse gases keep the temperature moderate so that organisms can live around the planet. Life evolved to match the conditions that were available and to some extent changed the atmosphere to suit its needs.

Composition of Air

Several properties of the atmosphere change with altitude, but the composition of the natural gases does not. The proportions of gases in the atmosphere are everywhere the same, with one exception. 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. This is called the ozone layer.

Nitrogen and Oxygen

Nitrogen and oxygen together make up 99% of the planet's atmosphere. Nitrogen makes up the bulk of the atmosphere but is not involved in geological or biological processes in its gaseous form. Nitrogen-fixing is described in "Concept Life on Earth." Oxygen is extremely important because it is needed by animals for respiration. The rest of the gases are minor components but sometimes are very important (see Figure below).

Nitrogen and oxygen make up 99% of the atmosphere; carbon dioxide is a very important minor component.

 

Water Vapor

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. When humidity is high, water vapor makes up only about 4% of the atmosphere.

Where around the globe is mean atmospheric water vapor higher and where is it lower (see Figure )? Why? 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 the air temperature is lower.

 

 

Mean winter atmospheric water vapor in the Northern Hemisphere when temperature and humidity are lower than they would be in summer.

Greenhouse Gases

Remember that greenhouse gases trap heat in the atmosphere. Important natural greenhouse gases include carbon dioxide, methane, water vapor, and ozone. CFCs and some other man-made compounds are also greenhouse gases.

 Be sure to watch this video "When CFCs Meet Ozone" about CFCs and the ozone layer. Links to an external site.

Particulates

Some of what is in the atmosphere are not gas. Particles of dust, soil, fecal matter, metals, salt, smoke, ash, and other solids make up a small percentage of the atmosphere and are called particulates. Particles provide starting points (or nuclei) for water vapor to condense on and form raindrops. Some particles are pollutants. 

What Is the Atmosphere?

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. Earth's atmosphere, along with the abundant liquid water at Earth's surface, is the key to our planet's unique place in the solar system. Much of what makes Earth exceptional depends on the atmosphere. For example, all living things need some of the gases in the air for life support. Without an atmosphere, Earth would likely be just another lifeless rock.

Let's consider some of the reasons we are lucky to have an atmosphere:

1. Gases 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?

2. Photosynthesis

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:

3. Respiration

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:

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.

 

 Chlorophyll indicates the presence of photosynthesizing plants as does the vegetation index.

 

4. Crucial Part of the Water Cycle

As part of the hydrologic cycle, water spends a lot of time in the atmosphere, mostly as water vapor. The atmosphere is an important reservoir for water.

5. Ozone 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 highest concentration of ozone is in the ozone layer in the lower stratosphere.

6. Keeps Earth's Temperature Moderate

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

7. Provides 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 through the atmosphere. Without an atmosphere, we could not hear a single sound. Earth would be as silent as outer space (explosions in movies about space should be silent). Of course, no insect, bird, or airplane would be able to fly, because there would be no atmosphere to hold it up.

 

 

How does the atmosphere resemble a greenhouse?

To extend the growing season, many farmers use greenhouses. A greenhouse traps heat so that days that might be too cool for a growing plant can be made to be just right. Similar to a greenhouse, greenhouse gases in the atmosphere keep Earth warm.

The Greenhouse Effect

The exception to Earth's temperature being in balance is caused by greenhouse gases. But first, the role of greenhouse gases in the atmosphere must be explained.

Greenhouse gases warm the atmosphere by trapping heat. Some of the heat that radiates out from the ground is trapped by greenhouse gases in the troposphere. Like a blanket on a sleeping person, greenhouse gases act as insulation for the planet. The warming of the atmosphere because of insulation by greenhouse gases is called the greenhouse effect (see Figure ). Greenhouse gases are the component of the atmosphere that moderate Earth's temperatures.

 

  

The Earth's heat budget shows the amount of energy coming into and going out of the Earth's system and the importance of the greenhouse effect. The numbers are the amount of energy that is found in one square meter of that location.

Greenhouse Gases

Greenhouse gases include CO₂, H₂O, methane, O₃, nitrous oxides (NO and NO₂), and chlorofluorocarbons (CFCs). All are a normal part of the atmosphere except CFCs. The table shows how each greenhouse gas naturally enters the atmosphere.

Greenhouse Gas	Where It Comes From
Carbon dioxide	Respiration, volcanic eruptions, decomposition of plant material; burning of fossil fuels
Methane	Decomposition of plant material under some conditions, biochemical reactions in stomachs
Nitrous oxide	Produced by bacteria
Ozone	Atmospheric processes
Chlorofluorocarbons	Not naturally occurring; made by humans

 

Different greenhouse gases have different abilities to trap heat. For example, one methane molecule traps 23 times as much heat as one CO₂ molecule. One CFC-12 molecule (a type of CFC) traps 10,600 times as much heat as one CO₂. Still, CO₂ is a very important greenhouse gas because it is much more abundant in the atmosphere.

Human Activity and Greenhouse Gas Levels

Human activity has significantly raised the levels of many greenhouse gases in the atmosphere. Methane levels are about 2 1/2 times higher as a result of human activity. Carbon dioxide has increased by more than 35%. CFCs have only recently existed.

What do you think happens as atmospheric greenhouse gas levels increase? More greenhouse gases trap more heat and warm the atmosphere. The increase or decrease of greenhouse gases in the atmosphere affects climate and weather the world over.

 

Be sure to watch the following video from NASA.

 

The Layers of Earth's Atmosphere

 

Earth's atmosphere includes the closest layer, the troposphere, where we live and weather occurs, but also includes outer layers that each has specific characteristics. 

 

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