15.2 Energy in the Atmosphere

Lesson Objectives

• Define energy.
• Describe solar energy.
• State how heat moves through the atmosphere.
• Describe how solar energy varies across Earth’s surface.
• Explain the greenhouse effect.

Vocabulary

• electromagnetic spectrum
• energy
• greenhouse effect
• greenhouse gas
• infrared light
• photon
• ultraviolet (UV) light
• visible light

Introduction

Picture yourself sitting by the campfire in Figure below. You and your friends are using the fire to heat soup in a pot. As the Sun goes down, the air gets chilly. You move closer to the fire. Heat from the fire warms you. Light from the fire allows you to see your friends.

These campers can feel and see the energy of their campfire.

What Is Energy?

What explains all of these events? The answer can be summed up in one word: energy. Energy is defined as the ability to do work. Doing anything takes energy. A campfire obviously has energy. You can feel its heat and see its light.

Forms of Energy

Heat and light are forms of energy. Other forms are chemical and electrical energy. Energy can’t be created or destroyed. It can change form. For example, a piece of wood has chemical energy stored in its molecules. When the wood burns, the chemical energy changes to heat and light energy.

Movement of Energy

Energy can move from one place to another. It can travel through space or matter. That’s why you can feel the heat of a campfire and see its light. These forms of energy travel from the campfire to you.

Energy from the Sun

Almost all energy on Earth comes from the Sun. The Sun’s energy heats the planet and the air around it. Sunlight also powers photosynthesis and life on Earth.

Photons of Energy

The Sun gives off energy in tiny packets called photons. Photons travel in waves. Figure below models a wave of light. Notice the wavelength in the figure. Waves with shorter wavelengths have more energy.

This curve models a wave. Based on this figure, how would you define wavelength?

Electromagnetic Spectrum

Energy from the Sun has a wide range of wavelengths. The total range of energy is called the electromagnetic spectrum. You can see it in Figure below.

Compare the wavelengths of radio waves and gamma rays. Which type of wave has more energy?

Visible light is the only light that humans can see. Different wavelengths of visible light appear as different colors. Radio waves have the longest wavelengths. They also have the least amount of energy. Infrared light has wavelengths too long for humans to see, but we can feel them as heat. The atmosphere absorbs the infrared light. Ultraviolet (UV) light is in wavelengths too short for humans to see. The most energetic UV light is harmful to life. The atmosphere absorbs most of this UV light from the Sun. Gamma rays have the highest energy and they are the most damaging rays. Fortunately, gamma rays don’t penetrate Earth’s atmosphere.

How Energy Moves Through the Atmosphere

Energy travels through space or material. Heat energy is transferred in three ways: radiation, conduction, and convection.

Radiation

Radiation is the transfer of energy by waves. Energy can travel as waves through air or empty space. The Sun's energy travels through space by radiation. After sunlight heats the planet's surface, some heat radiates back into the atmosphere.

Conduction

In conduction, heat is transferred from molecule to molecule by contact. Warmer molecules vibrate faster than cooler ones. They bump into the cooler molecules. When they do they transfer some of their energy. Conduction happens mainly in the lower atmosphere. Can you explain why?

Convection

Convection is the transfer of heat by a current. Convection happens in a liquid or a gas. Air near the ground is warmed by heat radiating from Earth's surface. The warm air is less dense, so it rises. As it rises, it cools. The cool air is dense, so it sinks to the surface. This creates a convection current, like the one in Figure below. Convection is the most important way that heat travels in the atmosphere.

Convection currents are the main way that heat moves through the atmosphere. Why does warm air rise?

Energy and Latitude

Different parts of Earth’s surface receive different amounts of sunlight. You can see this in Figure below. The Sun’s rays strike Earth’s surface most directly at the equator. This focuses the rays on a small area. Near the poles, the Sun’s rays strike the surface at a slant. This spreads the rays over a wide area. The more focused the rays are, the more energy an area receives and the warmer it is.

The lowest latitudes get the most energy from the Sun. The highest latitudes get the least.

How do the differences in energy striking different latitudes affect Earth? The planet is much warmer at the equator than at the poles. In the atmosphere, the differences in heat energy cause winds and weather. On the surface, the differences cause ocean currents. Can you explain how?

The Greenhouse Effect

When sunlight heats Earth’s surface, some of the heat radiates back into the atmosphere. Some of this heat is absorbed by gases in the atmosphere. This is the greenhouse effect, and it helps to keep Earth warm. The greenhouse effect allows Earth to have temperatures that can support life.

Gases that absorb heat in the atmosphere are called greenhouse gases. They include carbon dioxide and water vapor. Human actions have increased the levels of greenhouse gases in the atmosphere. This is shown in Figure below. The added gases have caused a greater greenhouse effect. How do you think this affects Earth’s temperature?

Human actions have increased the natural greenhouse effect.

Lesson Summary

• Energy is the ability to do work. Heat and light are forms of energy. Energy can change form. It can also move from place to place.
• Earth gets its energy from the Sun. The Sun gives off photons of energy that travel in waves. All the wavelengths of the Sun’s energy make up the electromagnetic (EM) spectrum.
• Energy moves in three ways. By radiation, it travels in waves across space. By conduction, it moves between molecules that are in contact. By convection, it moves in a current through a liquid or gas.
• Energy from the Sun is more focused at the equator than the poles. Differences in energy by latitude cause winds and weather.
• Greenhouse gases in the atmosphere absorb heat. This is called the greenhouse effect and it makes the planet warmer. Human actions have increased the greenhouse effect.

Lesson Review Questions

Recall

1. Define energy. List three forms of energy.

2. Describe the electromagnetic spectrum.

3. How is wavelength related to the energy of light?

4. What is the greenhouse effect?

5. List two greenhouse gases.

Apply Concepts

6. Look at Figure above. Apply lesson concepts to explain three ways that heat from the campfire can travel.

Think Critically

7. Why is Earth colder at the poles than the equator?

8. Explain how human actions have increased the greenhouse effect.

Points to Consider

Energy from the Sun heats the air in Earth’s atmosphere. You might predict that air temperature would increase steadily with altitude. After all, the higher you go, the closer you are to the Sun. But it’s not that simple.

• Besides the Sun, what might heat up the atmosphere?
• How do you think air temperature might change with altitude?