Energy in the Atmosphere
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C HAPTER
Chapter 1. Energy in the Atmosphere
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Energy in the Atmosphere
Lesson Objectives
•
•
•
•
•
Describe how energy is transmitted.
Describe the Earth’s heat budget and what happens to the Sun’s energy.
Discuss the importance of convection in the atmosphere.
Describe how a planet’s heat budget can be balanced.
Describe the greenhouse effect and why it is so important for life on Earth.
Vocabulary
•
•
•
•
•
•
•
•
•
albedo
electromagnetic waves
greenhouse effect
insolation
insulation
latent heat
reflection
specific heat
temperature
Introduction
Wind, precipitation, warming, and cooling depend on how much energy is in the atmosphere and where that energy
is located. Much more energy from the Sun reaches low latitudes (nearer the equator) than high latitudes (nearer the
poles). These differences in insolation — the amount of solar radiation that reaches a given area in a given time –
cause the winds, affect climate, and drive ocean currents. Heat is held in the atmosphere by greenhouse gases.
Energy, Temperature, and Heat
Energy
Energy travels through space or material. This is obvious when you stand near a fire and feel its warmth or when
you pick up the handle of a metal pot even though the handle is not sitting directly on the hot stove. Invisible energy
waves can travel through air, glass, and even the vacuum of outer space. These waves have electrical and magnetic
properties, so they are called electromagnetic waves. The transfer of energy from one object to another through
electromagnetic waves is known as radiation.
Different wavelengths of energy create different types of electromagnetic waves ( Figure 1.1).
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• The wavelengths humans can see are known as "visible light." These wavelengths appear to us as the colors
of the rainbow. What objects can you think of that radiate visible light? Two include the Sun and a light bulb.
• The longest wavelengths of visible light appear red. Infrared wavelengths are longer than visible red. Snakes
can see infrared energy. We feel infrared energy as heat.
• Wavelengths that are shorter than violet are called ultraviolet.
FIGURE 1.1
The electromagnetic spectrum;
short
wavelengths are the fastest with the highest energy.
Can you think of some objects that appear to radiate visible light, but actually do not? The moon and the planets do
not emit light of their own; they reflect the light of the Sun. Reflection is when light (or another wave) bounces back
from a surface. Albedo is a measure of how well a surface reflects light. A surface with high albedo reflects a large
percentage of light. A snow field has high albedo.
One important fact to remember is that energy cannot be created or destroyed – it can only be changed from one
form to another. This is such a fundamental fact of nature that it is a law: the law of conservation of energy.
In photosynthesis, for example, plants convert solar energy into chemical energy that they can use. They do not
create new energy. When energy is transformed, some nearly always becomes heat. Heat transfers between materials
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Chapter 1. Energy in the Atmosphere
easily, from warmer objects to cooler ones. If no more heat is added, eventually all of a material will reach the same
temperature.
Temperature
Temperature is a measure of how fast the atoms in a material are vibrating. High temperature particles vibrate
faster than low temperature particles. Rapidly vibrating atoms smash together, which generates heat. As a material
cools down, the atoms vibrate more slowly and collide less frequently. As a result, they emit less heat. What is the
difference between heat and temperature?
• Temperature measures how fast a material’s atoms are vibrating.
• Heat measures the material’s total energy.
Which has higher heat and which has higher temperature: a candle flame or a bathtub full of hot water?
• The flame has higher temperature, but less heat, because the hot region is very small.
• The bathtub has lower temperature but contains much more heat because it has many more vibrating atoms.
The bathtub has greater total energy.
Heat
Heat is taken in or released when an object changes state, or changes from a gas to a liquid, or a liquid to a solid.
This heat is called latent heat. When a substance changes state, latent heat is released or absorbed. A substance
that is changing its state of matter does not change temperature. All of the energy that is released or absorbed goes
toward changing the material’s state.
For example, imagine a pot of boiling water on a stove burner: that water is at 100°C (212°F). If you increase the
temperature of the burner, more heat enters the water. The water remains at its boiling temperature, but the additional
energy goes into changing the water from liquid to gas. With more heat the water evaporates more rapidly. When
water changes from a liquid to a gas it takes in heat. Since evaporation takes in heat, this is called evaporative
cooling. Evaporative cooling is an inexpensive way to cool homes in hot, dry areas.
Substances also differ in their specific heat, the amount of energy needed to raise the temperature of one gram of
the material by 1.0°C (1.8°F). Water has a very high specific heat, which means it takes a lot of energy to change the
temperature of water. Let’s compare a puddle and asphalt, for example. If you are walking barefoot on a sunny day,
which would you rather walk across, the shallow puddle or an asphalt parking lot? Because of its high specific heat,
the water stays cooler than the asphalt, even though it receives the same amount of solar radiation.
Energy From the Sun
Most of the energy that reaches the Earth’s surface comes from the Sun ( Figure 1.2). About 44% of solar radiation
is in the visible light wavelengths, but the Sun also emits infrared, ultraviolet, and other wavelengths.
When viewed together, all of the wavelengths of visible light appear white. But a prism or water droplets can break
the white light into different wavelengths so that separate colors appear ( Figure 1.3).
Of the solar energy that reaches the outer atmosphere, 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.
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FIGURE 1.2
An image of the sun taken by the SOHO
spacecraft. The sensor is picking up only
the 17.1 nm wavelength, in the ultraviolet
wavelengths.
FIGURE 1.3
A prism breaks apart white light.
• UVA: the lowest energy, travels through the atmosphere to the ground.
The remaining solar radiation is the longest wavelength, infrared. Most objects radiate infrared energy, which we
feel as heat ( Figure 1.4).
Some of the wavelengths of solar radiation traveling through the atmosphere may be lost because they are absorbed
by various gases ( Figure 1.5). Ozone completely removes UVC, most UVB, and some UVA from incoming
sunlight. O2 , CO2 and H2 O also filter out some wavelengths.
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Chapter 1. Energy in the Atmosphere
FIGURE 1.4
An infrared sensor detects different
amounts of heat radiating from a dog.
FIGURE 1.5
Atmospheric gases filter some wavelengths of incoming solar energy.
Yel-
low 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.
Solar Radiation on Earth
Different parts of the Earth receive different amounts of solar radiation. Which part of the planet receives the most
insolation? The Sun’s rays strike the surface most directly at the equator.
Different areas also receive different amounts of sunlight in different seasons. What causes the seasons? The seasons
are caused by the direction Earth’s axis is pointing relative to the Sun.
The Earth revolves around the Sun once each year and spins on its axis of rotation once each day. This axis of
rotation is tilted 23.5° relative to its plane of orbit around the Sun. The axis of rotation is pointed toward Polaris, the
North Star. As the Earth orbits the Sun, the tilt of Earth’s axis stays lined up with the North Star.
Northern Hemisphere Summer
The North Pole is tilted towards the Sun and the Sun’s rays strike the Northern Hemisphere more directly in summer
( Figure 1.6). At the summer solstice, June 21 or 22, the Sun’s rays hit the Earth most directly along the Tropic of
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Cancer (23.5°N); that is, the angle of incidence of the sun’s rays there is zero (the angle of incidence is the deviation
in the angle of an incoming ray from straight on). When it is summer solstice in the Northern Hemisphere, it is
winter solstice in the Southern Hemisphere.
FIGURE 1.6
Summer solstice in the Northern Hemisphere.
Northern Hemisphere Winter
Winter solstice for the Northern Hemisphere happens on December 21 or 22. The tilt of Earth’s axis points away
from the Sun ( Figure 1.7). Light from the Sun is spread out over a larger area, so that area isn’t heated as much.
With fewer daylight hours in winter, there is also less time for the Sun to warm the area. When it is winter in the
Northern Hemisphere, it is summer in the Southern Hemisphere.
FIGURE 1.7
In Southern Hemisphere summer, the
Sun’s rays directly strike the Tropic of
Capricorn (23.5°S). Sunlight is spread
across a large area near the South Pole.
No sunlight reaches the North Pole.
Equinox
Halfway between the two solstices, the Sun’s rays shine most directly at the equator, called an "equinox" ( Figure
1.8). The daylight and nighttime hours are exactly equal on an equinox. The autumnal equinox happens on
September 22 or 23 and the vernal or spring equinox happens March 21 or 22 in the Northern Hemisphere.
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Chapter 1. Energy in the Atmosphere
FIGURE 1.8
Where
equinox,
sunlight
reaches
summer
on
solstice,
spring
vernal
equinox, and winter solstice. The time is
9:00 p.m. Universal Time, at Greenwich,
England.
Heat Transfer in the Atmosphere
Heat moves in the atmosphere the same way it moves through the solid Earth (Plate Tectonics chapter) or another
medium. What follows is a review of the way heat flows, but applied to the atmosphere.
Radiation is the transfer of energy between two objects by electromagnetic waves. Heat radiates from the ground
into the lower atmosphere.
In conduction, heat moves from areas of more heat to areas of less heat by direct contact. Warmer molecules
vibrate rapidly and collide with other nearby molecules, transferring their energy. In the atmosphere, conduction
is more effective at lower altitudes where air density is higher; transfers heat upward to where the molecules are
spread further apart or transfers heat laterally from a warmer to a cooler spot, where the molecules are moving less
vigorously.
Heat transfer by movement of heated materials is called convection. Heat that radiates from the ground initiates
convection cells in the atmosphere ( Figure 1.9).
Heat at Earth’s Surface
About half of the solar radiation that strikes the top of the atmosphere is filtered out before it reaches the ground.
This energy can be absorbed by atmospheric gases, reflected by clouds, or scattered. Scattering occurs when a light
wave strikes a particle and bounces off in some other direction.
About 3% of the energy that strikes the ground is reflected back into the atmosphere. The rest is absorbed by rocks,
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FIGURE 1.9
Thermal convection where the heat
source is at the bottom and there is a
ceiling at the top.
soil, and water and then radiated back into the air as heat. These infrared wavelengths can only be seen by infrared
sensors.
The basics of Earth’s annual heat budget are described in this video (4b): http://www.youtube.com/watch?v=mjj2i
3hNQF0 (5:40).
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/1572
Because solar energy continually enters Earth’s atmosphere and ground surface, is the planet getting hotter? The
answer is no (although the next section contains an exception) because energy from Earth escapes into space through
the top of the atmosphere. If the amount that exits is equal to the amount that comes in, then average global
temperature stays the same. This means that the planet’s heat budget is in balance. What happens if more energy
comes in than goes out? If more energy goes out than comes in?
To say that the Earth’s heat budget is balanced ignores an important point. The amount of incoming solar energy is
different at different latitudes ( Figure 1.10). Where do you think the most solar energy ends up and why? Where
does the least solar energy end up and why? See Table 1.1
TABLE 1.1: The Amount of Incoming Solar Energy
Equatorial Region
Polar Regions
Day Length
Nearly the same all
year
Night 6 months
Sun Angle
High
Solar Radiation
High
Albedo
Low
Low
Low
High
Note: Colder temperatures mean more ice and snow cover the ground, making albedo relatively high.
FIGURE 1.10
The maximum land surface temperature
of the Earth, showing a roughly gradual
temperature gradient from the low to the
high latitudes.
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Chapter 1. Energy in the Atmosphere
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 radiation 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 ( Figure 1.11). Greenhouse gases are the component of the
atmosphere that moderate Earth’s temperatures.
FIGURE 1.11
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 include CO2 , H2 O, methane, O3 , nitrous oxides (NO and NO2 ), and chlorofluorocarbons (CFCs).
All are a normal part of the atmosphere except CFCs. Table 1.2 shows how each greenhouse gas naturally enters the
atmosphere.
TABLE 1.2: Greenhouse Gas Entering the Atmosphere
Greenhouse Gas
Carbon dioxide
Methane
Nitrous oxide
Ozone
Chlorofluorocarbons
Where It Comes From
Respiration, volcanic eruptions, decomposition of plant
material; burning of fossil fuels
Decomposition of plant material under some conditions, biochemical reactions in stomachs
Produced by bacteria
Atmospheric processes
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 has significantly raised the levels of many of greenhouse gases in the atmosphere. Methane levels
are about 2 1/2 times higher as a result of human activity. Carbon dioxide has increased more than 35%. CFCs have
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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 affect climate and
weather the world over.
This PowerPoint review, Atmospheric Energy and Global Temperatures, looks at the movement of energy through
the atmosphere (6a): http://www.youtube.com/watch?v=p6xMF_FFUU0 (8:17).
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/1576
Lesson Summary
• All materials contain energy, which can radiate through space as electromagnetic waves. The wavelengths
of energy that come from the Sun include visible light, which appears white but can be broken up into many
colors.
• Ultraviolet waves are very high energy. The highest energy UV, UVC and some UVB, gets filtered out of
incoming sunlight by ozone.
• More solar energy reaches the low latitudes and the redistribution of heat by convection drives the planet’s air
currents.
Review Questions
1. What is the difference between temperature and heat?
2. Give a complete description of these three categories of energy relative to each other in terms of their wavelengths
and energy: infrared, visible light, and ultraviolet.
3. Why do the polar regions have high albedo?
4. Give an example of the saying “energy can’t be created or destroyed.”
5. Describe what happens to the temperature of a pot of water and to the state of the water as the dial on the stove is
changed from no heat to the highest heat.
6. Describe where the Sun is relative to the Earth on summer solstice, autumnal equinox, winter solstice and spring
equinox. How much sunlight does the North Pole get on June 21? How much does the South Pole get on that same
day?
7. What is the difference between conduction and convection?
8. What is a planet’s heat budget? Is Earth’s heat budget balanced or not?
9. On a map of average annual temperature, why are the lower latitudes so much warmer than the higher latitudes?
10. Why is carbon dioxide the most important greenhouse gas?
11. How does the amount of greenhouse gases in the atmosphere affect the atmosphere’s temperature?
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Chapter 1. Energy in the Atmosphere
Points to Consider
• How does the difference in solar radiation that reaches the lower and upper latitudes explain the way the
atmosphere circulates?
• How does the atmosphere protect life on Earth from harmful radiation and from extreme temperatures?
• What would the consequences be if the Earth’s overall heat budget were not balanced?
References
1. Zachary Wilson. CK-12 Foundation . CC BY-NC 3.0
2. Courtesy of SOHO and NASA. http://sohowww.nascom.nasa.gov/gallery/SolarCorona/eit028.html . Public
Domain
3. User:Kelvinsong/Wikimedia Commons. . Public Domain
4. Courtesy of NASA/IPAC. http://commons.wikimedia.org/wiki/File:Infrared_dog.jpg . Public Domain
5. Courtesy of the US Government. http://commons.wikimedia.org/wiki/File:Radiation_Spectrum.png . Public
Domain
6. Sam McCabe. CK-12 Foundation . CC BY-NC 3.0
7. Sam McCabe. CK-12 Foundation . CC BY-NC 3.0
8. Tom Ruen, Full Sky Observatory, User:Saperaud/Wikimedia Commons. http://commons.wikimedia.org/wik
i/File:Seasonearth.png . Public Domain
9. User:Harroschmeling/De.Wikipedia. http://commons.wikimedia.org/wiki/File:Convection-snapshot.gif . Public Domain
10. Courtesy of Jesse Allen and Robert Simmon/NASA, using MODIS data. http://earthobservatory.nasa.gov/IOT
D/view.php?id=77779 . Public Domain
11. Jodi So. CK-12 Foundation . CC BY-NC 3.0
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