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Chapter 2
Warming the Earth and the Atmosphere
Chapter
Outline
Temperature and Heat Transfer
Temperature Scales
Latent Heat - The
Hidden Warmth
Conduction
Convection
Focus on a Special
Topic
Rising Air Cools
and Sinking Air Warms
Radiant Energy
Focus on an Environmental Issue
Sun Burning
and UV Rays
Radiation - Absorption, Emission, and Equilibrium
Selective Absorbers and the
Atmospheric Greenhouse Effect
Enhancement of the
Greenhouse Effect
Warming the Air from Below
Shortwave Radiation Streaming
from the Sun
Earth’s Annual Energy
Balance
Why the Earth Has Seasons
Seasons in the Northern
Hemisphere
Focus on a Special Topic
Space Weather and its Impact
on Earth
Seasons in the Southern
Hemisphere
Local Seasonal Variations
Focus on a Special Topic
Is December 21 Really the
First Day of Winter?
Summary
Key Terms
Questions for Review
Questions for Thought and Exploration
Summary
This chapter begins with definitions
of temperature and heat, and compares the Kelvin, Celsius, and Fahrenheit temperature scales. Heat transfer via the
processes of conduction, convection, latent heat, and radiation are described.
Students will learn, for example, that air is a relatively poor conductor of
heat but can transport energy efficiently over large distances by convection.
The latent heat energy associated with changes of phase of water is shown to be
a very important energy transport mechanism in the atmosphere. A
physical explanation of why rising air cools and sinking air warms is given.
The electromagnetic spectrum and the
physical laws which govern the emission of electromagnetic radiation are reviewed next. This provides sufficient background
for a detailed study of the atmospheric greenhouse effect and the exchange of
energy between the earth's surface, the atmosphere, and space. Students will
see that, because the amounts of energy absorbed and emitted by the earth are
in balance, the earth's average radiative equilibrium temperature varies little
from year to year. Students should understand that the energy the earth absorbs
from the sun consists primarily of short-wave radiation. A focus section
describes the sun's ultraviolet energy, and its damaging effects on living
cells. Essentially all of the energy
emitted by the earth is in the form of
infrared radiation. Selective absorbers in the atmosphere, such as water vapor
and carbon dioxide, absorb some of the earth's infrared radiation and then
radiate a portion of it back to the surface. Because of this effect, the earth's average surface temperature is
appreciably higher than would otherwise be the case. Results from recent
research relating to the effect of increasing concentrations of carbon dioxide
and other greenhouse gases and the effects
of clouds on the earth's energy balance are reviewed.
The final portion of this chapter
demonstrates that variations in the intensity of sunlight reaching the ground
and the length of the day caused by the changing tilt of the earth relative to
the plane of its orbit around the sun that
are the main causes of seasonal variations on the earth. We follow seasonal
changes for a full year in the Northern Hemisphere. By comparison,
seasons in the Southern Hemisphere are six months out of phase and moderated by the larger surface coverage by
oceans.
Teaching
Suggestions, Demonstrations and Visual Aids
1.
Heat a thin iron bar in a flame
(from a Bunsen burner or a propane torch).
Begin by holding the bar fairly close to the end of the bar. Students will see that heat is quickly conducted
through the metal when the instructor is forced to move his or her grip down
the bar. Repeat the demonstration with a
piece of glass tubing or glass rod.
Glass is a poor conductor and the instructor will be able to comfortably
hold the glass just 2 or 3 inches from the tip.
Ask the students if they believe energy is being transported away from
the hot glass and if so, how? Without
heat loss by conduction, the glass will get hotter than the iron bar and the
tip should begin to glow red - a good demonstration of energy transport by
radiation. Faint convection currents in
the air can be made visible if the hot piece of glass is held between an
overhead projector and the projection screen.
Ask the students what they would do to quickly cool a hot object. Many
will suggest blowing on it, an example of forced convection. Someone might suggest plunging the hot object
into water. This makes for a satisfying
end to the demonstration. Evaporating
water can be seen and heard when the hot iron rod is put into the water (the
glass will shatter if placed in the water).
The speed with which the rod is cooled is proof of the large amount of
latent heat energy associated with changes of phase.
2. Ask the students whether they believe
water could be brought to a boil most rapidly in a covered or an uncovered
pot. The question can be answered
experimentally by filling two beakers with equal amounts of water and placing
them on a single hot plate (to insure that energy is supplied to both at equal
rates). It is a good idea to place boiling
stones in the beakers to insure gentle boiling.
Cover one of the beakers with a piece of foil. The covered pot will boil first. Explanation: a portion of the energy added
to uncovered pot is used to evaporate water, not to increase the water's temperature.
3. The concept of equilibria is sometimes
difficult for students to grasp. Place a
glass of water on a table top and ask the students whether they think the
temperature of the water in the glass is warmer, cooler or the same as the
surroundings. Many will say it is the
same. Ask the students whether they
think there is any energy flowing into or out of the glass. With some encouragement, they will recognize
that the water is slowly evaporating and that this represents energy flow out
of the glass. Energy flowing out of the
glass will cause the water's temperature to decrease. Will the water just continue to get colder
and colder until it freezes? No, as soon
as the water's temperature drops below the temperature of the surroundings,
heat will begin to flow into the water.
The rate at which heat flows into the glass will depend on the
temperature difference between the glass and the surroundings. The water temperature will decrease until
energy flowing into the glass balances the loss due to evaporation.
4. Use a lamp with a 150 Watt reflector
bulb to help explain the concept of radiation intensity. Blind-fold a student and hold the lamp at
various distances from the student's back.
Ask the student to judge the distance of the bulb. Use the same lamp to illustrate the concepts
of reflection, albedo, and absorption by measuring the amount of reflected
light from various colored surfaces with a sensitive light meter. The reflectivity of natural surfaces outdoors
could be measured or form the basis for a student or group project.
5. A 200 Watt clear light bulb connected
to a dimmer switch can be used to illustrate how the temperature of an object
affects the amount and type of radiation that the object will emit. Explain that passage of electricity through
the resistive filament heats the filament.
The filament's temperature will increase until it is able to emit energy
at the same rate as it gains energy from the electric current. With the dimmer switch set low, the bulb can
be made to glow a dull red. At low
temperatures, the bulb emits low-intensity, longwave radiation. As the setting on the dimmer switch is
increased, the color of the filament will turn orange, yellow and then white as
increasing amounts of shortwave radiation are emitted. The intensity of the radiation will increase
dramatically.
6. Many students don't understand that a
colored object appears that way because it reflects or scatters light of that
color. The object isn't emitting visible
light (ask the student whether they would see the object if all the lights in
the room were turned off). Some students
have the misconception that a green object reflects all colors but green. Similarly it is important that students
understand that a red or green filter transmits red or green light. Put a red and a green (or blue) filter on an
overhead projector and draw a hypothetical filter transmission curve. Put the two filters together and show that no
light is transmitted. Ask the students
what happens to the light that is not transmitted by the filter.
7. Thought experiment to illustrate the
magnitude of latent heat of evaporation/condensation: Ask students to think
about taking a hot shower. Their body
temperature is ~ 100oF; the water temperature is > 100oF;
the air temperature in the room is ~75oF. Why, then, do you feel cold when you step,
dripping wet, out of the shower?
8. Begin the
lecture by drawing an ellipse on the blackboard with the sun positioned much
closer to one end of the
ellipse. On the other end of the ellipse, closest to the sun, make a dot for
the earth and label it, "January and winter." Then label the other
end "July and summer." Act confused and ask, "Wait a minute, is that correct?" Usually this is enough to start an
interesting discussion on what causes the seasons.
9. Explain the seasons by shining a fairly broad, collimated
beam of light onto a globe in a darkened room. Begin
by showing the earth with no tilt, then increase the tilt to 23.5°. Finally
increase the tilt to 45°. Explain how the
change in tilt would influence the average temperature measured in July and
January in the Northern Hemisphere.
Using the globe or
drawings, the students should understand whether they would need to look to the
south or north of overhead to see the sun at noon from different locations on the
earth at different times of year. They
should also understand whether it is necessary to look east, northeast, or
southeast to see the sun rise.
10. The
attenuation of light as is passes through a
scattering medium can be demonstrated by placing a photodetector at one end of an aquarium full of
water and a light source at the other end. Then begin to add milk in small
but measurable amounts. The signal at the detector will decay
exponentially with the amount of milk added. The decay will depart from the
exponential law when enough milk is added that appreciable multiple scattering begins to occur. The effect of
an absorbing medium can be demonstrated if India ink (diluted with water
if necessary) is used in place of milk.
Student
Projects
1. Items #2, 3, 9 and 10 above could form the basis of
student projects or experiments.
2. There are several interesting experiments in Chapter 4
(Heat) in Hands-On
Meteorology.
3. Solar irradiance (energy per unit time
per unit area) at the ground can be measured relatively easily. Begin with a rectangular piece of aluminum a
few inches on a side and 3/8 or 1/2 inch thick.
Drill a hole in one side so that a thermometer can be inserted into the
middle of the block. Paint one of the
two surfaces with flat black paint.
Position the block in a piece of styrofoam insulation so that the
painted surface faces outward and is flush with the styrofoam surface. Insert the thermometer into the side of the
block. Orient the block so that the
black surface is perpendicular to incident radiation from the sun. Note the time and measure the block
temperature every 30 seconds for 10 to 15 minutes. When plotted on a graph, students should find
that temperature, T, increases linearly with time, t. The slope of this portion of the graph can be
used to infer the solar irradiance, S, using the following equation:
 |
4. Use the Weather Underground web site
(www.wunderground.com) to examine the current weather at locations in both the
northern and southern hemispheres. Discuss the relationship between the
observed weather conditions and the current season.
5. Use the NASA web site http://weather.msfc.nasa.gov/GOES/ to
observe current visible and infrared satellite images. Comment on the differences between the
images, and suggest reasons for these differences.
Answers to
Questions for Review
1. Temperature is
the quantity that tells us how hot or cold something is relative to some set
standard value. Heat, on the other hand, is
energy in the process of being transferred from one object to another because
of the temperature difference between them.
2.
The temperature of air or any
other substance is a measure of the average speed of atoms and molecules, where
higher temperatures correspond to faster average speeds.
3. (a) conduction: The transfer of heat
from molecule to molecule within a substance.
(b) convection: The transfer of heat by the mass movement in liquids and
gases. (c) radiation: Heat transfer from
one object to another without the space between them necessarily being heated.
4. The heat energy required to change a
substance, such as water, from one state to another is called latent heat. When
water vapor condenses into clouds, latent heat is released into the
atmosphere. This provides a tremendous
amount of heat in storms, such as thunderstorms and hurricanes.
5. Each degree on the Kelvin scale is
exactly the same size as a degree Celsius, and a temperature of 0 K is equal to
–273°C. Converting from °C to K can be done by simply adding 273 to the Celsius
temperature.
6. The earth emits a lot less radiation
than the sun because it is cooler than the sun.
7. An object’s temperature has a direct
effect on the wavelengths of radiation that an object emits, and the
higher the object’s temperature, the shorter are the wavelengths of emitted
radiation. Similarly, as an object’s temperature increases, its peak emission
of radiation shifts toward shorter wavelengths. This relationship between
temperature and wavelength is called Wien’s law* (or Wien’s displacement law)
after the German physicist Wilhelm Wien (1864–1928).
In addition, objects that have a high temperature emit radiation at a greater
rate or intensity than objects with a lower temperature. Thus, as the
temperature of an object increases, more total radiation
is emitted each second. This relationship between temperature
and emitted radiation is known as the Stefan-Boltzmann law* after Josef Stefan
(1835– 1893) and Ludwig Boltzmann (1844–1906).
8. Because the earth is cooler, its
radiation is at longer wavelengths than that of the sun.
9. A radiative equilibrium temperature is
reached when the amount of radiation entering the surface of the body equals
the amount exiting the surface of the body.
10. Because they absorb radiation at certain
wavelengths and not others.
11. Water vapor, carbon dioxide, nitrous
oxide, methane, chloroflorocarbons.
12. The atmosphere allows visible radiation
to pass through, but inhibits to some degree the passage of infrared radiation
leaving the earth's surface.
13. CO2, methane (CH4),
nitrous oxide (N2O), and chlorofluorocarbons (CFCs).
14.
Averaged for an entire year, the
earth and its atmosphere (including its clouds), will redirect about 30 percent
of the sun’s incoming radiation back to space, which gives the earth and its
atmosphere a combined albedo of 30 percent.
15. Solar
energy is able to warm the surface air, and as it warms, it
actually becomes less dense than the air directly above it. The warmer air
rises and the cooler air sinks, setting up thermals, or free convection cells
that transfer heat upward and distribute it through a deeper layer of air. The
rising air expands and cools, and, if sufficiently moist, the water vapor condenses
into cloud droplets, releasing latent heat that warms the air. Meanwhile, the
earth constantly emits infrared energy. Some of this energy is absorbed by
greenhouse gases that emit infrared energy upward, and downward back to the
surface. Since the concentration of water vapor decreases
rapidly above the earth, most of the absorption occurs in a
layer near the surface. Hence, the lower atmosphere is mainly heated from the
ground upward.
16. The outpouring of solar energy constantly
bathes the earth with radiation, while the earth in turn constantly emits
infrared radiation. The rate of
absorption of solar radiation equals the rate of emission of infrared earth
radiation.
17. There is more insolation in summer
(longer days, larger sun angles).
18. Our seasons are regulated by the amount
of solar energy received at the earth’s surface. This amount is determined
primarily by the angle at which sunlight strikes the surface, and by how long
the sun shines on any latitude (daylight hours).
19. Summer.
20. The sun elevation is smaller and the
albedo is larger.
21. During the summer in far northern
latitudes, the sun is never very high above the horizon, so its radiant energy
must pass through a thick portion of atmosphere before it reaches the earth's
surface. Some of the solar energy that
does reach the surface melts frozen soil or is reflected by snow or ice. And, that which is absorbed is spread over a
large area.
22. Hills that face south receive more
sunshine and become warmer than north-facing hills, thus providing a different
environment for plants.
Answers to Questions for Thought
and Exploration
1. The bridge will become icy first
because it is able to lose heat energy over its entire surface; it
cools
on top, on the sides, and on the underside. The road, on the other hand, loses
heat energy quickly,
but
only at its upper surface. Also, when the road begins to cool heat may flow up
from warmer ground
below.
2. Heat energy is released into the air
during freezing.
3. The warm air coming out of the heat
registers will rise.
4. By infrared radiation.
5. Removing the water vapor, because water
vapor is a strong absorber of infrared radiation and
atmospheric
concentrations of H2O are much higher than concentrations of CO2.
6. Summers would be hotter, winters would
be colder.
7. An increase in cloud cover would
increase the earth-atmosphere albedo and, thus, less sunlight
would
reach the earth's surface. Depending on the height and thickness of the cloud
cover, the clouds
might
absorb more infrared earth radiation and, thus, tend to strengthen the
atmospheric greenhouse
effect.
8. The low cloud absorbs energy emitted by
the earth's surface and re-radiates infrared radiation
back
to the surface. A portion of the energy lost by the earth is returned.
9. No, because water vapor is a greater
contributor to the atmospheric greenhouse effect than CO2.
10. In the northern hemisphere, the sun
crosses the sky from east to west at a lower elevation in December than it does
in June.
11. This is due to the lag in seasonal
temperature. Although incoming solar
energy is the same on both days, in February outgoing energy still exceeds
incoming energy.
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