Performance Benchmark E.12.A.4
Students know convection and radiation play important roles in moving heat energy in the Earth
system. E/S
Radiant energy from the sun provides slightly more than 99.97% of the total energy found in
Earth’s atmosphere. The sun generates around 5.6 x 1027 calories every minute, though Earth
intercepts less than one part in a billion of this energy. The rate at which solar energy strikes the
Earth (perpendicular to the solar rays) is about 2.0 cal/cm2/min (1.4 kW/m2). The rate is
somewhat lessened as the radiation is intercepted at more oblique angles due to Earth’s
curvature.
For vital statistics of our sun, go to
http://solar-center.stanford.edu/vitalstats.html
As solar radiation (insolation) impinges on the atmosphere, four events typically occur (Figure
1). A portion of the radiation is reflected back into space. Some of the radiation is scattered by
the air. Clouds, greenhouse gases and particulates absorb a part of the insolation. The remainder
of the solar radiation will be transmitted through the atmosphere and reach Earth’s surface.
To learn more about atmospheric effects on incoming solar radiation, go to
http://www.physicalgeography.net/fundamentals/7f.html
Figure 1: Earth-Atmosphere Energy Balance (from NOAA JetStream,
http://www.srh.weather.gov/srh/jetstream/atmos/energy_balance.htm)
Of the energy which strikes Earth, some will be absorbed and some will be reflected. The albedo
(reflectivity) of Earth’s surface varies. The higher the albedo, the greater the percentage of
insolation reflected; the lower the albedo, the lower the percentage reflected. The albedo for
various surfaces associated with Earth’s atmosphere and surface are listed in Table 1.
Table 1: Reflectivity, or “Albedo,” of Various Surfaces
SURFACE
% REFLECTED
Clouds (depending upon cloud type and thickness)
Concrete
Crops, green
Forest, green
Meadows, green
Ploughed field, moist
Road, blacktop
Sand, white
Snow, fresh fallen
Snow, old
Soil, dark
Soil, light (or desert)
Water
25-84
17-27
5-25
5-10
5-25
14-17
5-10
30-60
80-90
45-70
5-15
25-30
8
Earth’s surface is comprised of many different materials, each with their own albedo. There are
large bodies of water (which absorb a great deal of insolation), sandy deserts, forests, fields of
crops, barren regions of basaltic lava flow, and expansive polar ice caps, just to name a few. It is
to be expected, then, that the Earth will not reflect back the solar radiation to equivalent degrees.
Figure 2: Surface Reflectivity of Earth (from PhysicalGeography.net, Chapter 7 Introduction to the Atmosphere,
http://www.physicalgeography.net/fundamentals/7f.html)
The depth to which the solar radiation can penetrate Earth’s surface varies. Regarding visible
light, 100% of the light penetrates water to a depth of 10 mm, 97% is left at a depth of 1.0 m,
73% remains as deep as 10.0 m, and just 6% of visible light has penetrated to a depth of 100.0 m.
The radiant energy which is absorbed by water helps drive ocean currents. Concerning soil and
sand, only 72% of visible light penetrates as deep as 0.5 mm, and at 1.0 mm deep, just 54% of
light remains. There is no further penetration of visible light once a depth of 5.0 mm has been
reached. In contrast to water, the absorption of solar radiation by soil and sand warms only a
thin surface at the top. Long-wavelength radiation (infrared) forms during absorption of visible
light. Most of the infrared formed in Earth’s soils and sand is directly adjacent the atmosphere
and readily re-emitted. Much of the infrared radiation formed in oceans remains deep below the
surface, so the energy absorbed is released more slowly from large bodies of water than from
expansive regions of soil and sand.
For additional information concerning the interaction of incoming solar radiation with the Earth
system, review the following websites:
http://www.everythingweather.com/atmospheric-radiation/absorption.shtml
http://www.lwr.kth.se/Grundutbildning/1B1292/Compendium_online/ch05s01s02.html
http://www.lwr.kth.se/Grundutbildning/1B1292/Compendium_online/ch05s02s01.html
The radiant energy which is changed into infrared radiation by Earth’s surface is involved in
further transfer of energy by the process of convection. It should be remembered that “radiation”
is the transfer of energy (from any wavelength of the electromagnetic spectrum) without the
involvement of a physical substance in the transmission. This is why solar radiation is able to
reach Earth as it travels through the vacuum of space. Convection, on the other hand, transmits
heat energy by transporting groups of molecules from one place to another within a substance.
Specifically, a fluid medium must be present in order for convection to occur. In the Earth
systems, convection within the core and mantle is driven by Earth’s internal energy. External
energy, from solar radiation, drives the convection found in oceans, lakes and ponds, and
convection within the atmosphere.
For more information on Earth’s atmosphere as a convective fluid heated from below, go to
http://www.etl.noaa.gov/about/eo/activities/convection.pdf
Convection occurs as a result of warmer, less dense portions of the medium rising while the
cooler, more dense portions sink. The warmer the fluid (liquid or gas) in contrast to the
temperature of the surrounding medium, the quicker and more forceful the convection which
ensues. In our atmosphere, slow convection might elicit, at most, a gentle breeze, but rapidly
circulating convection cells create strong winds. In effect, as the very warm air rises, a low
pressure center develops; as the cold air descends, high pressure centers are created. Winds blow
from the regions of high pressure towards the regions of low pressure. Convection can occur on
a small scale or a large scale. In our atmosphere, small scale convection might occur adjacent
land and water where the temperature of the land changes more quickly, during the course of a
day, than the temperature of the body of water. Though the temperature of large bodies of water
changes little from day to night, the land warms up quite a bit as it absorbs solar radiation.
During daytime, the warm, less dense air above land rises as the cooler, more dense air above the
ocean descends and rushes landward (a sea breeze). At night, the land cools rapidly, resulting in
colder air above the land compared to the temperature of the air above the ocean. So, during
nighttime, the comparatively warmer air above the oceans rise and the colder air above land
sinks, moving the air away from the land and out to the ocean, creating a land breeze (Figure 3).
Figure 3: Localized Atmospheric Convection (from Center for Science Education and Department of Geological
Sciences, University of South Carolina, High School Earth Science Project)
http://cse.cosm.sc.edu/hses/WindBelt/pages/convec.htm)
If Earth was smooth and had no interactions between land and ocean masses, two very large
convection cells would arise between the polar and equatorial regions. But, the Earth is more
complicated, thus smaller cells develop in both the northern and southern hemisphere, further
distributing heat energy (Figure 4).
Figure 4: Global Convection Cells (from National Center for Atmospheric Research)
http://www.ucar.edu/learn/1_1_1.htm and http://www.ucar.edu/learn/1_1_2_7t.htm)
Performance Benchmark E.12.A.4
Students know convection and radiation play important roles in moving heat energy in the Earth
system. E/S
Common misconceptions associated with this benchmark:
1. Students incorrectly believe solar radiation (electromagnetic energy) must be
transmitted through a physical medium.
Students experience heat transfer through touching hot items, feeling the heat blowing out of
a furnace register, and by noting the heat emitted from hot surfaces, such as a light bulb or
electric stove burner. They generally understand conduction is the transfer of heat between
matter in contact, such as their hand and a hot pan, and they may have seen convection as
water boils in a pot. But, many students are of the opinion that radiant heat needs a medium
of transmission. When asked to use radiation to explain how heat from an infrared bulb
warms a burger, they will discuss how heat from the bulb warms the air, which in turn warms
the burger. If pressed that the burger would warm up even in a vacuum, they reiterate the
necessity of air, and might offer the explanation of sound waves being transmitted through
air. Since their experiences occur within an atmosphere, it seems an important component in
the transmission of electromagnetic radiation. It helps to lead these students through a
discussion of how solar radiation reaches Earth via the vacuum of space, as well as
discussing the coldness of space (~3K) since solar radiation is barely absorbed by what little
gaseous matter is present in space.
The following websites address solar radiation and its propagation from Sun to Earth.
http://almashriq.hiof.no/lebanon/600/610/614/solar-water/unesco/19-20.html
http://almashriq.hiof.no/lebanon/600/610/614/solar-water/unesco/21-23.html
2. Students mistakenly believe solar radiation is radioactive.
Students confuse the meaning of “radiation” as it pertains to the electromagnetic spectrum
with “radiation” in reference to radioactive decay of unstable isotopes. When students hear
that Earth’s ozone layer helps filter out harmful ultraviolet radiation, which can cause skin
cancer, they remember, as well, that subatomic particles from nuclear radiation can cause
cancer. It helps to review, with these students, the sources and properties of electromagnetic
radiation and those of nuclear radiation.
The accompanying URL provides an opportunity to compare the meanings of radiation and
radioactivity. You are able to select terms (for example, radiation, radioactivity and solar
radiation) which are then compared side-by-side.
http://iaspub.epa.gov/trs/trs_proc_qry.alphabet?p_term_nm=R&p_reg_auth_id=1&p_data_id
=11607&p_version=1
3. Students incorrectly believe convection has no influence on the weather.
Students are taught about convection cells, and see how masses of air are cycled through
Earth’s atmosphere. However, they can fail to associate convection in the atmosphere with
the movement of fronts across the surface of Earth, the development of atmospheric
turbulence, or the introduction of moisture into the atmosphere which leads to precipitation.
This can be addressed by teaching students some basics of meteorology. Animations can
show students how fronts in the atmosphere and move across the land. Students might wish
to study severe cyclonic atmospheric disturbances (hurricanes and tornados) which result
from rapidly convecting air. And, students can be shown that the length of time a convecting
air cell spends over land versus water makes a difference in moisture content of the air,
effectively providing plenty of moisture for some land regions, such as during monsoon
seasons, or very limited moisture for other regions, creating desert climates.
More information regarding the influence of convection on Earth’s weather can be found at
the following:
http://www.wrh.noaa.gov/otx/outreach/ttalk/convect.php
http://www.phy6.org/stargaze/Lsun1litA.htm
http://www.research.umbc.edu/~tokay/chapter4.html
Performance Benchmark E.12.A.4
Students know convection and radiation play important roles in moving heat energy in the Earth
system. E/S
Sample Test Questions
1. By studying the accompanying diagram, we can discern that:
Figure reference: http://www.physicalgeography.net/fundamentals/images/cascade.GIF
a.
b.
c.
d.
Earth’s surface reflects more solar radiation than do the clouds
Earth reflects 100% of the solar radiation it receives
more solar radiation is absorbed just by the surface than is reflected in total
the atmosphere and clouds are able to absorb more solar radiation than Earth’s surface
2. As solar radiation reaches Earth’s atmosphere, all the following occurs EXCEPT:
a.
b.
c.
d.
some radiation is reflected
some radiation is scattered
some radiation gets absorbed by gases
some radiation decays into stable elements
3. The term __________ refers to the reflectivity of a surface or substance.
a.
b.
c.
d.
albedo
convection
insolation
radiation
4. Based on the accompanying graphic, which Earth surface has the highest albedo?
Figure Reference: http://www.geo.lsa.umich.edu/~crlb/COURSES/117-IntroductiontoGeology/Lec23/albedo.gif
a.
b.
c.
d.
forest
ocean
sand
snow
5. Convection distributes heat energy by:
a.
b.
c.
d.
transporting groups of molecules from one place to another within a substance
transferring energy without the involvement of a physical substance in the transmission
direct, point-to-point contact with static, neighboring molecules
emitting alpha and beta particles from unstable atomic nuclei
6. A sea breeze occurs during __________ as warm air above land rises and cold air over ocean
water sinks, causing a convection cell which moves air from the __________.
a.
b.
c.
d.
daytime / land out to the ocean
nighttime / land out to the ocean
nighttime / ocean in towards the land
daytime / ocean in towards the land
Use the diagram below to answer question #7.
Figure Reference: http://aquarius.nasa.gov/images/six_cell.jpg
7. Based on the diagram above, we can see that regions of convecting atmosphere where air is
rising are areas of _________ while the regions where air is descending are areas of
__________.
a.
b.
c.
d.
conduction / radiation
radiation / conduction
low pressure / high pressure
high pressure / low pressure
Performance Benchmark E.12.A.4
Students know convection and radiation play important roles in moving heat energy in the Earth
system. E/S
Answers to Sample Test Questions
1. (c)
2. (d)
3. (a)
4. (d)
5. (a)
6. (d)
7. (c)
Performance Benchmark E.12.A.4
Students know convection and radiation play important roles in moving heat energy in the Earth
system. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. Atmospheric Processes - Radiation
The National Center for Atmospheric Research developed a content site with lessons useful
to the understanding of Earth’s atmosphere. The following website provides content and
activities to help students understand heat transfer through radiation.
http://www.ucar.edu/learn/1_1_2_5t.htm
2. Earth’s Energy Cycle: Albedo
The National Center for Atmospheric Research also provides a lesson pertaining to albedo of
Earth surfaces. The following website links to an Adobe Acrobat document with a nice set
of lessons.
http://eo.ucar.edu/educators/ClimateDiscovery/ESS_lesson4_10.19.05.pdf
3. Clouds and Particles
Another site which will help students understand albedo is presented by Environmental
Science for Everybody Round the Earth (ESPERE).
http://www.atmosphere.mpg.de/enid/35598cfdc8dfc57bfc329c04dfea84a8,0/3__Sun_and_cl
ouds/-_Albedo_25w.html
4. Atmospheric Processes - Convection
Among the lessons from the National Center for Atmospheric Research, the following
provides content and activities to help students understand convection.
http://www.ucar.edu/learn/1_1_2_7t.htm
5. The following three websites provide a nice general overview of content on heat
transfer, as well as providing activities to reinforce that content.
Introduction to the Atmosphere
http://www.ucar.edu/learn/1_1_1.htm
Conduction, Convection and Radiation
http://aspire.cosmic-ray.org/labs/atmosphere/popcorn.html
Heat Transfer in the Atmosphere
http://www.cocorahs.org/Content.aspx?page=HeatTransfer