Content Benchmark E.12.A.4
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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.

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)

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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

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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. Earth’s surface reflects more solar radiation than do the clouds
b. Earth reflects 100% of the solar radiation it receives
c. more solar radiation is absorbed just by the surface than is reflected in total
d. 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. some radiation is reflected
b. some radiation is scattered
c. some radiation gets absorbed by gases
d. some radiation decays into stable elements

3. The term __________ refers to the reflectivity of a surface or substance.
a. albedo
b. convection
c. insolation
d. 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. forest
b. ocean
c. sand
d. snow

5. Convection distributes heat energy by:
a. transporting groups of molecules from one place to another within a substance
b. transferring energy without the involvement of a physical substance in the transmission
c. direct, point-to-point contact with static, neighboring molecules
d. 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. daytime / land out to the ocean
b. nighttime / land out to the ocean
c. nighttime / ocean in towards the land
d. 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. conduction / radiation
b. radiation / conduction
c. low pressure / high pressure
d. high pressure / low pressure

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)

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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_clouds/-_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

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