Agronomy 541 : Lesson 9a
Thermal Energy
Introduction
Developed by D. Todey and E. Taylor
It is suggested that you watch Video 9A and complete the exercise in the video before continuing with the
lesson.
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The ability to photograph infrared radiation allows one to collect data about the infrared emission and,
consequently, temperature of objects. This property of IR radiation has found usefulness in the agricultural
community, in diagnostic medicine, meteorology, and many other areas. This part of lesson 9 will examine IR
radiation and its use in assessing the temperature and health of a variety of agricultural areas.
What You Will Learn in This Lesson:
How objects emit radiation.
What IR radiation tells about objects.
How satellite technology can be used to determine surface characteristics.
Reading Assignments:
p. 371—Aguado and Burt
Agronomy 541 : Lesson 9a
Thermal Energy
Infrared Photographs
Figure 9.1 is a common infrared slide of a farm taken from an airplane. In the infrared slides of farms the
healthy vegetation usually appears as a bright red. Lack of vegetation appears either as black or as white for
bare soil. Unhealthy vegetation appears as some darker color, at least not a bright red color. This is a corn
field, showing unhealthy corn, lack of corn, and a spot of healthy corn showing up as a bright red color.
Fig. 9.1 Infrared image of a corn farm.
Other images from IR satellite photographs are very obvious (Fig. 9.2)
Fig. 9.2 IR photo of urban area
Study Question 9.1
What is pictured in the bottom left of Figure 9.2?
Check Answer
What is the health of the smaller field?
Good
Poor
Check Answer
These infrared pictures are taken with a standard camera. The standard camera has a glass lens, which does
not let heat through. These are not heat (or thermal infrared) pictures taken with a standard camera. They are
infrared pictures observing wavelengths barely beyond the visible. Just beyond the red color, which our eyes
can see, exists the "near visible" or the "near IR infrared" photos.
These have become quite important for imagery for local areas or areas as broad as the entire United States.
Vegetation is assigned a color, sometimes rather than being red, plants are made green. So healthy vegetation
may be green, and unhealthy vegetation, or lack of vegetation, may show up as red. Water will show up as
deep blue or black. The deserts of Utah, New Mexico, Arizona, and California have a great deal of bare soil. In
the irrigated valleys and coastal areas of California, we see limited but productive farmed areas.
IR photography was developed for detecting camouflage during the Second World War. If vegetation was not
healthy, it would show up as a different color than the healthy vegetation. Where vegetation had been cut to
hide a tent or a jeep or something, it showed up very obviously. When camouflage nets were used rather than
real vegetation, it also showed up. This near infrared film, seeing wavelengths just beyond where your eyes
see, could reliably detect healthy versus wilting or unhealthy vegetation.
IR photography is very useful meteorologically in sensing temperatures of a surface. The ocean surface, since
such large expanses cannot be measured easily, is scanned constantly to judge changing sea-surface
temperatures (Fig. 9.3).
Fig. 9.3 Thermal infrared picture of the gulf stream
off the east coast of the United States. Red colors
are warm temperatures. Blue colors are cooler
temperatures.
Another meteorological use is the sensing of land surface or cloud temperatures using satellite data. Figure 9.4
displays a different example of this technology. In this particular picture the state of Florida is shown. There are
some clouds on the right of the picture (east) and in the Gulf of Mexico (left). The temperature at the top is
colder than 32°F (0°C). Freezing conditions have reached north Florida and the Florida Panhandle. Cold
clouds appear as white in the Gulf of Mexico, shown in the lower left-hand side of the picture. Temperatures
greater than 46°F (8°C) exist from about central Florida to the South. The orange growing area has
temperatures between 33 and 45°F (0-8°C) in most of north Florida (around Gainesville). Clouds are seen as
small, oddly shaped orangish blobs in the figure. They are colder than the surface below.
Study Question 9.2
How could you use an IR satellite picture to forecast freezing temperatures if the area of interest is covered by
clouds?
You Punt
Extrapolate cloud temperatures to the surface
It's a trick question and would never happen
Check Answer
This was the first operational satellite picture (in 1976) of Florida temperatures used as an advisory to the
orange growers and vegetable farmers for freeze advisories in the winter. One of these was displayed on
television each hour to depict the encroaching cold air. As the yellow and red areas encroached upon the state
and producers' locality, the producers could blow the picture up to county size. When they saw the red spots
coming closer, they would know it was time to begin freeze protection for the crops, whether they were growing
low-growing crops or the orange trees.
Frost prevention methods can be found at FrostProtection
The satellite project cost $3,000,000 to taxpayers, and saved the people of Florida $7,000,000 that first night
of its operation. Since it was federally funded, everyone helped pay for it. Florida growers saved $7,000,000,
which in turn helped everyone on the very first night. Since that night the savings to agriculture are tens of
millions, if not hundreds of millions of dollars by having a satellite infrared picture of temperature provided
throughout the state.
Infrared pictures have become a valuable tool from space. There's a very strong medical value to these
images as well. Figure 9.5 is an IR picture of a person's face. Color differences indicate temperature
differences sensed by the IR radiometer. At times these can be used to evaluate circulation problems by
displaying cooler areas of limited blood flow. There is usefulness beyond the meteorology realm for these
instruments.
Fig. 9.5 IR pictures of a face a) and hands b). Hands indicate cold spots
where blood flow is restricted on a smoker's hands.
FYI : Infrared Pictures
IR satellite pictures can be found many places on the web. They are the only satellite picture available when the sun
is not shining on that side of the earth. You may find a current satellite picture, with sensed temperatures at:
http://www.rap.ucar.edu/weather/satellite/.
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Agronomy 541 : Lesson 9a
Thermal Energy
Radiation and Temperature
Discussed previously all things that have temperature radiate. The amount of energy given off by an object is
according to the fourth power of its absolute temperature
Equation 9.1
where T k is degrees Kelvin. To convert from a room temperature of 68°F (20°C) to absolute temperature (or
Kelvin scale), add 273 to the Celsius value. Therefore, the Kelvin scale room temperature is 293°K. To find the
amount of energy given off by something, take its Kelvin temperature to the fourth power. Then multiply it times
the Stefan-Boltzmann's constant, , named after the people who came up with this relationship, and a factor
called emissivity, . Emissivity is a number that varies from 0 to 1 (will be discussed later in the lesson). Thus,
the total energy radiated is given by the equation
Equation 9.2
The Stefan-Boltzmann constant converts to units of energy (watts, ergs or calories according to the unit system
being used). The important thing to remember is that the amount of energy given off by something is
proportional to the fourth power of its absolute temperature.
Study Question 9.3
How much energy is given off by a box at room temperature with an emissivity of 0.95?
Stephan-Boltzman constant: 5.67 x 10-8 Wm-2 K-4
Wm-2
Check Answer
How much energy is given off by the sun (temperature 6000 K) with an emissivity of 0.99?
Wm-2
Check Answer
This relationship explains many things about the amount of heat given off by items.
Quite often you won't need to know too much about the quantity of energy. What you need to know is the
temperature. If something is radiating according to this same rule, we will have to look at two temperatures. As
displayed in Equation 9.2, the emissivity reduces the energy output of the object. The temperature sensed will
be called the apparent temperature, T app. The apparent temperature will appear lower because of the
radiation reduction by the emissivity. The apparent temperature raised to the fourth power (T app4) will equal
the emissivity, times the true temperature (T true4) raised to the fourth power. The value that a satellite sees is
the apparent temperature. If a satellite was looking at the ground of actual temperature 294 K, the satellite
would see some apparent temperature (call it T app , without the 4th power) which equals the 4th root of the
emissivity of the surface of the fourth power of the true temperature.
Study Question 9.4
What temperature would a satellite see looking at a surface with an emissivity of 0.96 and a temperature of
270 K?
K
Check Answer
IN DETAIL : Heat Given Off
One question that I've been asked a great many times is, how come I need to have my house hotter in the winter than
in the summer to be comfortable? In the summer, I might be comfortable in the house if it was 68°F. In the winter it has
to be 72 or 73° F in the air temperature for comfort. Why would that be? The answer comes from thermal radiation. In
the winter, outside walls tend to be quite a bit cooler than in the summer because of the outside temperature. The
amount of energy coming to you is according to the fourth power of the absolute temperature of those walls. So the
type of problem that you might be seeing would say, "How much energy do you receive from a wall at 68°F (20°C) as
compared to a wall that is 15°C?" And how much warmer would the air temperature need to be to compensate for
that?
There is one other less important reason. In the winter the humidity is often lower causing you to feel colder. The
lower humidity permits more evaporative cooling through our clothes and at our skin in the winter than in the summer.
At least that's often the case. The big factor, though, is the thermal radiation from the objects around us. As my
graduate professor said. "We're seated in a blackbody cavity where our comfort is determined by the thermal
radiation of our environment." Blackbody cavity means a perfectly radiating environment, where radiation is the thing
that's determining our comfort.
Sufficient radiation can compensate for cold air by helping us feel warmer. If a warm pot-belly stove in the corner of
the room is radiating heat, you feel warm and don't really care what the air temperature is (Fig 9.6). Air temperatures
can be freezing. But as long as there is plenty of radiant energy, you'll be comfortable.
Figure 9.6 The amount of radiation given off is proportional to the 4th
power of temperature.
A number of stores have begun to place radiative heaters in their doorways and leave the doorway to the store open
in the winter. The circulation near the door is modified such that cold drafts into the store are limited. In the doorway
you have plenty of thermal radiation to compensate for the cold temperatures. These systems work fairly well.
For heating orchards, oranges, peaches and crops that are very sensitive to frost, smudge pots were often used to
heat the area. They created a great deal of smoke, limiting the freezing problem. But air pollution has become a
greater concern. As the population has grown, we've become more environmentally aware. Also, smoke itself
reportedly does little, if anything, to protect from freezing, because it does not have the same radiation effectiveness
as would a cloud of water vapor. Now, instead of smudge pots, producers put out radiative heaters to radiate heat to
the flower buds and to the leaves on the trees preserving them from frost by radiative energy. Not only is radiative
energy important for our comfort, it is important to agriculture.
As discussed in lesson 7 radiation is the means by which items cool down below air temperature and become subject
to radiative frost or to the cooling that can result in dew formation. We've already talked about the importance of
radiation.
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Agronomy 541 : Lesson 9a
Thermal Energy
Radiation Spectrum
Two kinds of infrared radiation exist, the near infrared and thermal infrared radiation. Looking at the energy
coming from the sun on some scale with the amount of energy called radiative intensity, the energy from the
sun may have a peak at about 0.5 µm (Fig. 9.7). The light that we see is between 0.4 and 0.7 µm. Solar energy
has its peak in the yellow of the visible light range. To the left of the visible at shorter wavelengths, solar
radiation drops off quite quickly. This is the ultraviolet, labeled as UV. To the right of the visible, beyond where
our eyes can see, is the IR, the near red IR. The wavelengths of the colors of the spectrum (violet, blue, green,
yellow, and orange in the middle, and red at the right end) are also depicted.
Fig. 9.7 Radiant energy over the solar spectrum. Wavelength is
micrometers to the left of the jagged green lines and meters to the
right of the lines.
Discussion Topic 9.1
What is the color of the sun during the daytime when we are receiving the maximum sunlight? Is
there a relationship here? What would the sun look like if its peak radiation was in the infrared
bands? Would we see the sun?
The amount of energy emitted by the sun at the 3 µm wavelength is very, very low. Solar energy at 1 µm is
relatively high. This is the IR that can get through a camera lens, and this is the IR that we were seeing in our
pictures of the vegetation taken by the satellites.
Infrared radiation, as you can tell by looking at the area under the curve, represents a great deal of the energy
coming from the sun, about 37%. If a plant or crop absorbed all of the energy from the sun, it would heat up to
a certain degree. If it only absorbed half of the energy from the sun, it would not heat up as much in the sun.
How much of the energy from the sun does a plant absorb? Imagine a typical soybean leaf with sun shining on
it. About one-fourth of the energy from the sun is reflected by the plant. About one-fourth passes through the
leaves (called transmitted), and one-half is absorbed into the leaf to be used in heating the leaf and some for
photosynthetic and photosensitive processes.
The percentage of energy from the sun that the plant absorbs is not uniform among wavelengths. What color
are the plants? They are green. The fact that they are green means that they are doing something strange with
the light. If they were treating all the light uniformly, the plant would be gray. Disproportionate absorption or
reflection makes the leaf have some color to it. Healthy plants, as mentioned, appear green. One might think
that plants reflect more green. It would be more accurate to say that healthy plants are absorbing the red. They
are using the red wavelength for photosynthesis. The plants are taking the red out of the light that is in our
environment and reflecting or transmitting most everything else. But because there is a lack of red, the plant
appears to be green. So we're not really seeing green when we look at a plant, we're seeing lack of red.
FYI : Micrometer
1 µm equals 10-6 m . Then the amount of energy received from the sun tapers off, approaching 0 at longer
wavelengths.
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FYI : Seeing Green
Sometimes you hear about psychedelic colors where you really are seeing green. It's possible to make something
absorb everything except green. In reflecting the green, we would have what is called "true spectral color." Quite often
these end up as a so-called psychedelic color because it really is green light. But with green plants it is not green light
we are seeing. What you are seeing is all colors except red. The complementary color is missing.
Some people's eyes seem able to discern pure colors from apparent colors.
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Agronomy 541 : Lesson 9a
Thermal Energy
Radiation Emission
Fig. 9.8 Variation of solar radiation over
wavelengths.
The output of the sun's energy also varies over wavelengths (Fig. 9.8). To understand the balance of radiation
we need to see what curve represents the energy given off by the earth. The earth is absorbing, as we already
discussed, quite a percentage of the solar radiation that strikes it. The albedo, or amount reflected by the
earth, is only about 30% of that reaching the earth. The earth is giving off infrared energy according to the
fourth power of the temperature of the earth. The peak of this energy given off is at 10 µm. The sun's peak is
somewhere around 0.5 µm wavelength (Fig. 9.9). Thus, the sun is a shortwave emitter. The earth absorbs this
shortwave and re-radiates as longwave radiation.
Fig. 9.9 Wavelength of peak radiation for the sun and the Earth.
An interesting thing, discovered many years ago, was that the amount of energy under the long-wave curve,
increases as something gets hotter (Fig. 9.10). That's easy to discover. As you build a bigger fire in a stove
and it gets hotter, you can feel more energy striking you. Obviously it gives off more energy. But the peak is not
simply higher; the peak moves to a shorter wavelength. Something at room temperature, 68° F (20 °C), has its
peak at 10 µm. If it is heated, the peak also moves. The area under the curve expands. When it is heated
more, the peak moves more, until you've heated this thing to around 6,000 K (3315° C). At this temperature the
peak moves to about 0.5 µm in the visible wavelength. Something which has been heated until it became "red
hot" can be seen.
Fig. 9.10 Shift of peak
wavelength of emission at
hotter temperatures.
An example would be a light bulb with a brightness control on it. Adjusting the brightness control on the light
switches the color from a nice white light, to yellowish light, and just before the bulb goes out, a red. Looking at
the filament of the light bulb, you would see it glowing a deep red just before it goes out and disappears.
Clearly there are two ways that we could utilize the characteristics of this curve to determine the temperature of
something. (1) Measure the area under the curve and say that represents a certain amount of energy that is
given off by the object. If the object is hotter, there will be a lot more energy under the curve. (2) Measure
where the peak of the energy emitted by an object is The first way that temperature was measured remotely
was by the area under the curve. When feeling warmth from the stove, you are feeling the energy under the
curve. When the stove is so hot it begins to be red hot, then you're seeing the peak. You can detect both
characteristics as things approach red hot.
Agronomy 541 : Lesson 9a
Thermal Energy
Plant-Radiation Relationships
While the whole spectrum of light reaches the plant, the plant may not use it effectively. The plant uses red
light, while the rest of the light is generally not valuable to the plant for photo-chemistry. There are some
exceptions. Far red light just beyond what we can see might have an effect on germination of seeds. Some UV
bands might have some effect on pollination and on insects. There are other colors of light that are utilized,
either in the chemistry of the plant or in its adaptation to its environment and to other things around it. But for
photo-chemistry and the growth of the plant, red is the most important color.
The value of the other energy comes into question. If the plant is too cold (growing in the Arctic tundra, for
example) absorbing some extra energy could be helpful. Little photosynthesis occurs when a plant is at or near
freezing point. If the leaf could be somewhat warmer than the air, that might be an advantage. Some of the
plants that grow in very cold environments not only absorb the red, but they absorb all other wavelengths very
well. Rather than absorbing half of the energy, as the soybean leaf, some leaves absorb 70% of the total
energy of the sun that is striking them. That extra energy absorbed will do little more than heat the leaf up,
causing it to be warmer and perhaps closer to its optimum temperature for growth and development. This has
been demonstrated in some cases.
At the other extreme, a plant growing in a very hot desert will heat up to a temperature greater than ideal.
Maybe it would heat up to near 40°C (104° F), where 30-35°C (86-95 °F) may be its optimum temperature for
photosynthesis. This plant would do well to absorb less solar energy. Indeed, a great many plants have
adapted to this. A barrel cactus is a good example or even the opuntia leaves on cactus (Fig. 9.11).
Fig. 9.11 Opuntia leaves on a cactus
Very little light, if any, is transmitted through them. The energy is absorbed, heating the plant. How much
energy is this plant absorbing? If it were like the soybean leaf, it would be absorbing half and reflecting a
quarter. Deep inside the plant, way down in the core, it would be absorbing that other 25% and heating it up to
the middle of the plant. But this is not so. In the chlorophyll layer there's still about half the energy being
absorbed. About 40% of the energy is being reflected rather than the 25% for the soybean leaf. We've got
about four-tenths being reflected off of the cactus, and about six-tenths being absorbed. Of the six-tenths
absorbed five-tenths are going into the photo layer to perform the chemistry and the things that need to
happen there. Only about one-tenth penetrates into the depth of the plant to heat it up. The cactus has
adapted to become much more effective at reflecting.
Some plants that have leaves that grow in a desert area are even more effective than that. They reflect about
40%, transmitting the other 10-12% through to the ground beneath them to keep from overheating. In fact, if
you could look in this infrared area with your eyes, these desert plants would look like hubcaps sitting out in the
desert since they would appear to be so shiny. Other plants would just look kind of dull and drab because they
do not reflect this IR.
This difference is quite dramatic when comparing the upland and swamp varieties of cypress tress in Florida.
The ones that grow in periodically dry areas have a much greater reflectance, reducing the heat load on them.
Figure 9.12 indicates that pond cypress b reflects more longwave radiation than bald cypress a.
Fig. 9.12 Reflectance difference between bald cypress (top) and
pond cypress (bottom). The middle line is the measured average
while the outer lines give an indication of the spread of the
measurements.
Study Question 9.5
Which reflects more long wave radiation?
Bald Cypress
Pond Cypress
Check Answer
Agronomy 541 : Lesson 9a
Thermal Energy
Emissivity
A perfect radiator has an emissivity of 1.0. This perfect radiator is termed a blackbody. So a blackbody
radiator has an emissivity of 1. That is, it doesn't make any difference in the equation. You're multiplying by 1
when you're using that equation. A perfect non-radiator has an emissivity of 0.0. So if something didn't radiate
at all, it would have an emissivity of 0. There are no perfect radiators, and there are no perfect non-radiators.
But we do get some things that are quite close to it.
Something that is silver such as aluminum foil has a rather low emissivity. Not only is this a good reflector to
your eye of visible light, it is a good reflector of heat. If you would feel the heat coming off something very hot
nearby, you could place some tin foil on the other side of your hand, feeling the heat on both sides of your
hand. You'd feel the heat coming from the heat source and also feel the heat reflecting from the tin. Tin foil
reflects heat, maybe even better than it reflects light. Being a very good reflector of heat, it has a low emissivity.
Fig. 9.13 Heat transfer from a lamp warms a hand. Not only will a hand
feel heat from the lamp, it will feel heat reflected from the foil.
These effects are complementary. If an object has a high emissivity, it doesn't reflect. If it has a low emissivity, it
has a high reflection. This assumes that transmission is neglected. The object is something heat cannot pass
through being either absorbed or reflected in this case.
FYI : Blackbody
Blackbody refers to the idea that black is the absence of all color while white contains all colors. A perfect blackbody
would absorb all radiation (which is not the case). If it did we would not be able to see a blackbody. Something that is
black absorbs all visible colors; something white reflects all visible radiation. In lesson 8 we discussed that an object
which absorbs well, also emits well. Thus a blackbody would not only absorb all radiation, it would emit all radiation.
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Agronomy 541 : Lesson 9a
Thermal Energy
Emissivity
Practical Examples
"Don't Cover A Radiator With Tin Foil"
Transcription of the audio.
Tin Foil Is Almost As Good As Gold.
Transcription of the audio.
Fig. 9.14 In winter heat is reflected by the foil inside the wall. In the
summer the foil reflects outside heat away from the house.
Discussion Topic 9.2
What would happen if the foil was installed backward? Would it change the insulating effect?
What occurs in the summer? With the sun beating on it, the outside wall gets hot and it heats up. Heat gets to
this aluminum and does not radiate into the room because aluminum is a poor radiator. Heat trying to get out of
the room is reflected into the room. Heat trying to get into the room can't get there.
There is a commercial product available called "low emissivity wall paper" which looks like regular wall paper.
They have taken mylar, backed with aluminum, and with a plastic type paint painted a design like any other
wallpaper on the front of it. It looks like normal wall paper at a glance, but reflects all of the heat back into the
room. This wall paper costs less than twice the price of normal wall paper, while cutting your heating bill by
75%. I saw it first in Popular Science magazine, wrote the company, got samples, and passed the samples
around in class. Everybody looked at them, chose one that we could tolerate, used it, and had perfect results
with it.
The foil used behind the wall must be kept clean to retain its low emissivity. If dust collects on the tin foil, then
the emissivity changes to that of dust, which would be close to 1 instead of 0.2.
The National Aeronautic and Space Agency used this concept idea to create warm suits for outer space. They
covered aluminum foil with mylar (actually they took the mylar and sprayed aluminum plating on it). When
placed on a person's skin, the mylar will let radiation through while the foil will reflect radiation escaping from
the skin. Repeating that process seven times will produce something that is only an eighth of an inch thick, but
will be as good an insulator as a coat of feathers 2" thick.
Baking Tin Emissivity
Transcription of the audio.
Fig. 9.15 A spot of flour on a baking tin will change the emissivity from
0.2 to 0.97 at that spot. Heat will be absorbed readily at that spot,
burning the bread.
FYI : Transcription of Audio
Aluminum will not radiate the heat away, and your radiator will stop radiating. A school in southern Iowa had doubled
their heating bill from one year to the next. They wanted to know if the weather had really been that much colder. The
principal of the school knew that the heating bill at his home had not doubled. In fact, the winter had been a little
warmer than the previous one. He said, "Boy, we've got trouble at the school. We've had them come in and check the
furnace and the system and the boiler and everything."
Just on a hunch, I asked, "Does your school have forced air or does it have radiators?" He said radiators. I said,
"When did you paint them?" "About a year ago,"he answered. "What color are they?" "They're silver." He painted
them with an aluminum paint.
The radiators previously were rusty and had an emissivity approximately equal to 0.99, being almost perfect radiators.
Now, aluminum colored, their emissivity was approximately 0.2. Of the energy that they should have been radiating,
they were only radiating 20% of it. They were heating some air adjacent to the radiators because of their fins. Air
rises between the fins, heating the air in the room somewhat. Radiators are most effective at radiating heat, but were
not radiating. Their heating bill had doubled because their radiators had now become convection ovens rather than
radiation heaters. Only the movement of air was heating the room. What was their answer? Get the paint off of them.
Paint them with something that is a blackbody. He said, "You mean we should have left them rusty?" Absolutely, he
should have left them rusty. But since they didn't leave them rusty, what could they do?
There was a John Deere plant located in Ottumwa. My recommendation was to contact the heavy equipment plant
and get some paint used to paint engine blocks. Engine blocks are painted with a thermally black paint, though it may
appear some other color to our eyes . The paint is a high-emissivity paint, with an emissivity as close to 1 as is
possible. The engine blocks will radiate and cool as well as possible. The school painted their radiators with engine
block paint; their heating bills went back down.
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FYI : Transcription of Audio
Tin foil is almost as good as gold. It is not worth as much, but almost equal in its low emissivity. A castle in England
had a room that was covered with tapestry. The guide said, "This room is the summer room." The next room in the
castle was covered with gold. Ornate gold leaf covered the ceiling and walls. Everything but the floor was gold. The
guide said, "This is the winter room." Someone in the group said, "That doesn't make sense. That other room has the
nice, warm tapestries on the walls. This room is of cold metal." And the guide just sort of shrugged her shoulders,
"Well, this is the winter room. That one is the summer room." Did the people in the castle know what they were doing?
Let's look at it for just a minute. In the summer, when temperatures are hotter and the sun is beating on the rocks on
the outside of the castle, the rocks become warm. They may transmit some of that heat into the castle. In England
rocks never get very warm and the inside of the castle is always cold. But in the winter temperatures become
especially cold. With those very cold walls in the winter, what happens by covering them with gold or aluminum foil in
this case?
Now any heat, a lighted candle in a room, or a person in the room gives off heat according to the fourth power their
temperature. What happens to that heat? It radiates in all directions, strikes the wall, and 80% of it or more is
reflected. Striking the other wall 80% of it or more is reflected. Finally, the reflected radiation hits the person, having
an emissivity of about 0.99 (absorptivity of 0.99), and all that heat is reabsorbed. No matter what the air temperature
is in that room, a person is warm because most body heat is being reflected back. Putting a little candle to
supplement heat would produce a warm room.
One of the students in a previous class went out to K-Mart and bought rolls of aluminum foil. She covered the outside
walls and the ceiling of her apartment by stapling aluminum foil over them. She kept the house at the same
temperature that she had been keeping it. Her heating bill dropped by 75%. She was perfectly comfortable and had
an ugly house. The heating bill was cut by 75% by putting up aluminum wall paper.
In construction aluminum is sometimes placed on one side of insulation. Most builders who use this call that the
aluminum vapor barrier. Of course plastic placed on the outside of the insulation makes a perfectly good vapor
barrier where one is necessary. If aluminum is not a vapor barrier, what function does it serve? A layer of aluminum
reflects the heat escaping through the walls of a house. It strikes the aluminum and bounces back into the room.
Fig. 9.14 In winter heat is reflected by the foil inside the wall. In the
summer the foil reflects outside heat away from the house.
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Agronomy 541 : Lesson 9a
Thermal Energy
Satellite Applications
With the coming of the satellite and infrared technology in the early 1960s the first weather satellite, Tyros, was
placed into orbit. It had a visible and an infrared capability. The infrared camera used was the bolometer, which
will be discussed at the end of the lesson. The bolometer could sense the infrared energy coming from the
surface creating a picture.
Some curious things appeared on the earth's surface. One of the things noticed quite quickly was that the
interpretation of the infrared data on the Tyros satellite was more dependent on the thermal emissivity of the
earth's surface than it was on the actual temperatures. Some very strange effects started appearing. There
were some observations of temperature in the Sahara Desert. Little cold spots began appearing in the desert.
The scientists wondered why there were cold spots in the desert. They went out to look at them, finding they
were oases, wet vegetated areas in the desert. This didn't make sense to them. What was the answer? The
emissivity of the dry sand and the wet sand had to be considered. The emissivity of water is very near 1 at all
wavelengths. The emissivity of dry sand, quartz sand over all wavelengths 0.85.
Fig. 9.16 Sahara desert oasis
Figure 9.17 displays emissivities for 0 to 1 for several common minerals varying by wavelength. While
substances are usually given an emissivity over all wavelengths, they often vary by wavelength of radiation.
Remember, radiation coming from the earth has a peak at about 10 µm, well into the infrared bands. A satellite
thermal sensor measures radiation from 8.5 to 9.75 µm. If the sensor looks at water, it's seeing something with
an emissivity of 1. If it's looking at dry quartz sand it is looking at something that has an emissivity from 0.2-0.4.
The sand would appear very cold, because it wouldn't be emitting energy very effectively.
Fig. 9.17 Variation of emissivity over
wavelength for various minerals
The satellite sensors from Tyros weren't that sensitive. They measured from about 7 to 13µm, covering a large
wavelength band. The emissivity averaged to about 0.85, because of the variation over these wavelengths.
Different soils and different minerals have different values. Feldspar, dunite, and calcite have different
emissivities because they have different characteristics of the amount of heat they would reflect at different
wavelengths. Another interesting thing occurs with calcite (Fig. 9.17). At low temperatures, calcite has an
emissivity very near 1. Remember the peak of emitted radiation wavelength raises with temperature. Looking at
a very warm surface of calcite, its emissivity would drop when the peak reached 7 µm.
Usually these temperature changes are not significant when dealing with the earth and with agriculture.
Everything stays at a moderate temperature, somewhere between 20 and 40° C. Objects are measured at
normal temperatures and given a characteristic emissivity. Things that operate at high temperatures such as jet
engines that need to be very careful of the temperature effect on emissivity as things go through a wide variety
of temperatures.
Study Question 9.6
The satellite over the Sahara was seeing 28°C temperatures with 0.85 emissivity. What actual temperatures
were existing?
K
Check Answer
Figure 9.18 displays the mistake that a satellite could make looking at different land surfaces. These surfaces
are at true temperatures of 30°C and would appear to the satellite at that temperature if their emissivity was
1.0. Limestone that was really at 30° would appear to the satellite to be at 28.5°C, whereas pine trees at 30°C
would still look like they were at 30°C. Oak trees would look like they were 29.5°. Alkali Flats, at Great Salt Lake
where they have the speed races and set all of the land speed records would look like it was 28.5° because it
has an emissivity of around .94. The white sands at White Sands National Monument would be around 28°C
with an emissivity of about .92. And dry quartz sand would appear to be 26° C in this case.
Fig. 9.18. Emissivities and apparent temperatures of various surfaces
when the objects are actually at 30°C.
Agronomy 541 : Lesson 9a
Thermal Energy
Radiation Measurement
Some years ago significant advances were made in the field of radiation physics. A fellow by the name of
Stewart came up with the law of absorption and emission of heat. Nobody paid any attention to it except
another scientist by the name of Kirkov. By 1859 he published Kirkov's Law of Radiation stating that for
radiation at the same wavelength and at the same temperature, the ratio of the emission and the absorption
power is the same for all bodies. Or in other words, a good absorber is a good emitter. About the time scientists
started to understand radiation, Langley in 1881 invented an instrument, now called the bolometer (Figure
9.19) to measure radiation. It is one of the great significant advances in scientific instrumentation.
Fig. 9.19 Bolometer
The bolometer was developed using a couple of fine wires, one made of platinum and one of iron. These wires
were connected by a little piece of platinum wire. This can also be done with copper and constantan or chrome
and iron or platinum and iron or platinum and copper. It is necessary that the metals be dissimilar. Whenever
dissimilar objects touch each other, while at different temperatures, they will generate an electric current. If the
junctions are at different temperatures, there will be some electrical difference across the wires. Placing a
volt meter on the circuit can measure the voltage.
Langley took advantage of this electrical potential caused by dissimilar metals. He placed a well insulted box or
case around one end (Fig. 9.20). At the other end he put a similar box with one end open to allow the sun to
shine in. The sun shining on one end creates the temperature difference. The voltage measured will indicate
how much energy is being absorbed by the exposed end. More than two little junctions were necessary. He put
about fifty of them together to raise the voltage. With a 50-junction "thermopile", it was called, the temperature
difference would be proportional to the amount of energy being absorbed. This is called a passive cavity
bolometer (or radiometer since it is looking at the sun).
Several uses were found for using this bolometer. By making it absorb all heat at all wavelengths, it could be
used to measure radiation from any source. It could measure how much heat was radiating from the pot-belly
stove in the corner. It could measure the amount of heat being radiated from a wall. It could measure the
amount of heat being radiated from a person. He pointed it at the moon and found that the moon wasn't
radiating enough energy to produce light, indicating that the moon's light was reflected. He pointed it at a firefly
and found that light can be emitted without heating something up to 6,000°F. And he made the first
discernment that some things give off light without heat. He found out a great deal about the composition of the
sun. His instrument was used to dispel the concept that the Earth's atmosphere acted exactly like a greenhouse
by showing that the atmosphere selectively absorbs radiation, different from the glass of a greenhouse.
Figure 9.20 Bolometer with the case removed.
A bolometer, very much like the one just mentioned, is pictured in Figure 8.20. Here you see the box and a little
window covered with plastic. The plastic keeps the air currents out but lets infrared through. Inside that little
window is a red shutter which will close the window to keep the light out, allowing the bolometer to equilibrate to
room temperature. Allowing the light in at the front permits infrared (heat) radiation to pass through. Then you
can see a wire that is going into the center. In the center is the thermometer to measure the temperature inside
this bolometer. At the very back is a spherical mirror, beyond that thing that looks like a doughnut that you see
in the middle. This spherical mirror with aluminum "silver" facing focuses all of the heat that enters the
bolometer on the thermometer so that it's concentrated onto the thermopile, measuring the differences of the
temperatures by what it's seeing.
Beyond research the IR bolometer is used to measure effectiveness of air-conditioning systems, water use of
crops, look for a "fever" in people and many other applications (Fig. 9.21).
Figure 9.21 IR thermometer measuring leaf temperature.
An infrared picture, displayed on TV or on the web, uses the same technology just discussed. A camera in a
satellite scans the clouds recording the temperatures of what it senses. Compiling them into one photograph
and colorizing according to temperature creates the image seen.
FYI : Electrical Difference
If you have a filling made of metal in your teeth, you can come to an understanding of the effect of dissimilar metals
very well.Get a little piece of tin foil and bite it on the top of your filling, and you'll measure this voltage. Because of
the difference in temperature and the dissimilar metal, you'll "measure" the voltage. You won't bite it for very long
because there is electricity conducted.
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Agronomy 541 : Lesson 9b
Air Masses, Fronts, and Mid-Latitude Cyclones
Introduction
Developed by D. Todey and E. Taylor
It is suggested that you watch Video 9B and complete the exercise in the video before continuing with the
lesson.
Podcast Version
Full Podcast List
Leading edges of air masses are called fronts. The edges of air masses are where interactions between air
masses occur. Usually, some significant weather, varying from clouds to precipitation to storms, will be
associated with these. Certain conditions are expected with the passage of a front. The unifying feature of
fronts is a low pressure area, or mid latitude cyclone, around which the fronts are oriented. The connection of
these with some weather maps and forecasting will be discussed in this lesson.
What You Will Learn in This Lesson:
About the structure of a mid-latitude cyclone.
How to determine what different fronts look like at the surface.
How to read data from a surface map.
Reading Assignments:
pg. 236-255—Aguado and Burt
pg. 262-266—Aguado and Burt
Agronomy 541 : Lesson 9b
Air Masses, Fronts, and Mid-Latitude Cyclones
Mid-Latitude cyclones
Air masses form in seasonally predictable locations. The mass of air acquires the characteristics of the locality
of origin, be it hot or cold, dry or moist. The spreading and movement of air masses provide a variety of
meteorological conditions to vast areas of the planet. Unsettled conditions are likely at the edge of an air mass
that is in the vicinity of a front.
When two air masses collide, it is uncommon for them to simply meld together. The attributes of one air mass
usually differs from another. A difference in temperature or moisture content is the usual reason that fronts
form where air masses meet. The difference in temperature or moisture across a front is sometimes very small.
The discussion here will focus on stronger more obvious fronts.
If one imagines a region of high pressure as a dome or a pile of air on the Earth's surface, it is easy to visualize
that where two such dome-like structures touch there occurs a line of pressure lower than at the center of
either high. There are several reasons for unsettled weather where air masses meet: first, both air masses
slowly rotate in a clock-wise direction, and where they meet, winds from opposite directions collide; second,
when air masses meet, one tends to dominate and the front moves. If cold air is displacing warm air, the contact
is said to be a cold front. Likewise, if warm air is displacing cold air, it is said to be a warm front. A front is
identified by an abrupt change in air temperature, dew point, and wind direction. Occasionally only one of these
is discernible.
When two air masses and areas of high pressure are in contact with each other, the slightly lower pressure
area along the boundary can be thought of as a trough. Along this trough will probably be a front of some sort
and the focusing mechanism for more significant weather. A front is defined as the boundary (or transition
zone) between two air masses. Where this boundary meets the ground is where the front is drawn, but the
boundary between air masses has a different shape for different air masses. When two air masses are in
contact without one moving against the other, the boundary is called a stationary front. A common stationary
frontal situation is pictured in Figure 9.22. Continental polar cP air resides north of the stationary front with
warm moist mT air to the south.
Fig. 9.22 A stationary front forms
along the boundary between
differing air masses which are not
moving or moving very little.
While no major weather events are expected along a stationary front, air is still converging along the front.
Some cloudiness and precipitation can be expected. Little change is observed for a period of time. These types
of fronts are the precursors to cyclone formation in the mid-latitudes. They may also be observed along the
Front Range of the Rockies when cold dense air is shoved against the mountains. The cold air is too dense to
be pushed over the mountains, remaining dammed against the mountains until it is eventually modified by solar
heating or pushed away by a warm air mass.
When an upper level disturbance or trough in the upper atmosphere passes along a stationary front, it will
provide some additional lift to the rising motion already occurring along the front. This lift creates an area of low
pressure. The rising motion will cause air to flow in toward the center of the low at the surface. Airflow toward
the center of the low will be turned by an imbalance of forces to cause the counter-clockwise circulation
associated with low pressure (Fig. 9.23).
Fig. 9.23 Vertically flowing air will produce an area of low pressure at
the surface. The atmosphere will try to balance the pressure
difference by allowing air to flow toward the low. The resulting airflow
is the counter-clockwise wind around a low seen in b).
This is the initial phase of a mid-latitude cyclone life cycle. The low will continue to strengthen creating a
stronger pressure gradient between the low and the highs nearby. The strengthening pressure gradient will
cause stronger winds. As the counter-clockwise circulation strengthens, it begins to draw the warm moist air
northward via southerly winds ahead of the low and pull the cold, dry air southward behind the low and the cold
front. This situation begins to form an open wave stage of a mid-latitude cyclone (Fig. 9.24).
Fig. 9.24 The open wave stage of a midlatitude cyclone sees strengthening
wind flow and the formation of distinct
warm and cold fronts. The winds
moving the air masses create these
new boundaries out of the existing
stationary front.
The fronts begin to take on their characteristics, which will be discussed in the next section. Not only is there
rising motion near the center of the low, there is rising motion along the fronts. These are the prime locations
for precipitation. Moisture from the tropical air is lifted above the warm front and the cold front shoves
underneath the warm moist air.
As the circulation continues, the warm front moves slowly northward while the cold front progresses steadily
behind the low. Since the cold air is more dense, it is more efficient at moving the warmer air in front of it. The
warm air along the warm front rises over the colder air in a sloping fashion. Thus, it progresses very slowly.
Eventually, the cold front "catches up" with the warm front, entering the occluded stage of a cyclone (Fig. 9.25).
Fig. 9.25 When the cold front
is brought around the low, it
lifts the warm air south of the
low creating the occluded
front or occluded stage of a
cyclone.
Study Question 9.7
What are the air masses ahead of and behind the low at the surface in the occluded stage?
mT and cP
cP and mT
mT and mT
cP and cP
Check Answer
The life cycle from stationary front to occlusion and dissipation takes several days. During this several day
period, the upper air (or jet stream) winds will guide the whole system eastward across the country. The size of
these systems is several hundred miles across and can affect areas up to 1000 miles across during very strong
systems. The strength of the system is based on the strength of the jet stream above it. Faster winds aloft
provide more energy. Weaker winds allow less development.
Fig. 9.26 Generalized life cycle following along with a mid-latitude
cyclone.
Agronomy 541 : Lesson 9b
Air Masses, Fronts, and Mid-Latitude Cyclones
Cold Fronts
Dense, dry, cold air massed in Canada is occasionally observed to "flow" under warmer moist air that is
stagnated in the central United States. This happens because the dense air simply displaces less dense air.
Often in this situation weather reports give no indication of wind, and weather maps show no front. The cold air
near the ground simply flows because there is nothing to oppose its doing so. Fog may form where the cold air
mixes with the warmer air. When mixing fog develops, it is not unlike the mixing that allows one's "breath" to be
seen on a cold day. The cold air usually remains in a shallow pool near the ground. Often, unexpected spring
freezing weather occurs this way.
Cold air commonly enters the central United States arriving as the leading edge of a moving air mass. Usually
the movement of the mass of air is as fast or faster than the spread of cold air would be without wind.
Accordingly, there is a deep layer of cold air pushing against the warmer air that occupied its path.
The warmer air will be forced up and over the encroaching cold air layer. The rapid lifting of the warm air often
results in strong thunderstorms. Lines of thunderstorms may occur at the cold front and scattered cumulus (fair
weather) clouds trail behind it (Figure 9.27).
Fig. 9.27 Cold dense air (moving left to right) lifts warm,
moist air at the front.
The cold front is depicted on a weather map as a blue line with triangles or as simply a blue line (Figure 9.28).
A cold front, as discussed in the previous section, is the leading edge of colder air brought southward by winds
around an area of low pressure. These fronts are most common during the active weather times of fall, winter,
and spring.
Winds ahead of the cold front are southwesterly in the warm sector of the mid-latitude cyclone. After the cold
front passes a point, winds turn to the west, northwest, or north. Since the cold air is very dense it is very
effective at displacing the warm air ahead of it. The dense cold runs under the warm air lifting it. The lifting of
warm moist air usually causes cloudiness at the least. If the air is moist and unstable enough, rain and
thunderstorms can accompany the passage of the front. Air pressure usually falls as a cold front approaches,
rising rapidly after passage as the dense cold air moves in. The dew point falls indicating the change to a dry
air mass. Usually there is little local observational evidence of a cold front approaching.
A surface map depicting a cold front is shown in Figure 9.28. Station data indicate the real difference
between the air masses ahead of the front and behind the front. Notice where the cold front (solid blue line)
is.
Fig. 9.28. Surface map with fronts, radar, and station data plotted. Click
image to enlarge.
Notice that in Oklahoma and Texas the air temperatures are in the 50°F range and dewpoints are in the 40°F.
Behind the front the temperature has dropped to 25°F with a dewpoint of 14°F in North Platte, Nebraska. An
occluded front is indicated by the lavender line over Minnesota and Iowa. Cloudiness is indicated by the filled
circles throughout the Midwest. Only a few stations are reporting any rain. Most of the moisture supply for rain
from the Gulf of Mexico is being transported to the southeastern United States, where Florida is receiving
heavy rain.
Review the In Detail and answer the following study questions.
Study Question 9.8
What is the temperature at Bismark, ND?
°F
Check Answer
Study Question 9.9
What is the wind speed?
knots
Study Question 9.10
The sky cover is:
clear
overcast
Check Answer
Check Answer
IN DETAIL : Station Data
Explanation of surface data plotted in a station model.
Weather symbols
Wind symbols
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Agronomy 541 : Lesson 9b
Air Masses, Fronts, and Mid-Latitude Cyclones
Warm Fronts
Warm and/or moist air encroaching on a mass of cooler or drier air results in a warm front. Warm and moist air
is naturally less dense than cold and dry air and will easily "ride" over the top of cool air. Accordingly, a warm
front does not represent an abrupt or sudden contrast, as does a cold front (Figure 9.29).
Fig. 9.29 Warm moist air easily rides over colder
air when impinging upon it.
Warm fronts are represented on weather maps as a red line with "half suns" on the leading edge (Figure 9.30).
Fig. 9.30 Depiction of warm front in a mid-latitude
cyclone.
The leading edge of a warm front may have a 30,000 foot or greater altitude and be hundreds of miles in
advance of the line at ground level separating warm and cold. As the warm air lifts over the cool air, it too cools
and at some level condensation begins. The first indication of an approaching warm front may be cirrus clouds
(or persistent contrails) followed in time by altostratus and then stratus clouds (Figure 9.31), and possibly
widespread and general rain (or snow). Although not considered as violent as weather events associated with a
cold front, warm fronts are usually the cause of glaze or freezing rain that can cripple movement of traffic and
break trees and power lines. Also, heavy general rains that can occur may result in widespread flooding, as
opposed to intense flash flooding often caused by a thunderstorm.
Fig. 9.31 Side view of a warm front. The cloud deck gradually lowers
and can produce rain.
The wind change around a warm front goes from easterly or southeasterly ahead of the front to southerly of
southwesterly behind it. Unlike the cold front, advance notice of a warm front is very apparent. Cirrus clouds
portend the advance of a warm from 24-36 hours ahead. These clouds are produced by the warm air being
lifted to very high levels in the atmosphere. Behind the warm front the dew point and temperature increase as
the air mass changes from a cP air mass to a mT air mass. Precipitation can occur, but is more showery type
as opposed to the thunderstorm type of the cold front. The extensive cloudiness ahead of the warm front
quickly gives way to clear skies. The lift disappears after the front passes. Pressure is usually falling until the
front passes, then it may rise slightly as the warm sector of the cyclone passes over.
Fig. 9.32 A warm front is depicted from southern Canada across the
Dakotas. Click image to enlarge.
Usually, the contrast across the warm front is not as striking as across a cold front. But changes can be seen.
Ahead of this warm front there is some cloudiness, but not as extensive as usually occurs. This is probably
because the air here is very dry after coming off the mountains. The fronts on this map are drawn from 2 hours
prior to these observations. Therefore, areas of the eastern Dakotas are probably behind the warm front.
Study Question 9.11
What is the warmest temperature behind the warm front?
°F
Check Answer
Study Question 9.12
What is the highest dewpoint behind the warm front?
°F
Check Answer
Study Question 9.13
Is there a wind direction change across the front?
Yes
No
Check Answer
Agronomy 541 : Lesson 9b
Air Masses, Fronts, and Mid-Latitude Cyclones
Occluded Fronts
Because cold air is more efficient at moving warm air out of its way, it is more effective than the converse; a
cold front associated with cyclonic activity tends to move faster than associated warm fronts. When a cold front
overtakes a warm front, the fronts are said to be "occluded." In early stages of occlusion, heavy rain and
thunderstorms may follow a typical warm front pattern. The activity with the occlusion is usually relatively shortlived as it involves the rapid lifting of a finite mass of warm air. At the time of occlusion, the pressure is at its
lowest and storm intensity is at its greatest. However, it also signals the gradual end of the frontal activity and
the beginning of cyclonic weakening. Often another wave will form along the frontal boundary and begin again
the genesis of a cyclonic pattern. The occluded front may have the character of a cold front if the coldest air in
the system is behind the encroaching cold front. The characteristics will be less severe if the encroaching cold
front is not the coldest air in the system (that is, if the warm front was encroaching on the coldest air in the
system). The process of occlusion serves to lift warm air near the low aloft as the cold air is wrapped around
the low (Figure 9.33).
Fig. 9.33 Side view of an occluded system looking north. The cold
air has lifted the warm air aloft. An occluded front can be seen in
Figure 9.28.
Web sites with these types of maps are available at:
UCAR
Unisys weather (formerly Purdue)
Weather Channel
Agronomy 541 : Lesson 9b
Air Masses, Fronts, and Mid-Latitude Cyclones
Identification and Forecasting
These various observations of storms can be used to create a point forecast of the weather. Looking at what is
happening to the weather nearby can give an indication of what changes may be coming.
If the wind is blowing you can always tell where the high and the low pressure are. The rule is "Stand with your
back to the wind, and your left hand will point toward the low." Technically you need to rotate a little bit. Stand
with your back to the wind, and turn about 45° to your right. The turn compensates for the turning of the wind
caused by friction.
What is the value of this? These systems tend to move toward the east. Sometimes they move north, and
sometimes they move south. But most of the time, they move almost directly to the east. A good way to
determine what is going to happen with the weather is to observe what's happening 1500 miles to the west of
you. That is what will happen to you somewhere along the line. In fact, that's the way the National Weather
Service started off initially. People used the telegraph capability to find out what the weather was to the west.
If you find the wind is at your back out of the north, a high is to your west. Likely, a cold front has passed by
recently. The winds are probably going to ease up because near the center of the high, winds are usually calm
where the pressure gradient becomes very weak. You will probably not have more significant precipitation, and
perhaps have a still and cold night.
If you have the wind at your back coming out of the south, the high is to your east. You don't necessarily know
when a front is getting there. As the wind gets stronger and stronger, the front is nearby. A low may be to your
left. You may have a storm coming with that low. Or you may have a front coming without a storm associated
with it, where the wind will just change directions. To determine which condition will exist will entail watching the
clouds.
Using these various observations can give a good idea of upcoming weather.
Most people who have lived in an area for a long time know several things about the effect of wind direction,
pressure, or clouds. They have observed that under certain wind and pressure conditions, certain weather will
occur. This is a way that a lot of point forecasting was done in earlier days. People living at one point didn't
know conditions anywhere else. The only information available on what usually happens with high and low
pressures was from your point. Point forecasting is still useful when isolated or without access to weather
observations or forecasts.
Other methods of forecasting are also used. A persistence forecast is one where the current conditions are
expected to continue. No change is expected. Another type is using an analogue. When one pattern looks
similar to one which has occurred previously, the conditions which occurred then would be expected to occur
again. A much poorer forecast would be a climatological forecast. This type assumes the average condition for
the time period would be expected. This will happen periodically. But rarely are conditions average. True
forecasts involve these methods while relying heavily on numerical models which forecast conditions based on
equations describing the motions of the atmosphere.
Current National Weather Service forecasts may be found at :http://iwin.nws.noaa.gov
Assignment 9.1
Click here for Assignment 9.1
Lesson 9 Reflection
Why reflect?
Submit your answers to the following questions in the Student Notebook System.
1. In your own words, write a short summary (<150 words) for this lesson.
2. What is the most valuable concept that you learned from the lesson? Why is this concept valuable to
you?
3. What concepts in the lesson are still unclear/the least clear to you?
4. What learning strategies did you use in this lesson?
Agronomy 541 : Lesson 9a
Thermal Energy
Introduction
Developed by D. Todey and E. Taylor
It is suggested that you watch Video 9A and complete the exercise in the video before continuing with the
lesson.
Podcast Version
Full Podcast List
The ability to photograph infrared radiation allows one to collect data about the infrared emission and,
consequently, temperature of objects. This property of IR radiation has found usefulness in the agricultural
community, in diagnostic medicine, meteorology, and many other areas. This part of lesson 9 will examine IR
radiation and its use in assessing the temperature and health of a variety of agricultural areas.
What You Will Learn in This Lesson:
How objects emit radiation.
What IR radiation tells about objects.
How satellite technology can be used to determine surface characteristics.
Reading Assignments:
p. 371—Aguado and Burt