Solar Radiation, Albedo, and Light
Transmission in Seawater
Equipment needed for each student group:
1 microscope light
1 portable ‘lux’ meter (digital light meter)
1 aquarium
1 stopwatch
1 globe
1 meter stick
1 mini secchi disk (2 cm diameter) taped to a 30 cm ruler
For general class use:
Powdered silica (<0.0004 cm diameter)
3 color filters (blue, green and red)
1.5 meter long tubes filled with water
4 dishes of surface types and albedo setup
(overhead lights off during this laboratory; shades drawn)
DISCUSSION:
Solar Radiation
In order to discuss solar radiation we need to first define a calorie. A calorie is the amount
of heat energy required to raise the temperature of 1 gm of freshwater one degree Centigrade.
We also need to establish two points about the relationship between the sun and the earth. First,
the sun is so far away from the earth that its rays are essentially parallel as they reach the earth
(Fig. 1). Secondly, the earth is tilted on its axis of rotation. As it revolves around the sun (once
each year) this tilt causes the sun to be overhead at 23.5oN at summer solstice (June 21) and to be
overhead at 23.5oS at winter solstice (December 22), and overhead at the equator on March 21
(vernal equinox) and September 23 (autumnal equinox). These two factors cause an unequal
heating of the earth's surface.
The solar constant is the rate at which heat energy reaches the earth's upper atmosphere and
is so named because it is very constant at 2.0 cal/ cm2/min. There are several factors that affect
the actual amount of energy that reaches the surface of the earth
(http://oceanz.tamu.edu/~wormuth/physicalproperties/eartheatbudget.gif).
During this lab exercise, we will use a light meter to measure incoming radiation. If we
consider the autumnal or vernal equinox, when the sun is directly overhead at the equator, a flat
surface of fixed dimensions will receive more energy than would the same surface at a higher
latitude because of the curvature of the earth's surface (Fig. 1).
EXERCISE 1:
Measuring a ‘Solar Constant’
1. For your first two exercises, you need to emulate the sun/earth relationship at autumnal or
vernal equinox, as shown in Figure 2. To do this, put your globe in the dish provided and
position it so that the equator is level and the Pacific Ocean faces the light source. Measure the
height of the equator from the bench top and adjust the center of the light source to be at the
same height. This should make the maximum light level fall at the equator. Move the globe
away from the light source until there is a slight halo of light on the wall behind the globe. Once
this distance is established, keep it constant for Exercise 1 and 2.
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FIGURE 1
2. It will be necessary to use the lux meter (Fig. 3) to confirm this by moving it slightly north
and south of the equator. If the light level is higher in either direction, adjust the height of the
light source to make the maximum occur at the equator (Fig. 4). The lux meter should be set on
the lowest scale (x 1). Record the light level on your forms as the solar constant and as the
equator (0o) value.
FIGURE 2
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FIGURE 3
FIGURE 4
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EXERCISE 2:
Latitude Effect on the Incoming Radiation
As Fig. 1 shows, the same amount of solar energy reaching the surface of the earth when the
sun is directly overhead at the equator is spread out over a larger surface area at higher latitudes,
and, therefore, there is less heating per unit area at higher latitudes than at lower latitudes.
Ocean currents redistribute this unequal heating, carrying colder water towards the equator and
warmer water toward the poles (http://oceanz.tamu.edu/~wormuth/geostrophic/pacificsurfacecurrents.gif).
This exercise will allow us to make our own measurements to show this phenomenon.
1. To demonstrate the effect of latitude, measure the light levels along 180o longitude in the
Pacific Ocean (the International Date Line) every 15o as indicated in Fig. 5 and record them
in Table 1 on your form. Make sure your measurement is along the same line of longitude
and hold the lux meter sensor tangent to (flat against) the surface of the globe at each
latitude. Graph your results in Table 2. To do this, create your own scale on the y-axis
based on the range of lux readings you recorded. For example, if your values we between
200 and 4, you would assign a value of 0 for the bottom, 200 for the top and each increment
would increase 10. After complete the graph, answer the following questions on your forms:
FIGURE 5
TABLE 1
0o
15o
30o
60o
75o
90o
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45o
TABLE 2
Answer the following questions:
A. Is this a linear relationship indicating a constant change in radiation with latitude? Why or
why not?
B. During the northern hemisphere winter, the distance between the earth and the sun is at a
MINIMUM; during the summer the distance is at a MAXIMUM, yet the summers are warmer
and the winters are colder. How would you explain this?
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DISCUSSION:
Albedo
A portion of the incident radiation that comes to earth is reflected back into the atmosphere
(http://oceanz.tamu.edu/~wormuth/physicalproperties/eartheatbudget.gif). The ratio of the
reflected radiation divided by the incident radiation is called the albedo and it varies depending
on the surface material: its color, composition and texture. Exercise 3 will compare the albedo of
the following common materials found on land and on the surface of the ocean:
Snow covered surface (represented by light colored sand)
Soil covered surface (represented by dark colored sand)
Water covered surface (represented by dark colored sand in 5-10 cm of water)
Glaciers or Ice sheets (represented by 5-10 cm thick ice block)
If we consider the following albedo equation, which material will you expect to have the greatest
albedo and why? Which should have the least and why?
albedo = reflected radiation
incident radiation
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EXERCISE 3:
Albedo
The overhead lights should be on for this portion of the lab
1. A small dish will be used to hold the four various earth materials over which the albedo is
measured.
2. Using the ambient light in the room, measure the INCIDENT RADIATION that will hit the
surface of the material by holding the lux meter face up above the center of the dish.
3. One dish will contain white sand (collected off the beach in Clearwater, Florida [not
financed by your lab fees]).
4. Next measure the REFLECTED RADIATION by holding the lux meter sensor down, 5-10
cm above the center of dish. Record the maximum reflection from the surface of the white
sand on your forms. Calculate the albedo according to the formula above.
5. Replace the dish containing the white sand with the dish containing the dark colored sand:
repeat the albedo experiment. You need only measure the reflected light since the incident
light will be the same for all four materials. The vertical distance should be the same for
both meters. NOTE: IT IS VERY IMPORTANT TO TAKE READINGS FROM THE
SAME SPOT FOR EACH ALBEDO! Fill in the blanks on Table 3 for the reflected light
and the albedo.
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6. Replace the cover containing the dark sand with the dish containing the dark sand and water:
repeat the albedo experiment. Record the data on Table 3. Be careful not to get the lux
meter wet!
7. Finally, replace the cover containing the dark sand and water with the cover containing the
dark sand and ice (mimicking ice-covered shallow water) and repeat the albedo experiment.
Again, record the data on Table 3 and calculate the albedo.
TABLE 3
INCIDENT RADIATION
CRUSHED ICE/LIGHT SAND
(SNOW)
reflected radiation
(LUX)
DARK SAND
(SOIL)
reflected radiation
albedo
albedo
WATER WITH BLACK SAND
(OCEAN)
reflected radiation
SOLID ICE
(GLACIER)
reflected radiation
albedo
albedo
Answer the following questions:
A. Which would be the better reflector of incident radiation - soil or water? Is this to be
expected? Explain!
B. From your albedo experiments, can you ‘guess’ why it took such a long time for the
continental glaciers to melt once they had become established?
C. How will the melting of the ice caps over the North Pole change the overall albedo of the
earth?
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DISCUSSION:
Qualitative Changes of Light in Seawater
As light strikes the surface of the ocean and penetrates below the surface, there are two types
of changes that take place: qualitative changes and quantitative changes. In Exercise 4 we will
look at the qualitative changes and in Exercise 5 we will look at the quantitative changes.
Light is made up of many wavelengths. Most of these wavelengths fall in the visible part of
the spectrum, which we see as ‘white light’. This light can be split into component colors by a
prism or by raindrops in the atmosphere to form a rainbow. When light strikes the surface of the
ocean, some wavelengths can't penetrate very far below the surface of the ocean. These
wavelengths can still be important (i.e., UV light which can cause skin cancer and is affected by
ozone, or lack thereof, in the atmosphere). The selective transmission of various wavelengths
influences colors of organisms as they appear to us at the depths where they live, and that has
had an important evolutionary influence on the photosynthetic pigments in oceanic plants.
EXERCISE 4:
Transmission of Visible Light as a Function of
Wavelength
1. Fill a 1.5 meter tube sealed on one end with water. In the room you will find a light, colored
wavelength filter, tube, stand and lux meter in the arrangement shown in Figure 6. You will
be responsible for changing the colored filters out and for recording the readings. Hold the
light meter sensor directly against the acrylic sealed end when making light measurements.
FIGURE 6
Lamp
Filter
Open end
NOTE: Items are not to scale
Lux Meter
Sealed end
Support Stand
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2. Make sure that the light source and filter are as close as possible to the top of the water
surface without getting the filter wet. It is important to not move the light during all of your
measurements with the tube.
3. First, measure the values for transmitted radiation, I1. Start with the white light passing
through the tube when there is no filter. Record this lux meter reading on your forms in
Table 4.
4. Without moving either the light source or the light sensor, slide one of the colored filters
between the light source and the water. Record the reading for transmitted radiation on your
forms in Table 4. Repeat this for the other color filter, making sure to record the data.
5. After finishing all the transmitted values with the tube, measure the incident radiation, I0, for
each color and white light. To do this, hold the sensor at a maximum distance of 5 inches
from the lamp with nothing between the filter and the sensor. Enter the results in Table 4 on
your form.
6. In Table 4, calculate the percent of light transmission by comparing the light passed through
the tube for each colored filter and for white light with the direct light measurement for each
respectively. The percent transmission is calculated by the following equation where I1 is the
measurement through the tube of water and I0 is the direct light measurement:
% Transmission =
I1
X 100
I0
TABLE 4
I1
I0
% Transmission
white light
violet light
green light
red light
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7. Plot the % of the light transmission of the various colors on the bar graph below in Table 5.
TABLE 5
Answer the following questions:
A. From your experiments can you tell which of these three colors would be less detectable if
you were looking down from a glass bottom boat at objects 10 meters deep in the water?
Explain.
B. If you took a flash photo at close range with an underwater camera of a red and green
starfish, would the colors in the photo be the real colors? Why or why not?
C. You collect a bright red shrimp in a deep sea trawl at 200m depth. It would seem that this
shrimp would be very conspicuous to its predators, yet isn't. Why might this be so?
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DISCUSSION: Quantitative Changes of Light in Seawater
One of the measurements oceanographers (particularly biological oceanographers) take is
that of water clarity. There is an equation that governs the vertical extinction (or loss) of light as
it penetrates down into the water column. This equation allows us to estimate the light intensity
at any given depth once we know the surface light intensity and a coefficient of extinction for
that particular water column. The form of the equation is a negative exponential. This means the
quantitative loss of light is not a linear loss, but is greater in the upper portion of the water
column and becomes less as one goes deeper.
The loss of light in water is due to several factors. First is the scattering of light by particles,
both living and non-living. Additionally, light absorption by living particles (small plants)
decreases the amount of light that reaches depths. Also, the conversion of light to heat energy
contributes to light loss in water by raising the temperature of the water slightly.
Many particles are vertically stratified in the water column. Plants occur only in the upper
150 m or so, and they depend on downwelling light for photosynthesis. Biologists measure the
penetration of light VERTICALLY in the water column to relate it to plant growth. They
sometimes use a secchi disc to accomplish this. A secchi disc is a flat disc, usually with
alternating quadrants of black and white. An observer watches the secchi disc as it is lowered
down into the water, recording the depth at which it just disappears from sight. Interestingly
enough, studies have shown that this depth is relatively independent of the diameter of the disc or
the visual acuity of observers with ‘normal’ eyesight.
There are oceanographers who are interested in the HORIZONTAL transmission of light.
For their measurements, they use a transmissometer, an instrument that utilizes a horizontally
oriented light source and a horizontally oriented light sensor. These are usually separated by 1 m
or less. Once calibrated, the transmissometer measures the % light transmission at various
depths as it is lowered down through the water column.
In the lab today we will use a very small secchi disc to make horizontal determinations of
light transmission in different concentrations of suspended silica. Since the light meters are not
waterproof, we are not able to make vertical measurements.
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EXERCISE 5:
Turbidity and Secchi Disk Measurements
1. Fill the provided aquarium with water to 19 cm depth. Measure the white light intensity (in
LUX) over the 49.5 cm path length of the aquarium. Place the mini secchi disk in the
aquarium and slide it from the source of light toward the far end (Fig. 7). Record the point at
which the secchi disk is no longer visible when looking from the end of the aquarium with
the light source (note: without silica in the water you will see it all the way back to the far
side of the aquarium). Enter the light attenuation value in Table 6 on your forms.
FIGURE 7
2. Add exactly 2.4 gms of powered silica to the aquarium, and mix it thoroughly. Note that the
silica tends to clump on the bottom when it is first dropped in the water, so be sure to stir all
the way to the bottom. Each aquarium will contain 16 liters of water (at 19 cm water depth),
so the concentration of the silica suspension is 150milligrams/liter when thoroughly mixed.
Wait a few seconds for the water in the aquarium to calm down. Then, using the digital lux
meter, measure the light which passes through the 49.5 cm path length. Record the data in
Table 6 on your forms.
3. Add an additional 1.2 gms of silica, mix, and measure the light attenuation and ‘depth of
visibility’ once again. (This is equivalent to 225mg/liter or 3.6 gms).
4. Add a final 1.2 gms of silica and repeat the experiment. Record all the data in on your form.
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TABLE 6
sediment concentration
( gms, milligrams/liter)
light intensity
(LUX)
limit of visibility
(centimeters)
49.5 cm
0 gms, 0mg/l
2.4 gms, 150mg/l
3.6 gms, 225mg/l
4.8 gms, 300mg/l
5. Plot the data on from Table 6 onto Table 7. Don’t plot the first point at zero silica
concentration. Instead, you will set your own scale for the 3 different densities (150mg/l,
225mg/l, and 300mg/l). To do this, fill in the scale on the left y-axis using the range of
values you recorded. For example, if your lux readings were between 200 – 350, your bottom
value would be 150, the top value 400 and each increment would increase 50. You will
graph 2 lines, so use two different symbols: ‘X’ for light attenuation and ‘O’ for depth of
visibility. Connect the Xs with one straight line and the Os with another.
TABLE 7
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Answer the following questions:
A. In your opinion, which is the more accurate measure of turbidity: measuring light loss with
the lux meter or using the mini Secchi disk? Justify your choice.
B. Suppose that rather than silica particles (about 40 microns), you had added the same
concentration (150, 225 and 300mg/l) of sand sized particles to the aquarium. Would you expect
the turbidity (i.e., light attenuation) to be the same, greater or less? Justify your answer.
DISCUSSION: LIGHT ATTENUATION AND SETTLING RATES
Turbidity is a physical property of seawater that changes with time. For example, if a severe
coastal storm causes flooding, river systems almost immediately transport suspended sediment
into the estuaries, thereby raising the turbidity of the seawater. The amount of time the
suspended sediment stays in the water column is a function of the sediment size and its settling
rate. In addition, the common suspended sediment size along the Texas coast is in the clay sized
particles. Examples of this can be seen in the Brazos and Mississippi rivers. When clay-sized
particles enter seawater, the flat, plate-like clay particles tend to clump together through a
process called flocculation. Once this happens, the ‘effective particle size’ is increased and the
particles settle out more rapidly than would otherwise be expected.
Phytoplankton are also affected by ‘sinking’ which is a function of size and density
difference between a particular species and the seawater in which it lives. This means that the
same species will sink more slowly in colder water (higher latitudes) than in warmer water
(lower latitudes) of the same salinity because the density difference will be less. If the
phytoplankton sink out of the photic zone, productivity ceases. To combat sinking, some species
have developed adaptations to decrease their settling rates (i.e., oils to reduce bulk density and
spines to increase their surface area to volume ratio, thereby increasing frictional forces).
EXERCISE 6:
Light Attenuation and Settling (Lamp intensity same as in
Exercise 5)
1. The aquarium still has 300mg/l suspended sediment from the previous experiment. Turn on
the microscope light, and resuspend the sediment with the secchi. Wait a few seconds for the
surface of the water to stabilize, then begin recording the light intensity (lux) along the 49.5
cm path length at one minute intervals for 0-10 minutes using the stopwatch (record your
data in Table 8 on your forms).
2. Fill in the suspended sediment column by estimating the correct values from the light
intensity curve you prepared in Table 7 from Exercise 5.
3. Plot the suspended sediment concentrations as a function of time from 0-10 minutes in Table
9.
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TABLE 8
time
light intensity (LUX)
suspended sediment
0
1 min
2 min
3 min
4 min
5 min
6 min
7 min
8 min
9 min
10 min
TABLE 9
Answer the following question:
A. Examine the sedimentation concentration curve in Table 9. Explain why there is not a
straight line decrease over the 10 minute period!
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