Marine Science Laboratory Solar Radiation
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Name ________________________Lab Section _____________Date _____________.
Grade 10/10
MS20 Laboratory: Solar Radiation and Light Transmission
Record all data using appropriate metric units (centimeters, grams, etc.). Remember to
use significant figure rules and to indicate appropriate units (if the scale reads 13.4 g,
your answer is not 13.4, but 13.4 g (or 13.4 grams).
A.
Measuring a “solar constant”
Your numbers will differ because light sources and light meters vary. However, your
numbers may be used compare trends shown here.
Table one:
Distance from light source
Measured
Light intensity
Theoretical (equation 1)
50 cm
500 lux
488 lux
100 cm
122 lux -Æ
122 lux
150 cm
60 lux
54.2 lux
200 cm
34 lux
30.5
Are your measured values of light intensity similar to your calculated (theoretical)
values? List several possible sources of error.
Meter not held at proper distance or angle. Stray light in the room. Batteries weak, light
source inconsistent etc.
Earth’s solar constant is 1366 W/m2 (watts per square meter). The planet Venus is
72% of the distance from the Earth to the sun, and the planet Mars is 152%. Ignoring
clouds and reflection (albedo), how much more sunlight might you expect to receive at
the surface of Venus? How much less light might you expect to receive at the surface of
Mars? (Hint: use equation 1 and remember the denominator is the ratio of the
distances)
Light intensity of x = Light Xo intensity of standard / (ratio of the distance of X divided by standard
Xo)2
Light Venus= 1366/ (.72) 22
= 2635 Watts/M2 Light at Venus is 2635/1366 = 92% greater than Earth
Light Mars = 1366/(1.52) 2
= 591 Watts/M2 Light at Mars is 591/1366 = 43% less than Earth
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In our model, the light source is 200 cm from the globe, and the globe has a radius of 15
centimeters. A flea sitting on the globe is 15 cm closer to the light source at noon than
he is at sunrise or sunset. How much does the light intensity drop as our flea is rotated
from noon to sunset? (Hint: use equation 1 again).
Light @ 185 = Light at 200/(185/200) 2
= Light at 200/(.925) 2
= Light at 200/.856
if the light at 200 = 1
then 1/.856 = 1.16 the light intensity drops by 16%
The sun is 150 million kilometers from the Earth, and the Earth’s equatorial radius is
6378 km. How much would you expect the sun’s intensity to drop, based only on this
additional distance? (Hey, how about using equation 1 again).
Light @ 185 = Light at Earth Surface/(150,000,000/150,006,378) 2
= Light at X/(.99999) 2
= Light at X/.99999
if the light at Earth’s closest point = 1
then 1/.9999 = 1.000 drops by 0%
The amount of light difference is very little (not significant).
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B.
Effect of latitude and season on incoming solar radiation
Table two:
Latitude of measurement
Light intensity
Axis vertical
Axis tilted
90° N (north pole)
0
0
66.5° N (arctic circle)
13
1
45° N
24
12
23.5° N (T of Cancer)
29
24
0° (equator)
36
29
23.5° S (T of Capricorn)
30
36
How do the measurements compare between latitudes when the tilt of the globe is
changed? What day(s) of the year are represented by the globe with its axis vertical?
What day(s) are represented by the globe with its axis tilted?. With the Earth vertical,
the Equator is closest to the light source and therefore is brightest. With the Earth tilted,
the Tropic of Capricorn is closest to the light source and therefore is brightest.
Axis tilted = Equinox (March 22 & Sept. 22)
Axis vertical = 1st day of Summer/ winter (shortest & longest days of the year (June and
Sept 22)
Compare the decrease in light intensity from the equator to 23.5 N with the decrease
between 45° N and 66.5° N. Since the changes in latitude are similar, why is the change
in intensity so much different?
The angle at which the sun’s rays strike the globe is different.
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In the northern hemisphere winter, Earth is near perihelion (closest approach to the
sun), at 147 million km, while during the northern hemisphere summer Earth is at
aphelion (furthest from the sun) at 152 million km. Why are the summers hotter than the
winters in the northern hemisphere?
The tilt of Earth’s axis is more significant than the elliptical orbit.
C.
Albedo
Incident Light 320
Table three:
Surface
White
Black
Water
Ice
Reflected Radiation
60
7
10
40
Albedo (equation 2)
18%
2%
3%
12.5%
If you had a lava flow made of fresh, black basalt partially covering a light-colored
granite surface, which rock would have the greater albedo? What happens to the
radiation that is incident to the surface but is not reflected back into the atmosphere
(hint: think about walking across an asphalt parking lot in bare feet in the summer.)?
The light colored rock has the greater albedo (reflectivity). Heat is absorbed by the
darker color.
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Which surface was more reflective, the water or the ice? If continental glaciers and sea
ice were to begin to expand on the Earth at the expense of rock and ocean, what effect
might this have on global temperatures? Currently, as glaciers melt and sea ice retreats
further into the arctic regions each year, what effect are we likely to see on global
temperatures?
Ice has a much higher albedo. If ice expands, so does albedo hence more light is
reflected and global temperatures would drop. As ice retreats, global temperatures will
rise.
On the diagram below, sketch the earth's axis at the winter and summer solstice, and
indicate at what latitude the solar constant is at maximum during these periods.
Would the earth's albedo be higher or lower in the southern hemisphere compared
with the northern hemisphere? Explain! (hint: consider the distribution of land
mass/water in the two hemispheres)
The albedo in the southern hemisphere would be lower because a higher percentage of
the Earth’s surface is ocean than in the north. Water absorbs rather than reflects light.
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C.
Turbidity in Seawater
Table four:
Sediment concentration
Light intensity (lux)
Visibility limit (cm)
0 grams
326
60 cm
1 gram (50 mg/liter)
23
54
2 gram (100 mg/liter)
7
27
3 gram (150 mg/liter)
4
16
Which is the more accurate measure of turbidity: measuring light loss with the lux meter,
or using the mini Secchi disk? Explain!
Measuring with the Lux Meter is theoretically more accurate because it measures light in
the same way every time (unless the batteries are weak or there is contamination on the
sensitive meter. Also, you may see the Secchi disk differently than your partner
because the eyesight of individuals differs. However, many of you did very well with the
Secchi disk!
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Suppose rather than fine grained clay/silt sized particles, you had added the same
concentrations (50 mg/liter, 100 mg/liter, and 150 mg/liter) of sand sized particles to
the aquarium. Would you expect the turbidity (i.e., light attenuation) to be the same,
greater, or less? Explain!
At first, the light attenuation may be the same (fewer large particles versus more small
particles,) but settling would occur much more quickly with the sand and so the water
would clear faster.
C.
Light Attenuation and settling rates
Table five:
Time
Light intensity (lux)
Suspended sediment (mg)
0 minutes
2
150
1 minutes
4
145
2 minutes
6
137
3 minutes
9
117
4 minutes
10
114
5 minutes
11
112
6 minutes
12
110
7 minutes
13
104
8 minutes
13
104
9 minutes
14
100
10 minutes
14
100
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From your laboratory experiments, can you tell if water with higher suspended
sediment concentrations would heat up faster than clear water? Explain! (Hint:
consider the light absorption experiments, not the albedo!)
As the sediments absorb light the water should heat up faster. Light is energy; light is
absorbed and converted to heat. However, the temperature may not rise much
because of water’s high heat capacity.
Examine the Sediment Concentration vs. Time Graph below. This settling graph is similar
to your laboratory experiment. Which of the two curves (A or B) represents the coarser
sediment? Explain!
B settles faster and is therefore coarser.
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Primary productivity is enhanced by light. The depth of the photic zone is measured by the
depth of light penetration. Can productivity affect the depth of the photic zone? Explain!
Yes, productivity affects the depth of the photic zone. High productivity means “more
plankton”; more plankton absorb and reflect more light. So the light does not penetrate
as deeply when productivity is high.
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