36
Exercise 3
Cgs units should be used throughout. In our case, where A z = Ao and Zg = mean depth
of a lake basin, or zm/2 for the aquarium,
(for our purposes it is sufficient simply to sum densities for every 1 cm of depth and
divide the total by zm).
The value of S is determined most easily by plotting Z on the vertical axis (set the
origin at zero) and the values for (z - Zg) x (pz - Pm) on the horizontal axis. The area
enclosed by the curve is equal to S and may be determined planimetrically or by
counting squares. Calculate stability for your model at the time of maximum thermal
stratification (p. 32, #4). How do you regard the efficacy ofthis calculation as a measure
of "stability"?
Note: It is suggested that the class divide into thirds and each group use Lake Models
2, 3, and 4. The results for each model then can then be presented orally to the entire
class.
LAKE MODEL 2: TROPICAL LAKE
Purpose
The physical properties of a tropical lake will be illustrated. It will be demonstrated
how small temperature differences at high temperatures can generate the same stability
in a lake as much greater temperature differences did at lower temperatures. The
physical importance of temperature profiles thus is not temperature itself, but rather
the thermally produced density differences.
Procedures
1. Allow the water (uniformly mixed at a temperature of about 200q to stand until no
major currents are visible. Distribute over the surface of the water afew particles of
catechol violet and allow the streaks to form. Introduce a moderate wind (lamp is off
at this time) and observe that the waters will mix completely.
2. Record temperatures as a function of depth. Turn on the lamp, noting this time as
the beginning of the experiment. Measure temperatures after 5 min, 15 min, and
30 min. Plot temperatures as a function of depth at the time they are taken, using the
spare thermometer to obtain carefully the intermediate readings in the near-surface
region.
3. Spread particles of catechol violet liberally over the surface as in Lake Model 1 and
allow the streaks to descend.
4. Turn off the light. Introduce a moderate wind so as to bring the zone of
homogeneous red color down to a depth of about 5 to 7 cm. Record temperatures
and the depth of the red layer. Measure the periods of the internal seiche as in Lake
Modell.
5. With a slightly more forceful wind, bring the homogeneous red layer down to
include the upper two-thirds of the "lake." Record temperatures and the depth ofthe
homogeneous red layer. Now, with forced oscillation from the wind, cause another
internal seiche to form and measure the period. Record temperatures and the depth
of the homogeneous red layer again.
6. With a strong wind, bring about complete mixing. Record the final temperatures.
Exercise 3
Cgs units should be used throughout. In our case, where A z = Ao and Zg = mean depth
of a lake basin, or zm/2 for the aquarium,
(for our purposes it is sufficient simply to sum densities for every 1 cm of depth and
divide the total by zm).
The value of S is determined most easily by plotting Z on the vertical axis (set the
origin at zero) and the values for (z - Zg) x (pz - Pm) on the horizontal axis. The area
enclosed by the curve is equal to S and may be determined planimetrically or by
counting squares. Calculate stability for your model at the time of maximum thermal
stratification (p. 32, #4). How do you regard the efficacy ofthis calculation as a measure
of "stability"?
Note: It is suggested that the class divide into thirds and each group use Lake Models
2, 3, and 4. The results for each model then can then be presented orally to the entire
class.
LAKE MODEL 2: TROPICAL LAKE
Purpose
The physical properties of a tropical lake will be illustrated. It will be demonstrated
how small temperature differences at high temperatures can generate the same stability
in a lake as much greater temperature differences did at lower temperatures. The
physical importance of temperature profiles thus is not temperature itself, but rather
the thermally produced density differences.
Procedures
1. Allow the water (uniformly mixed at a temperature of about 200q to stand until no
major currents are visible. Distribute over the surface of the water afew particles of
catechol violet and allow the streaks to form. Introduce a moderate wind (lamp is off
at this time) and observe that the waters will mix completely.
2. Record temperatures as a function of depth. Turn on the lamp, noting this time as
the beginning of the experiment. Measure temperatures after 5 min, 15 min, and
30 min. Plot temperatures as a function of depth at the time they are taken, using the
spare thermometer to obtain carefully the intermediate readings in the near-surface
region.
3. Spread particles of catechol violet liberally over the surface as in Lake Model 1 and
allow the streaks to descend.
4. Turn off the light. Introduce a moderate wind so as to bring the zone of
homogeneous red color down to a depth of about 5 to 7 cm. Record temperatures
and the depth of the red layer. Measure the periods of the internal seiche as in Lake
Modell.
5. With a slightly more forceful wind, bring the homogeneous red layer down to
include the upper two-thirds of the "lake." Record temperatures and the depth ofthe
homogeneous red layer. Now, with forced oscillation from the wind, cause another
internal seiche to form and measure the period. Record temperatures and the depth
of the homogeneous red layer again.
6. With a strong wind, bring about complete mixing. Record the final temperatures.