32
Exercise 3
phenomena of internal seiches and convective sinking during night or autumnal
cooling. Calculations will be made of the "annual" heat budget, the stability of the
"lake", the thermal resistance to mixing for different layers, and internal seiche periods.
Procedures
1. Prepare a chart with columns for time (minutes elapsed) and temperatures at depths
of 1, 3, 5, 9, 13, 17, and 21 cm. Leave a space at the extreme right for inserting
temperatures at other (variable) depths.
2. Fill the aquarium with cold tap water and mix to 4°C with ice. Allow the system to
come to rest (3 to 5 min) with a small amount of ice floating on the surface. Then
introduce a moderate wind (first wind) on the top, blowing from one side and then
the other. After a minute or two you will notice that the temperature at 1 cm has
dropped to 2 to 3°e. Stop the wind and record the temperatures at the surface and at
each fixed depth. Next, remove the excess ice as gently as possible and turn on the
heat lamp. Record this time as the beginning ofthe experiment. With the light on and
no wind, record temperatures after approximately 5 min and every 5 min thereafter
for approximately 50 min, taking special care to get additional temperature readings
in regions where temperatures change rapidly with depth (e.g., upper 5 cm). Plot
temperature changes as a function of depth on graph paper as they are recorded.
You will observe an exponential decrease in temperature with depth. Why?
3. With the lamp still on, drop some crystals of catechol violet more or less randomly
over the surface. Allow them sufficient time to sink so that most of the descending
trails are in the upper third of the aquarium with a few extending into the middle
third, and still fewer to the bottom. Observe the positions of the trails and the
complexities of the water motion, particularly near the surface. What can you infer
about the water currents in the "lake"?
4. Now introduce a light to moderate wind (second wind) blowing first from one side
and then the other so as to mix the surface waters into one homothermal mass.
A void too strong a wind which could destroy the thermocline that you arc now
creating. The epilimnion (with homogeneous distribution of color) should now
be about 5- to 7-cm thick. Turn off the light and allow the currents to slack; record
the depth of the homogeneous red layer and temperatures as a function of
depth.
5. Leave the light off. Using the same wind speed (third wind), impart a slope of 10 to
ISO to the thermocline by blowing from one side only. This slope can be achieved
best in stages, allowing successive winds with intervening periods of quiescence to
build up the internal seiche. When the desired slope to the thermocline has been
obtained, stop the wind and measure the periods of the internal seiche (average
approximately five measurements). When an electrical thermometer is suspended at
the end of lake at the depth of the upper portion of the metalimnion, the periodic
temperature changes of the oscillating internal seiche can be recorded directly.
Allow the system to come to rest, record the temperatures at each depth, and
measure the depth of the homogeneous red layer.
6. Next, carefully add a layer of ice cubes to the surface with minimal disturbance of the
water. It will be necessary to cover about three-fourths of the surface if small cubes or
crushed ice are used. Observe the descending convection cells sinking through the
thermocline region, which in turn cause the thermocline to descend. When the
thermocline has descended to about mid-depth in the tank, carefully remove the
remaining ice with as little turbulence as possible. With a wind (fourth wind) from
Exercise 3
phenomena of internal seiches and convective sinking during night or autumnal
cooling. Calculations will be made of the "annual" heat budget, the stability of the
"lake", the thermal resistance to mixing for different layers, and internal seiche periods.
Procedures
1. Prepare a chart with columns for time (minutes elapsed) and temperatures at depths
of 1, 3, 5, 9, 13, 17, and 21 cm. Leave a space at the extreme right for inserting
temperatures at other (variable) depths.
2. Fill the aquarium with cold tap water and mix to 4°C with ice. Allow the system to
come to rest (3 to 5 min) with a small amount of ice floating on the surface. Then
introduce a moderate wind (first wind) on the top, blowing from one side and then
the other. After a minute or two you will notice that the temperature at 1 cm has
dropped to 2 to 3°e. Stop the wind and record the temperatures at the surface and at
each fixed depth. Next, remove the excess ice as gently as possible and turn on the
heat lamp. Record this time as the beginning ofthe experiment. With the light on and
no wind, record temperatures after approximately 5 min and every 5 min thereafter
for approximately 50 min, taking special care to get additional temperature readings
in regions where temperatures change rapidly with depth (e.g., upper 5 cm). Plot
temperature changes as a function of depth on graph paper as they are recorded.
You will observe an exponential decrease in temperature with depth. Why?
3. With the lamp still on, drop some crystals of catechol violet more or less randomly
over the surface. Allow them sufficient time to sink so that most of the descending
trails are in the upper third of the aquarium with a few extending into the middle
third, and still fewer to the bottom. Observe the positions of the trails and the
complexities of the water motion, particularly near the surface. What can you infer
about the water currents in the "lake"?
4. Now introduce a light to moderate wind (second wind) blowing first from one side
and then the other so as to mix the surface waters into one homothermal mass.
A void too strong a wind which could destroy the thermocline that you arc now
creating. The epilimnion (with homogeneous distribution of color) should now
be about 5- to 7-cm thick. Turn off the light and allow the currents to slack; record
the depth of the homogeneous red layer and temperatures as a function of
depth.
5. Leave the light off. Using the same wind speed (third wind), impart a slope of 10 to
ISO to the thermocline by blowing from one side only. This slope can be achieved
best in stages, allowing successive winds with intervening periods of quiescence to
build up the internal seiche. When the desired slope to the thermocline has been
obtained, stop the wind and measure the periods of the internal seiche (average
approximately five measurements). When an electrical thermometer is suspended at
the end of lake at the depth of the upper portion of the metalimnion, the periodic
temperature changes of the oscillating internal seiche can be recorded directly.
Allow the system to come to rest, record the temperatures at each depth, and
measure the depth of the homogeneous red layer.
6. Next, carefully add a layer of ice cubes to the surface with minimal disturbance of the
water. It will be necessary to cover about three-fourths of the surface if small cubes or
crushed ice are used. Observe the descending convection cells sinking through the
thermocline region, which in turn cause the thermocline to descend. When the
thermocline has descended to about mid-depth in the tank, carefully remove the
remaining ice with as little turbulence as possible. With a wind (fourth wind) from