|Life depends on an essentially continuous exchange of
mass and energy between living organisms and their environment.
Human impact on this vital exchange has occurred on a global or
macroclimate scale. Understanding the physical principles
involved in heat transfer and absorption in the atmosphere is critical
to understanding how these physical factors affect living
organisms. The specific objectives of this section are to explain
the properties of heat transfer, and to describe laboratory activities
that can be used at a variety of academic levels with only slight
Described below are three series of experiments performed in
the laboratory to address questions that emphasize the underlying
principles of heat transfer. These hands-on experiments focused
on principles that relate to conduction and convection. The object was to identify the method of heat transfer
through solids, liquids, gases, and between boundaries.
Understanding these concepts gave us a better understanding of how heat
is transferred between our environment and living
organisms. These experiments were used as an integral part
of the workshop, which consisted of reflections on redesigning or
modifying lab exercises to fit personal needs of workshop
teachers. These exercises could be adapted for middle
school, high school, and college level courses.
The methods utilized for the three experiments involved increasing
or decreasing the temperature of a solid or liquid, and where
applicable, observing the motion of a dye caused by the changes in
temperature and density of the medium.
|Modes of Heat Transfer:
- Conduction: heat transfer resulting from direct contact
between substances of different temperatures; heat is transferred
from the high-temperature substance to the low by direct molecular
- Convection: heat transport by a moving fluid (gas of
liquid). The heat is first transferred to the fluid by
conduction, but the fluid motion carries the heat away.
- Radiative exchange: heat transfer via electromagnetic
waves, the amount of radiant energy emitted, transmitted, or
(Figure from Microsoft Encarta)
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Laboratory Apparatus for Labs 1-3
|Lab 1: Heating
from Below: Convection
In this experiment, water was heated from below to produce
convection. Although the atmosphere is composed of air, this
experiment was relevant to atmospheric motion as well. The lower
atmosphere (troposphere) is mostly heated from below because the
oceans and continents absorb radiation from the sun and then transfer
some of the resulting heat energy to the lower atmosphere.
In Lab 1, a beaker was heated (see figure below). Thermometers were placed in 1/2 cm
below water surface and 1/2 cm above the bottom of the beaker.
The temperature was recorded at 30 second intervals. Drops of
dye were added to the bottom of the beaker between intervals.
After three minutes the beaker was removed from the hot plate and
temperature reading recorded for another five minutes.
Convection was visualized by observing the motion of the
The motion of the dye was circular from bottom to top and returning to
the bottom of the beaker. The energy from heating created a less
dense liquid at the bottom, thus causing the upward motion of the
dye. Upon reaching the surface, the dye was now in the denser
medium and therefore returned to the bottom. This motion is an
example of convection. This phenomenon is evident in the motion
of wind. The difference in densities and kinetic movement of the
water molecules driven by temperature change resulted in the movement of
air molecules. This lab can be used at lower levels to
demonstrate simple properties of heat transfer and convection.
At higher levels, this lab illustrates these basic principles, and
could be extended to address more complex applications related to
convection such as the Coriolis
1. Explain the process by which the water is heated.
2. Describe the motion of the water as made visible by the
3. Why does convection occur?
4. Did convection cease? When? Why?
Environmental Applications of Principles
of Radiative Exchange, Conduction and Convection (Figure from E. Zerba, Princeton University; email@example.com)
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|Lab 2: Conduction
Comparison of this experiment with the first illustrated the
difference between the rate of heat transfer by conduction and that of
convection. It also illustrated the difference in heat
capacities between water and the solid materials of the
Lab 2 was configured similarly to Lab 1, but looked at the effect of
heating and cooling temperature difference using sand of equal weight
as water used in experiment 1. No dye was used in this experiment, as convection was not a
The temperature difference between the top and bottom layers of sand
indicated that sand heats and cools at a faster rate compared to
water. When the beaker was removed from the heat, the
temperature continued to increase via conduction from the bottom of the
beaker. This lab exercise is useful for demonstrating the concept of conduction to lower level students. Upper level
students can use this lab to make the connections between conduction and
heat capacity of various substances related to heat transfer that occurs between the
earth's surfaces and the surface of living organisms.
1. Is there any convection in the sand? Explain.
2. Why did the temperature recorded by the lower thermometer
continue to rise dramatically after the heating ceased?
3. On the basis of heat capacity, explain why the temperature
changes for the sand and water were different.
4. Using what you have observed in the two experiments, predict
whether a cold front will lower temperatures more at inland locations
or on the coast. Explain your answer.
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|Lab 3: Cooling From Above
In lakes and oceans, convection is generally the result of cooling
from above rather than heating from below. This was demonstrated
by adding ice to the water.
Using an experimental setup that allowed
measurement of temperature at the top and the bottom of a beaker of
water, ice was added to the top of the beaker. This experiment
illustrated the concept that at 4 °C, water
has higher density and sinks. Convection was
visualized by the movement of dye added to the bottom of the beaker
which was displaced by the cooler more dense water.
This lab demonstrates several physical principles associated with heat
transfer, including density, kinetic molecular theory, and
convection. On a larger scale, this laboratory exercise
demonstrates the process by which seasonal turnovers occur in ponds
and lakes. At
lower levels, teachers may choose to discuss physical principles of
heat transfer only, while at upper levels, teachers may choose to
integrate this small-scale investigation with the study of climate
processes and lake nutrient stratification and mixing.
1. Why does ice float?
2. Is there any evidence of convection? Why does or does
it not occur?
3. Draw a diagram to explain how seasonal turnover occurs in a
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