• Explain the processes involved when water changes state
• Use a psychrometer or hygrometer and appropriate tables to determine the relative humidity and dew-point temperature of the air
• Explain the adiabatic process and its effect on cooling and warming the air
• Calculate the temperature and relative humidity changes that take place in air as the result of adiabatic cooling
• Describe the global patterns of precipitation and its variability
Materials Needed:
• Lab Manual
• Textbook
• Pencil
• Colored pencils
• Calculator
• Laptop
• Ruler
• Digital Psychrometer
• Hot plate
• Beaker
• Thermometers
• Water and ice
SECTION 5.1 – ATMOSPHERIC MOISTURE AND PHASE CHANGES OF WATER
(29 points total)
By observing, recording and analyzing weather conditions, meteorologists attempt to define the principles that control the complex interactions that occur in the atmosphere. One important element, temperature, has already been examined.
However, no analysis of the atmosphere is complete without an investigation of
atmospheric moisture and, more importantly, processes that create clouds and
eventually precipitation.
Water vapor, an odorless, colorless gas produced by the evaporation of water,
comprises only a small percentage of the lower atmosphere. However, it is an
important atmospheric gas because it is the source of all precipitation, aids in the
GE101 Natural Environments: The Atmosphere Laboratory 54
heating of the atmosphere by absorbing radiation and is the source of latent heat
(hidden or stored heat).
Changes of State
The temperatures and pressures that occur at and near the Earth’s surface allow
water to change readily from one state of matter to another. The fact that water can
exist as a gas, liquid or solid within the atmosphere makes it one of the most unique
substances on Earth. Use Figure 1 to answer questions 1-4.
Figure 1. Changes of state of water.
1. To help visualize the processes and heat requirements for changing the state of
matter of water, write the name of the process involved (choose from the list below)
and whether heat is absorbed or released by water during the process at the
indicated location by each arrow in Figure 1. [12 pt]
Freezing
Evaporation
Deposition
Sublimation
Melting
Condensation
2. To melt ice, heat energy must be (absorbed, released) by water molecules. [1 pt]
3. The process of condensation requires that water molecules (absorb, release) heat
energy. [1 pt]
4. The energy requirement for the process of deposition is the (same as, less than)
the total energy required to condense water vapor and then freeze the water. [1 pt]
Latent Heat Experiment
This experiment will help you gain a better understanding of the role of heat in
changing the state of matter. You are going to heat a beaker that contains a mixture
of ice and water. You will record temperature changes as the ice melts and continue
to record the temperature changes after the ice melts. Conduct the experiment by
completing the following steps.
GE101 Natural Environments: The Atmosphere Laboratory 55
5. Prior to starting the experiment, write a brief hypothesis of how you think the
temperature of ice-water mixture will change when heat is added. Use your
knowledge of phase changes and latent heat to help with your hypothesis. [1 pt]
Step 1: Turn on the hot plate and set the temperature dial to about three-quarters
maximum strength (7 on a scale of 10).
Step 2: Fill a Mason jar approximately half full with ice and add enough COLD water
to cover the ice. Insert a temperature through the hole in the Mason jar’s lid.
Step 3: Gently stir the ice-water mixture in the Mason jar. After 15 seconds, record
the temperature of the mixture. This is the “starting” temperature at time 0. Record
this value in an Excel table. (If you did not bring your laptop, record temperatures on
a piece of paper and entered into Excel after lab).
Step 4: Place the Mason jar with the ice-water mixture on the hot plate and while
stirring constantly, record the temperature of the mixture at one minute intervals.
Record all temperatures in your Excel table. Students will take turns reading the
temperature of the ice-water mixture. Continue until ice has completely melted. Note
this time in your Excel table.
Step 5: Continue stirring the mixture and recording the temperature for five minutes
in one minute intervals after the ice has melted. Record all temperatures in your
Excel table. Be sure you get all temperatures before leaving lab!!
Step 6: In Excel, make a graph of the temperature pattern of the ice-water mixture
over time. Perform this analysis on one graph with a line plotted including the data
points). Be sure to label your axes and include a title of the experiment. Include
both the table and graph with your submitted lab! [6 pt, 3 pt for table and 3 pt for
graph]
6. How did the temperature of the mixture change prior to, and after, the ice melted.
Describe the trend you plotted in your graph. [2 pt]
7. Calculate the average temperature change per minute of the ice-water mixture
prior to the ice melting and the average rate after the ice had melted. [2 pt]
Average rate prior to melting: __________________________________
Average rate after melting: _____________________________________
8. With your answers to question 6 and 7 in mind, write a statement comparing the
results of this experiment to your initial hypothesis. Explain. [1 pt]
GE101 Natural Environments: The Atmosphere Laboratory 56
9. With reference to the absorption or release of latent (hidden) heat, explain why
the temperature changed at a different rate after the ice melted as compared to
before all the ice had melted. [2 pt]
SECTION 5.2 – WATER–VAPOR CAPACITY OF AIR
(17 points total)
Any measure of water vapor in the air is referred to as humidity. The amount of
water vapor required for saturation is directly related to temperature.
The mass of water vapor in a unit of air compared to the remaining mass of dry air is
referred to as the mixing ratio. Table 1 presents the mixing ratios of saturated air
(water vapor needed for saturation) at various temperatures. Use the table to
answer questions 10-13.
Table 1. Amount of water vapor needed to saturate a kilogram of air at
various temperatures, the saturation mixing ratio.
Temperature
(°C) (°F)
Water vapor content at
saturation (g/kg)
-40 -40 0.1
-30 -22 0.3
-20 -4 0.75
-10 14 2
0 32 3.5
5 41 5
10 50 7
15 59 10
20 68 14
25 77 20
30 86 26.5
35 95 35
40 104 47
10. To illustrate the relation between the amount of water vapor needed for
saturation and temperature, prepare a graph in Excel of water vapor content at
saturation and temperature (°C). Label axes and include a title. Submit the graph
with your completed lab. [3 pt]
11. Using Table 1 and/or your graph from question 10, write a statement that
relates the amount of water vapor needed for saturation to temperature. How does
water vapor change as you change the temperature? [2 pt]
GE101 Natural Environments: The Atmosphere Laboratory 57
12. Using Table 1 data and/or the graph from question 10, what is the water vapor
content at saturation of a kilogram of air at each of the following temperatures?
[4 pt]
-5°C: ____________________________________
12°C: ____________________________________
21°C: ____________________________________
34°C: ____________________________________
13. Using Table 1 data and/or the graph from question 10, what is the air
temperature (°C) at each of the following saturated water vapor contents?
[4 pt]
0.3 g/kg: ____________________________________
8 g/kg: ____________________________________
17 k/kg: ____________________________________
35 g/kg: ____________________________________
14. From Table 1, raising the air temperature of a kilogram of air 10°C, from 15°C to
25°C, (increases, decreases) the amount of water vapor for saturation (10, 20)
grams. However, raising the temperature from 25°C to 35°C (increases, decreases)
the amount by (5, 15) grams. [4 pt]
SECTION 5.3 – MEASURING HUMIDITY
(19 points total)
Relative humidity is the most common measurement used to describe water vapor
in the air. In general, it expresses how close the air is to reaching saturation at that
temperature. Relative humidity is a ratio of the air’s actual water vapor content
(amount actually in the air) compared with the amount of water vapor required for
saturation at that temperature (saturation mixing ratio), expressed as a percentage.
The general formula is:
For Example, from Table 1, the saturation mixing ratio of a kilogram of air at 25°C
would be 20 grams per kilogram. If the actual amount of water vapor in the air was
5 grams per kilogram (the water vapor content), the relative humidity of the air
would be calculated as follows:
GE101 Natural Environments: The Atmosphere Laboratory 58
15. Use Table 1 and the formula for relative humidity to determine the relative
humidity for each of the following situations with identical temperatures. [3 pt]
Air Temperature Water Vapor Content Relative Humidity
30°C 5 g/kg
30°C 10 g/kg
30°C 20 g/kg
16. From question 15, if the temperature remains constant, adding water vapor will
(raise, lower) the relative humidity, while removing water vapor will (raise, lower)
the relative humidity. [2 pt]
17. Use Table 1 and the formula for relative humidity to determine the relative
humidity for each of the following situations of identical water vapor content. [3 pt]
Air Temperature Water Vapor Content Relative Humidity
25°C 0.75 g/kg
15°C 0.75 g/kg
5°C 0.75 g/kg
18. From question 17, if the amount of water vapor in the air remains constant,
cooling will (raise, lower) the relative humidity, while warming will (raise, lower) the
relative humidity. [2 pt]
19. In the winter, air is heated in homes in colder climates. What effect does heating
have on relative humidity inside the home? What is a possible solution to lessen this
effect? [2 pt]
20. Explain why the air in a cool basement is humid (damp) in the summer. [2 pt]
21. What are two ways that the relative humidity of the air can be changed? [2 pt]
One of the misconceptions concerning relative humidity is that it alone gives an
accurate indication of the actual quantity of water vapor in the air. For example, on a
winter day if you hear on the radio that the relative humidity is 90%, can you
conclude that the air contains more water vapor than on a summer day that records
a 40% relative humidity? Completing question 21 will help you find the answer.
GE101 Natural Environments: The Atmosphere Laboratory 59
22. Use Table 1 to determine the water vapor content for each of the following
situations. As you do the calculations, keep in mind the definition of relative
humidity. [2 pt]
Summer Winter
Temperature (°C) 25 -20
Relative Humidity (%) 75 75
Content (g/kg)
23. Explain why relative humidity does not give an accurate indication of the actual
amount of water vapor in the air. [1 pt]
SECTION 5.4 – DEW–POINT TEMPERATURE
(31 points total)
Air is saturated when it contains all the water vapor that it can hold at a particular
temperature. The temperature at which saturation occurs is called the dew-point
temperature. Put another way, the dew-point is the temperature at which relative
humidity is 100%.
A kilogram of air at 25°C, containing 5 grams of water vapor had a relative humidity
of 25% and was not saturated. When the temperature was lowered to 5°C, the air
had a relative humidity of 100% and became saturated. Therefore, 5°C is the dew-
point temperature of the air in the example.
24. By referring to Table 1, what is the dew-point temperature of a kilogram of air
that contains 2 grams of water vapor? [1 pt]
Dew-point temperature = _______________°C
25. What is the relative humidity and dew-point temperature of a kilogram of air at
20°C that contains 3.5 grams of water vapor? [2 pt]
Relative Humidity = _______________%
Dew-point temperature = _______________°C
26. If the air parcel in question 25 retains its water vapor content and decreases to
10°C, what is the new relative humidity and dew-point temperature? [2 pt]
Relative Humidity = _______________%
Dew-point temperature = _______________°C
27. Is the air parcel in question 26 approaching saturation? Explain. [2 pt]
