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Name: __________________________________ Date: __________________
Grade: ___________________________
_______________________________________________________
Lab 5
Atmospheric Moisture _______________________________________________________ (From Applications and Investigations in Earth Science, Fifth Edition, Edward J. Tarbuck, Frederick K. Lutgens, Dennis Tasa and Kenneth G. Pinzke. Copyright © by Pearson Education Inc. Published by Prentice-Hall, Inc. All rights reserved.)
Lecture Reference Material:
• Chapter 5 (Atmospheric Moisture)
Lab Objectives:
• 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
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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.
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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]
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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]
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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:
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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.
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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]
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Using a Psychrometer or Hygrometer The relative humidity and dew-point temperature of air can be determined by using a psychrometer or hygrometer and appropriate charts. The sling psychrometer consists of two thermometers mounted side by side on a handle. One of the thermometers, the dry-bulb thermometer measures the actual air temperature. The other thermometer, the wet-bulb thermometer, has a piece of wet cloth wrapped around its bulb. As the psychrometer is spun through the air, water on the wet-bulb thermometer evaporates and cooling results. This is continued until all water has evaporated and the cloth is dry. In dry air, the rate of evaporation will be high and a low wet-bulb temperature will be recorded. After using the instrument and recording both the dry- and wet-bulb temperatures, the relative humidity and dew-point temperatures can be determined using charts similar to those seen in Table 2 (Relative humidity (percent)) and Table 3 (Dew-point temperature). With a hydrometer, relative humidity can be measured directly without the use of tables. In today’s lab, we will use digital psychrometers that report dry- and wet-bulb temperatures by circulating air through a vent in the instrument.
28. Use Table 2 to determine the relative humidity for each of the following psychrometer readings. [4 pt] Reading 1 Reading 2
Dry-bulb temperature (°C) 14 32
Wet-bulb temperature (°C) 11 18
Wet-bulb depression (°C) Relative humidity (%)
29. From question 28, what is the relation between the difference in the dry-bulb and wet-bulb temperatures (wet-bulb depression) and the relative humidity of the air? [2 pt]
30. Which reading is closer to saturation? Why? [2 pt]
31. Use Table 3 to determine the dew-point temperature of each of the following psychrometer readings. [4 pt] Reading 1 Reading 2
Dry-bulb temperature (°C) 30 30
Wet-bulb temperature (°C) 15 20
Wet-bulb depression (°C) Dew-point temperature (°C)
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32. Which reading was likely taken in a dry air environment? Why? [2 pt]
33. Digital Psychrometer Exercise Students will break up into groups of 3-4. Using the digital psychrometer, record a dry-bulb and wet-bulb temperature inside the building and outside the building. When looking at the face of the psychrometer, the dry-bulb temperature is on the left and the wet-bulb temperature is on the right. Place your readings in the blanks provided in table below and proceed to complete the remainder of the table using Tables 2 and 3. [10 pt]
Inside Outside Dry-bulb temperature (°C) Wet-bulb temperature (°C)
Wet-bulb depression (°C)
Dew-point temperature (°C)
Relative humidity (%)
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Table 2. Relative humidity (percent)*
*To determine the relative humidity and dew-point temperature, find the air (dry-bulb) temperature on the vertical axis (far left) and the wet-bulb depression on the top horizontal axis. Wet-bulb depression is given in the heading above. Where the two intersect in the chart, the relative humidity or dew-point can be determined. For example, use a dry-bulb temperature of 20°C and a wet-bulb temperature of 14°C. From Table 2, the relative humidity is 51% and from Table 3, the dew-point is 10°C. Note: given the wet- bulb temperature, you still need to calculate the wet-bulb depression.
Table 3. Dew-point temperature (°C) (see footnote above)
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SECTION 5.5 – DAILY TEMPERATURE AND RELATIVE HUMIDITY (10 points total)
Figure 2 shows the typical daily variations in air temperature, relative humidity and dew-point temperature during two consecutive spring days at a mid-latitude city. Use the figure to answer questions 34-38.
34. Relative humidity is at its maximum at (6 A.M., 3 P.M.) on Day (1, 2). [2 pt]
35. The lowest temperature over the two-day period occurs at (6 A.M., noon, 3 P.M.) on Day (1, 2). [2 pt]
36. The lowest relative humidity occurs at (6 A.M., noon, 3 P.M.) on Day (1, 2). [2 pt]
37. Write a general statement describing the relationship between temperature and relative humidity throughout the time period shown in this figure. [2 pt]
38. Did a dew or frost form on either of the two days in this figure? If so, list the time it occurred and explain how you arrived at your answer. [2 pt]
Figure 2. Typical variations in air temperature, relative humidity and dew- point temperature for a mid-latitude city.
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SECTION 5.6 – CONDENSATION (6 points total)
Condensation is the process where water vapor converts into liquid water (gas to liquid). This occurs in the atmosphere when the air is cooled below the dew-point temperature. In the atmosphere, the condensation centralizes onto tiny, free-floating particles called condensation nuclei. Condensation nuclei include dust, dirt, smoke and sea salt particles. Rapid condensation may result in the formation of dew or frost on the ground and clouds and fog in the atmosphere. Continual condensation can lead to precipitation.
39. How many grams of water vapor will condense on a surface if a kilogram of air at 86°F with a relative humidity of 100% is cooled to 59°F? Refer to Table 1. [1 pt]
___________ grams of water will condense
40. Assume a kilogram of air at 30°C contains 10 grams of water vapor. Using Table 1, determine how many grams of water vapor will condense out if the air’s temperature is lowered to each of the following temperatures. [2 pt]
15°C: ____________ grams of condensed water
5°C: ____________ grams of condensed water
41. When condensation occurs, what three (3) conditions must be achieved in the atmosphere? (Hint: Look in previous sections for the answers) [3 pt]
SECTION 5.7 – THE ADIABATIC PROCESS AND CLOUDS/PRECIPITATION (24 points total)
As you have seen, the key to causing water vapor to condense, which is necessary before precipitation can occur, is to reach the dew-point temperature. In nature, when air rises and experiences a decrease in pressure, the air expands and cools. The reverse is also true of sinking air parcels where air is compressed and warmed. Temperature changes brought about solely by expansion and compression are called adiabatic temperature changes. Air with a temperature above its dew point (unsaturated air) cools by expansion or warms by compression at a rate of 10°C per 1000 meters (1°C per 100 meters) of changing altitude. This is called the dry adiabatic lapse rate. After the dew-point temperature is reached and condensation has occurs, latent heat that has been stored in the water vapor will be liberated. The heat being released by the condensing water slows down the rate of cooling of the air. Rising saturated air will continue to cool by expansion, but at a lesser rate of about 5°C per 1000 meters (0.5°C per 100 meters) of changing altitude. This is called the wet (moist) adiabatic lapse rate.
Cloud Formation There are two important criteria for the formation of clouds: 1) temperatures must be cooler than the dew-point temperature and 2) cloud condensation nuclei must be
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present. When air temperatures drop below the dew-point temperature, the air is said to be super-saturated. Physically, the air temperature can not be lower than the dew-point temperature. To restore equilibrium, the air must condense out water vapor which in return warms the surrounding air. As a result, the temperature will increase back up to the dew-point temperature. The condensation of water around nuclei within the atmosphere leads to the formation of clouds. The generation of a cloud begins at the lifted condensation level (LCL), the point where saturation occurs. Cooling of air at the moist adiabatic lapse rate builds cloud heights.
Figure 3 illustrates a kilogram of air at sea level with a temperature of 30°C and a relative humidity of 75%. The air is forced to rise over a 5,000 meter mountain and descend to a plateau 2,000 meters above sea level on the opposite (leeward) side. To help understand the adiabatic process, answer questions 42-53 by referring to Figure 3.
Figure 3. Adiabatic processes associated with a mountain barrier.
42. What is the saturation mixing ratio, content and dew-point temperature of the air at sea level? [3 pt]
Saturation mixing ratio: _____________ g/kg of air
Content: _____________ g/kg of air
Dew-point temperature: _____________°C
43. The air at sea level is (saturated, unsaturated) [1 pt]
44. The air will initially (warm, cool) as it rises over the windward side of the mountain at the (moist, dry) adiabatic rate, which is (10, 5)°C per 1000 meters. [3 pt]
45. What will be the air’s temperature at 500 meters? [1 pt]
_____________°C at 500 meters
Temperature: 30°C Relative Humidity: 75%
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46. The rising air will reach its dew-point temperature at _______________ meters and water vapor will begin to (condense, evaporate). [2 pt]
47. From the altitude where condensation begins to occur to the summit of the mountain, the rising air will continue to expand and will (warm, cool) at the (moist, dry) adiabatic lapse rate of about _____________°C per 1000 meters. [3 pt]
48. The temperature of the rising air at the summit of the mountain will be _____________°C. [1 pt]
49. Assuming the air begins to descend on the leeward side of the mountain, it will be compressed and its temperature will (increase, decrease). [1 pt]
50. Assume the relative humidity of the air is below 100% during its entire descent to the plateau. The air will be (saturated, unsaturated) and will warm at the (wet, dry) adiabatic rate of about _____________°C per 1000 meters. [3 pt]
51. As the air descends and warms on the leeward side of the mountain, its relative humidity will (increase, decrease). [1 pt]
52. The air’s temperature when it reaches the plateau at 2,000 will be _____________°C. [1 pt]
53. Explain why mountains might cause wet conditions on their windward side and dry conditions on their leeward sides (i.e. adiabatically). Describe the land type you might find on each side of the mountain barrier. [4 pt]
SECTION 5.8 – GLOBAL AND REGIONAL PATTERNS OF PRECIPITATION (14 points total)
Precipitation varies greatly worldwide. Figure 4 shows a map of global average annual precipitation in centimeters. Lines of constant precipitation, isohyets, are drawn across the land masses showing locations with similar precipitation measurements. Using maps like that shown in Figure 4 and 5, meteorologists and climatologists can make general inferences about precipitation patterns which are important for designating ecosystems and biomes as well as monitoring droughts and floods. Use Figure 4 to answer questions 54-57 and Figure 5 to answer questions 58-60.
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Figure 4. Global average annual precipitation (mm) from 1980 to 2004. (Source: GPCC – Visualizer).
54. Analyzing Figure 4, where are the highest and lowest global average annual precipitation measurements? Give the geographical region in the space below. [2 pt]
Highest measurement:__________________________________________________
Lowest measurement: __________________________________________________
55. In general, the polar regions of the Earth have (high, low) average annual precipitation. [1 pt]
56. According to Figure 4, (continents, oceans) experience more precipitation annually? Explain your reasoning. [2 pt]
57. It is possible to distinguish different land types using precipitation measurements. Given the color scale in Figure 4, where would you expect deserts and rainforests to be present? Give specific locations. [2 pt]
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Figure 5. Average annual precipitation for the United States (1971-2000) produced from data from the National Weather Service (NWS). (Source: http://www.erh.noaa.gov/btv/images/us_pcpn.png).
58. Analyzing Figure 5, where is the highest and lowest average annual precipitation measurements in the U.S.? Name each state. [2 pt]
Highest measurement:__________________________________________________
Lowest measurement: __________________________________________________
59. With U.S. geography in mind, the highest precipitation falls (along the coast, in the interior of the country). Explain your reasoning. [2 pt]
60. Describe the general pattern (west to east) of annual precipitation across the United States. What is the major reason for the aridity of the dry areas in the U.S.? [3 pt]
