Explain the processes involved when water changes state

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

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)

GE101 Natural Environments: The Atmosphere Laboratory 61
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 (%)

GE101 Natural Environments: The Atmosphere Laboratory 62
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)

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

GE101 Natural Environments: The Atmosphere Laboratory 65
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%

GE101 Natural Environments: The Atmosphere Laboratory 66
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.

GE101 Natural Environments: The Atmosphere Laboratory 67
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]