ESCI 1010 Lab 3
Atmospheric Moisture
Before Lab: Review pages 112-125 in your Weather and Climate textbook. Pay special attention to the
section entitled “Water’s Changes of State”, the terms relative humidity and dew point, and the sling
psychrometer, wet-bulb temperature, and dry-bulb temperature discussion on pages 123-125.
Summary: This lab will give you hands-on experience collecting data that can be used to determine two
key moisture variables that are often used by meteorologists and climatologists—the relative humidity and
the dew point. By the end of the lab, you should be familiar with how to use a sling psychrometer to
determine the relative humidity and the dew point, the relationship between relative humidity and dew
point, and the heat exchanges taking place as water changes phase.
LAB EXERCISE
1. Note Figure 5-3 on page 114 in your textbook. Using that figure, fill in the following table
Water’s Phase Change Heat (released or consumed)
Ice Water (liquid)
Water (liquid) Ice
Ice Water Vapor
Water Vapor Water (liquid)
Water Vapor Ice
Water (liquid) Water Vapor
2. Using a sling-psychrometer, you will go outside and take measurements of the wet-bulb and dry-bulb
temperature in order to determine the relative humidity and the dewpoint:
Dip the sock, located on one of the thermometers, into the container of water provided in lab.
Go outside and have one person in your group sling the psychrometer around (as demonstrated
by your lab instructor) with the other keeping time.
Sling the psychrometer for one minute, and look at the readings on the thermometers.
Sing the psychrometer for another 30 seconds, and determine if there has been any change in
the wet-bulb or dry-bulb readings. If there is no change, you may stop and record your values. If
the reading has changed, the repeat this step by slinging the psychrometer for another 30
seconds and check the values again. Record your dry-bulb and wet-bulb temperatures in the
table on the next page under “Your Group.”
Once you are back in the lab, your instructor will ask for the wet-bulb and dry-bulb temperatures
from each group and those values will be noted on the board.
Compute a class average for the wet-bulb and dry-bulb temperatures and record those values in
the table on the next page under “Class Average.”
1
, Your Group Class Average
Wet-bulb Temperature (F)
Dry-bulb Temperature (F)
3. Were all group observations within a degree or two of the class average? Why might you expect
differences?
4. Unless the air outside is completely saturated, the wet-bulb temperature likely declined as the
psychrometer was slung. Why does the temperature of the wet-bulb go down when the air is not
saturated (hint: remember heat exchanges when water changes state from question 1; refer back to the
textbook on page 123-125)?
5. Using the wet-bulb and dry-bulb temperatures you recorded in the table above, the relative humidity
can be determined, but first you must calculate a variable known as the wet-bulb depression. It is
computed by subtracting the wet-bulb temperature from the dry-bulb temperature (i.e. dry-wet). Compute
the wet-bulb depression using the “Class Average” data you recorded in the table above.
6. Think about low wet-bulb depression values vs. high wet-bulb depression values as they relate to the
air saturation. Is the air closer to saturation when you see low values or high values?
7. Find the relative humidity. Use your computed wet-bulb depression value from question 5 above, the
“Class Average” data in the table above, and Table 1 on page 5 of this lab. You may need to interpolate
between the values in Table 1.
8. Use the value for relative humidity you found in question 7 above, the “Class Average” data in the
table above, and Table 2 on page 6 of this lab to find the dew point temperature. Once again, you may
need to interpolate between the values in Table 2.
2
Atmospheric Moisture
Before Lab: Review pages 112-125 in your Weather and Climate textbook. Pay special attention to the
section entitled “Water’s Changes of State”, the terms relative humidity and dew point, and the sling
psychrometer, wet-bulb temperature, and dry-bulb temperature discussion on pages 123-125.
Summary: This lab will give you hands-on experience collecting data that can be used to determine two
key moisture variables that are often used by meteorologists and climatologists—the relative humidity and
the dew point. By the end of the lab, you should be familiar with how to use a sling psychrometer to
determine the relative humidity and the dew point, the relationship between relative humidity and dew
point, and the heat exchanges taking place as water changes phase.
LAB EXERCISE
1. Note Figure 5-3 on page 114 in your textbook. Using that figure, fill in the following table
Water’s Phase Change Heat (released or consumed)
Ice Water (liquid)
Water (liquid) Ice
Ice Water Vapor
Water Vapor Water (liquid)
Water Vapor Ice
Water (liquid) Water Vapor
2. Using a sling-psychrometer, you will go outside and take measurements of the wet-bulb and dry-bulb
temperature in order to determine the relative humidity and the dewpoint:
Dip the sock, located on one of the thermometers, into the container of water provided in lab.
Go outside and have one person in your group sling the psychrometer around (as demonstrated
by your lab instructor) with the other keeping time.
Sling the psychrometer for one minute, and look at the readings on the thermometers.
Sing the psychrometer for another 30 seconds, and determine if there has been any change in
the wet-bulb or dry-bulb readings. If there is no change, you may stop and record your values. If
the reading has changed, the repeat this step by slinging the psychrometer for another 30
seconds and check the values again. Record your dry-bulb and wet-bulb temperatures in the
table on the next page under “Your Group.”
Once you are back in the lab, your instructor will ask for the wet-bulb and dry-bulb temperatures
from each group and those values will be noted on the board.
Compute a class average for the wet-bulb and dry-bulb temperatures and record those values in
the table on the next page under “Class Average.”
1
, Your Group Class Average
Wet-bulb Temperature (F)
Dry-bulb Temperature (F)
3. Were all group observations within a degree or two of the class average? Why might you expect
differences?
4. Unless the air outside is completely saturated, the wet-bulb temperature likely declined as the
psychrometer was slung. Why does the temperature of the wet-bulb go down when the air is not
saturated (hint: remember heat exchanges when water changes state from question 1; refer back to the
textbook on page 123-125)?
5. Using the wet-bulb and dry-bulb temperatures you recorded in the table above, the relative humidity
can be determined, but first you must calculate a variable known as the wet-bulb depression. It is
computed by subtracting the wet-bulb temperature from the dry-bulb temperature (i.e. dry-wet). Compute
the wet-bulb depression using the “Class Average” data you recorded in the table above.
6. Think about low wet-bulb depression values vs. high wet-bulb depression values as they relate to the
air saturation. Is the air closer to saturation when you see low values or high values?
7. Find the relative humidity. Use your computed wet-bulb depression value from question 5 above, the
“Class Average” data in the table above, and Table 1 on page 5 of this lab. You may need to interpolate
between the values in Table 1.
8. Use the value for relative humidity you found in question 7 above, the “Class Average” data in the
table above, and Table 2 on page 6 of this lab to find the dew point temperature. Once again, you may
need to interpolate between the values in Table 2.
2
