Name_____________________________________________Date__________________Per.______
Relative Humidity and Dew Point Lab
Background Review:
1.
What happens to the amount of water vapor that can be present in air as it warms up?
2. Air conditioners are always vented to the outside because water condensation usually forms.
Explain why this usually occurs and what would affect whether condensation forms.
3.
The dew point is the temperature at which condensation will form as air cools. If air contains
18 g/cubic meter water vapor, what is its dew point?
4.
Relative humidity is the amount of water vapor in the air compared to the maximum amount of
water vapor that can be “held” in the air at that given temperature. The formula to calculate
relative humidity is:
Relative Humidity (%) = (Actual Water Vapor Density ÷ Maximum Water Vapor Density) x 100
Calculate the relative humidity for air that contains 9 g/cubic meter water vapor at the
temperatures given below. In order to calculate the relative humidity, use the graph to
determine the maximum water vapor density (saturation quantity). Show your work.
A. 10 °C
B. 40 °C
5.
Explain how temperature relates to relative humidity for a given water vapor density as the
temperature increases.
6.
On a September day the temperature in Seattle is 20oC and the relative humidity is 60%. The
air in Seattle travels west and is forced to rise when it reaches the Cascade Mountains. As it
rises, it cools.
A.
Use the graph determine the maximum water vapor content of20oC air and then
calculate the actual water vapor content of the 60% humidity air using the relative
humidity formula from the previous page. Show your work.
B. Use the graph to determine at what temperature the air will become saturated and
clouds will form as the air cools.
7. The air at Snoqualmie pass is 5oC (41oF) and has a relative humidity of 80%. As the air descends
into Eastern Washington, it warms up to 25oC (77oF).
A.
Using the graph, calculate the actual water vapor content at 5oC. Show your work.
B.
Using the graph and the relative humidity equation, calculate what the relative
humidity of this air will be in Eastern Washington when the air warms up to 25oC.
Show your work.
8. Explain why the weather in Eastern Washington will have less precipitation than Western
Washington, and why the humidity will generally be lower in Eastern Washington than in
Western Washington.
Background on Dew Point:
The temperature to which the air must cool before condensation forms is called the dew point. The
dew point is dependent upon the relative humidity and starting temperature of the air. If the air has
100% humidity, any cooling of the air will result in condensation. However, in the air has a low humidity
there must be a greater drop in temperature before the air exceeds its water vapor-holding capacity and
condensation forms. In this lab, you will try to determine the dew point for the air and compare this to
the relative humidity and temperature of the initial air sample.
Materials:
- small or medium can or beaker
-room temperature water
- crushed ice
-spoon
-thermometer
-Proquest probe with temperature and relative humidity sensors
Procedure:
1. Fill the can about half full with room temperature water.
2. Use the Proquest probe to determine the relative humidity and temperature in the classroom.
Record in the data table.
3. Add a spoonful o f crushed ice to the container. Watch for condensation on the sides of the can. Use
a spoon to stir the ice water mixture. Do not stir with the thermometer.
4. Continue adding spoonfuls of ice, and continue to watch for condensation. Record the temperature
at which condensation forms as the dew point.
5. Use the graph on the following page to determine the actual dew point for the initial air sample after
rounding the relative humidity to the nearest 10%. (see color version in document file on my website to
better determine the line for each level of relative humidity ).
Data chart:
Starting temperature
Relative humidity
Dew Point Measured
Dew Point Actual (From chart)
Review Questions
1. If air has an initial temperature of 40° Celsius, what is the dew point of the following relative humidity
levels:
90% relative humidity _________________
50% relative humidity _________________
10% relative humidity _________________
2. How close was your lab results for the dew point to the expected dew point? Explain possible
sources of error.