Dew Point and Relative Humidity
Objectives:
1. Observe dew formation
2. Determine relative humidity
3. Compare methods of finding relative humidity
Part 1: Dew Point Method
Materials:
 Thermometer
 2 beakers
 Room temperature water
 Ice



Stirring rod
Spoon
Water Vapor Capacity Table
Procedure:
1. Use a thermometer to measure the classroom air temperature. Record the temperature in the
Dew Point Method data table.
2. Use the Water Vapor Capacity table to find the capacity of air to hold water for the
temperature. Record the capacity.
3. Fill a beaker halfway with water. Place a thermometer in the water. Add a small amount of ice.
Stir the water with the stirring rod.
4. Watch for the first appearance of dew on the outside of the beaker. At the instant you see dew,
record the temperature.
5. Confirm the accuracy of your dew point readings by repeating steps 3 and 4 using the second
beaker. Average your two dew point values and record the average.
6. Use the Water Vapor Capacity table to find the capacity of air to hold water for the average dew
point temperature. Your value for air capacity at the dew point equals the specific humidity of
the air. Record the value.
7. Use your values and the formula in the data table to compute the relative humidity of the air as
a percentage.
Part 2: Wet-Bulb Thermometer Method
Materials:
 Thermometer
 Cotton gauze
 Room temperature water


Piece of paper
Relative Humidity Chart
Procedure:
1. Use a thermometer to measure the classroom air temperature. Record the temperature in the
Wet-Bulb Thermometer Method data table.
2. Create a fan from your piece of paper.
3. Wrap the gauze around the bulb of the thermometer and secure with a rubber band.
4. Dip the gauze wrapped bulb in room temperature water.
5. Fan the bulb until the temperature stabilizes. Record the temperature in the Wet-Bulb
Thermometer Method data table.
6. Subtract the wet-bulb temperature from the dry-bulb temperature. Record this value.
7. Use the Relative Humidity Chart to determine the relative humidity of the classroom air. Record
the value.
Questions:
1.
2.
3.
4.
What is relative humidity?
What is the dew point?
What is the relationship between air’s capacity to hold water vapor and temperature?
Compare your two relative humidity values for the classroom. Are the two values the same? If
the values are different, which do you think is more accurate?
5. Suppose you are looking at clouds that have just formed on a hot summer day. What do you
know about the relative humidity of the air at the bottom of the cloud?
6. Imagine that, early one morning, you discover that the wet-bulb and dry-bulb temperatures are
the same.
a. What conclusion can you draw about evaporation from the wet-bulb in this case?
b. What conclusion can you draw about the relative humidity in this case?
c. Would this be a good time for hanging the laundry outdoors to dry? Explain your
answer.
Data Tables:
Dew Point Method
Air Temp. of Classroom (oC)
Water vapor capacity of air at
classroom temp. (g/kg)
Dew point from trial 1 (oC)
Dew Point from trial 2 (oC)
Average dew point (oC)
Specific humidity-water vapor
capacity at the dew point (g/kg)
Relative humidity =
𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑢𝑚𝑖𝑑𝑖𝑡𝑦
∗ 100
𝑟𝑜𝑜𝑚 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦
Wet-Bulb Thermometer Method
Dry-bulb temperature of
classroom (oC)
Wet-bulb temperature of
classroom (oC)
Difference between dry-bulb
and wet-bulb temperatures (oC)
Relative humidity