HONORS EARTH SCIENCE
RELATIVE HUMIDITY LAB
PART 1- cup method
NAME _______________
DATE ______ HOUR ___
PURPOSE:
To calculate dew point temperature and relative humidity
MATERIALS:
glass, ice cubes, Celsius thermometer, water
PROCEDURE:
1. Use a thermometer to measure the classroom air temperature in Celsius degrees.
2. Use your chart for water vapor capacity. Find the capacity of air to hold water vapor for the
temperature in your classroom. Record the capacity.
3. Fill the glass halfway with water. Place a thermometer around the outside of the glass. Add a
handful of ice. Note- if dew does not form within 5 minutes, add more ice.
4. Swirl the water and ice mixture.
5. Watch for the first appearance of dew on the outside of the glass. At the instant you see or feel
dew, record the dew point temperature in Data Table A.
6. Use the chart for water vapor capacity, to determine the water vapor content. Record.
7. compute the relative humidity of air at room temperature. Record this value in Data Table A
Data table A
Temperature of classroom air (°C)
Capacity of air at classroom air temperature (g/kg)
(from chart)
Dew point (°C)
Water vapor conent (g/kg)
(from chart @ dew point temp)
Relative humidity = water vapor content
Capacity of air at room temp
Part 2- SLING PSYCHROMETER
MATERIALS
Sling psychrometer, eye dropper, water, wet/dry bulb chart
PROCEDURE
1. Read the air temperature from the dry bulb on the psychrometer. Record In Data Table
2. Place 2-3 drops of room-temperature water on the cloth-covered end of the thermometer.
This is the wet-bulb thermometer. DO NOT GET THE DRY BULB WET!!
3. Carefully swing the Sling-Psychrometer for 3 minutes (without stopping).
4. Subtract the wet-bulb temperature from the dry-bulb temperature and record this value in
Data Table B. Repeat steps 1-3 outside if time permits.
5. Using the relative humidity table (at your lab station and in appendix D of your text),
locate the dry-bulb temperature and the difference between the wet-bulb and dry-bulb
readings. Determine the relative humidity. Record this value in Data Table B.
Data table B
Dry bulb temperature (°C)
Wet bulb temperature (°C)
Dry-bulb (minus) wet-bulb
temperatures (°C)
Relative humidity (%)
Inside
Outside (if
time permits)
QUESTIONS; (pages 436-441 may be helpful)
1. Which method (glass or psychrometer) do you think was more
accurate?_____________ Support your answer (discuss possible errors)
2. If the room temperature was higher (assume same amount of water vapor), would the
relative humidity be higher or lower than what you calculated in lab today? (think of the
beakers)
Explain why.
3. What would happen if we cooled the room to the dew point temperature?
What temperature would this room have to be to reach 100% humidity? _________
(hint: use data table A)
4. Dew point temperature is often given on local weather forecasts. Explain why
someone would want to know this temperature? Be specific
5. As air temperature gets closer to dew point temperature, humidity __________
6. In YOUR OWN WORDS explain how we measured the water vapor CONTENT
7. Why did the wet bulb temperature decrease? (think about phase changes)
8. Imagine that early one morning you use a psychrometer outdoors and discover that the
wet-bulb and dry-bulb temperatures are the same. What conclusion can you draw about
the relative humidity?
9. On a hot summer (30º C) day the relative humidity is 90%. On a cool (10º C) winter
day the relative humidity is 90%. Use the formula for relative humidity to determine how
much water vapor is in the air on each day?
Show your work:
Summer day:
Winter day: