Humidity Lab Relative Humidity table on the next page to figure the relative
◆ Pre-Lab Discussion humidity. Remember that the dry-bulb measures current air
Even without rain, the air can be very wet because it contains invisible temperature.
water vapor. The amount of water vapor in the air is known as
humidity. As air gets warmer, it can hold more moisture. Sample Inside Outside
Meteorologists usually speak of relative humidity—the amount of Data Data Data
water vapor in the air compared to the maximum amount that air can Dry-bulb reading 21°C
hold at a particular temperature. Wet-bulb reading 15°C
You can measure relative humidity with a psychrometer, which
consists of two thermometers. One thermometer has a dry bulb, and
one has a wet bulb. A piece of wet cloth surrounds the bulb of the Relative humidity (%) 53%
wet-bulb thermometer. When the wet bulb is exposed to air, water in
the cloth evaporates, just as it does from wet clothing. Water 5. Repeat steps 1 through 5 outdoors. You may need to re-wet
evaporation requires heat energy, so it cools the wet bulb. your gauze.
Problem: How can you use a psychrometer to find the relative Data Table 1: Relative Humidity (%)
humidity of the classroom and outside?
Materials: (per group) psychrometer water
Safety Handle the psychrometer carefully. If it breaks, tell your
teacher. Do not pick up broken glass.
CAUTION: Handle the psychrometer carefully; it’s breakable. Keep
in mind the “you break, you buy” policy!
1. Carefully wet your gauze on one of the thermometers. Do
NOT WET the other thermometer.
2. Read the current temperatures on each thermometer, but you
don’t have to record them.
3. Swing the psychrometer CAREFULLY until the temperature
of the wet-bulb thermometer stops dropping. Continue
swinging and checking the wet bulb until the temperature
appears to stop dropping. Sometimes you have to swing the
psychrometer for a few minutes.
4. Once you feel that your wet bulb temperature has stopped
dropping, record your finding on the data table. Use the
Conclusion Answer the following questions in complete sentences on
a separate sheet of paper. You may type it for +5.
1. Which of the two thermometers measures the air temperature? Dry
2. What is the relationship between evaporation and relative
humidity? (like the more humid, the more/less evap.?)
3. Predict the difference between the dry-bulb and wet-bulb readings
when the relative humidity is 100%. Give a reason for your answer.
4. Would you feel more comfortable in a desert where the temperature
is 35°C or in a rain forest where the temperature is 35°C? Give a
reason for your answer.