Relative Humidity. In this experiment, you will use two Temperature Probes to determine relative humidity at several different locations.

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1 Relative Humidity Computer 26 If you live in a place where it gets hot and muggy, you know about humidity. Humidity is a measure of the amount of water vapor in the air. When meteorologists report humidity, however, they are usually referring to the relative humidity. Relative humidity is the amount of water vapor in the air compared to the maximum amount it can hold. The maximum amount of water vapor air can hold varies depending on its temperature warm air can hold more water than cold air. One way to measure relative humidity is by observing how easily water evaporates. The drier the air, the faster water will evaporate. Have you ever noticed that you feel cooler when you have just stepped out of the water? This is because as water evaporates, it cools the surface it evaporated from. If we measure how much a Temperature Probe cools as water evaporates off of it, we can calculate the rate of evaporation and therefore the relative humidity. In this experiment, you will use two Temperature Probes to determine relative humidity at several different locations. OBJECTIVES In this experiment, you will Use a computer to measure temperature. Determine relative humidity. Explain your results. MATERIALS computer piece of shoelace, 5 cm long Vernier computer interface masking tape Logger Pro beaker 2 Temperature Probes water Evaluation copy temperature probe masking tape Figure 1 shoelace Earth Science with Vernier 26-1

2 Computer 26 Choosing a Method Method 1 uses remote data collection. It should be used if you have a LabPro interface with a desktop computer. Method 2 requires that the computer remains attached to the interface. This method should be used if you have a Vernier computer interface with a laptop computer that can be taken outside. METHOD 1: Using LabPro Remotely 1. Connect the Temperature Probes to LabPro. 2. Slip Probe 2 into a 5 cm piece of shoelace until the probe tip is at the middle of the shoelace piece as shown in Figure 1. Fasten the shoelace to the probe with masking tape. 3. Prepare the computer for data collection by opening the file 26 Relative Humidity in the Earth Science with Vernier folder. 4. Wet the shoelace on Probe 2 by placing it into a beaker or cup of water that is at or above room temperature. Probe 1 is to stay dry. 5. Click to begin data collection. Take a probe in each hand and gently wave the probes in the air. 6. Continue waving the probes until the temperatures measured by both probes stop changing. Then click to end data collection. Record both final temperatures in the data table. 7. Choose Store Latest Run from the Experiment menu to store your data. 8. Choose Remote Remote Setup from the Experiment menu. A summary of your setup will be displayed. 9. Click. The interface can now be disconnected from the computer. Important: (1) Be very careful not to press the START/STOP button on the interface until you are ready to begin collecting data, and (2) do not close the Logger Pro computer program. 10. Resoak the shoelace on Probe 2 when you are at the new location. Allow approximately one minute for the Temperature Probes to adjust to the temperature at the new location before beginning data collection. 11. When everything is ready, press the START/STOP button on the LabPro interface. The green light on LabPro should begin to blink. Take a probe in each hand and gently wave the probes in the air. Continue waving the probes for 5 minutes (the green light will stop blinking when data collection ends). 12. When data collection is finished, go back to the computer and reattach the interface. If a Remote Data Available window appears, simply click the YES button and choose to retrieve remote data into the current file. If a window does not appear when the interface is reconnected, choose Remote Retrieve Remote Data from the Experiment menu. 13. Choose Store Latest Run from the Experiment menu to store your data Earth Science with Vernier

3 Relative Humidity 14. Repeat Steps 8 12 for a third site. If this site is at an outside location, be sure to use water that is at or above the air temperature. (For example: If the outside air temperature is 22 C, the water temperature must be 22 C or higher.) METHOD 2: Interface Remains Attached to Computer 1. Connect the Temperature Probes to the Vernier computer interface. 2. Slip Probe 2 into a 5 cm piece of shoelace until the probe tip is at the middle of the shoelace piece as shown in Figure 1. Fasten the shoelace to the probe with masking tape. 3. Prepare the computer for data collection by opening the file 26 Relative Humidity in the Earth Science with Vernier folder. 4. Wet the shoelace on Probe 2 by placing it into a beaker of water that is at or above room temperature. Probe 1 is to stay dry. 5. Click to begin data collection. Take a probe in each hand and gently wave the probes in the air. 6. Continue waving the probes until the temperatures measured by both probes stop changing, and then click to end data collection. Record both final temperatures. 7. Depending on the equipment you are using, you will either collect data at another location or stop data collection now. Read the following two options to determine which you will do. If you are using a LabPro interface and a laptop computer or if you are using a battery-powered Serial Box Interface and a laptop computer, proceed to Step 8. If you are using a desktop computer or your interface is not battery-powered, proceed directly to the Processing the Data section. 8. Choose Store Latest Run from the Experiment menu to store your data. 9. Repeat Steps 3 7 at another location inside your school. Be sure to resoak the shoelace on Probe 2 when you are at the new location. Allow approximately one minute for the Temperature Probes to adjust to the temperature at the new location before beginning data collection. 10. Repeat Step 4 6 outside your school at a third location. Use water that is at or above the air temperature at the outside location. (For example: If the outside air temperature is 22 C, the water temperature must be 22 C or higher.) Earth Science with Vernier 26-3

4 Computer 26 DATA Site 1 Site 2 Site 3 Classroom Dry probe temperature ( C) Wet probe temperature ( C) Temperature difference ( C) Relative humidity (%) Table 1: Relative Humidity Dry Probe Dry Probe Temperature Minus Wet Probe Temperature ( C) Temperature C C C C C C C C C C C C C C C C C C C C C Earth Science with Vernier

5 PROCESSING THE DATA Relative Humidity 1. In the space provided in the data table, subtract to find the difference between the dry-probe (Probe 1) and wet-probe (Probe 2) temperatures at each site. 2. Determine the relative humidity at each site using Table 1 Relative Humidity. a. Find the temperature difference you calculated in Question 1 at the top of the table. Keep one finger on this number. b. Find the dry probe (Probe 1) temperature in the first column of the table. c. Look across the row until you find the column marked with your finger. This is the relative humidity. d. Record this number in the data table. 3. How did the wet probe temperature compare with the dry probe temperature at each site? Explain why. 4. Which site had the highest relative humidity? 5. Which site had the lowest relative humidity? 6. Explain what could cause the relative humidity to be different at these locations on the same day. EXTENSIONS 1. Compare relative humidity values at sunny and shaded sites outdoors. 2. Compare relative humidity values at breezy and protected sites outdoors. Earth Science with Vernier 26-5

6 Vernier Lab Safety Instructions Disclaimer THIS IS AN EVALUATION COPY OF THE VERNIER STUDENT LAB. This copy does not include: Safety information Essential instructor background information Directions for preparing solutions Important tips for successfully doing these labs The complete Earth Science with Vernier lab manual includes 33 labs, 6 projects, and essential teacher information. The full lab book is available for purchase at: Vernier Software & Technology S.W. Millikan Way Beaverton, OR Toll Free (888) (503) FAX (503) info@vernier.com