Name_________________________ Date: _______Period______ Unit Three: Meteorology Lab Activity: How Can We Measure Relative Humidity? The amount of water vapor in air compared with the maximum amount of water vapor that the air can hold at a given temperature is called relative humidity. Relative humidity can be measured with a sling psychrometer. A psychrometer makes use of the cooling effect of evaporation. As water evaporates from a surface, it cools the surface. Water will evaporate more quickly into dry air than into humid, moist air. The more a wet surface is cooled by evaporation, the more quickly evaporation is taking place – the drier the air. Objective: In this activity you will use a sling psychrometer to determine the relative humidity of the air Skills: measuring, interpreting a chart, comparing Materials: sling psychrometer, water, timing device Procedure: 1. The thermometer with the cloth is called a wet-bulb thermometer. Wet the cloth with room temperature water. 2. Spin the psychrometer for 20 seconds. Read both thermometers. Record the temperatures in the Table 1. Subtract the wet-bulb temperature from the dry bulb temperature and record the difference in the Table 1. 3. Continue spinning the psychrometer and checking the temperatures until there is no further change or until two minutes have past. Record those temperatures as final temperatures in Table 1. Subtract the wet-bulb temperature from the dry-bulb temperature. Record the difference in Table 1. 4. In Table 1, circle the two numbers that represents the final dry-bulb temperature the final temperature difference. 5. On the humidity chart provided, locate the two circled values from table 1. Dry-bulb temperature is on the left hand side and the temperature difference is along the top. 6. Follow the temperature difference down and the dry-bulb temperature to the right until the two lines meet. Circle this value. This value represents the relative humidity of the air. Observations: Table 1 - Measuring Relative Humidity (Inside) Dry-Bulb Temperature (°C) Wet-Bulb Temperature (°C) Temperature Difference (°C) After 20 Seconds Final (after 2 minutes) Table 2 - Measuring Relative Humidity (Outside) Dry-Bulb Temperature (°C) After 20 Seconds Final (after 2 minutes) Wet-Bulb Temperature (°C) Temperature Difference (°C) 7. Record your data for final dry-bulb temperature, final temperature difference, and relative humidity in line #1 of Table 2. 8. Repeat steps 1 – 7 for outside measurement. 9. Record both sets of data from Table 2 onto the board. As other groups record their data on the board record their information on the remaining lines on Table 2. Table 2: Group Data (Inside) Dry Bulb Temperature Temperature Difference Table 2: Group Data (Outside) Relative Humidity Dry-Bulb Temperature #1 #1 #2 #2 #3 #3 #4 #4 #5 #5 #6 #6 #7 #7 #8 #8 #9 #9 # 10 # 10 # 11 # 11 # 12 # 12 # 13 # 13 Temperature Difference Relative Humidity Questions: 1. What was your value for relative humidity? Inside: __________ Outside: _________ 2. Calculate the average relative humidity of all groups. How close was this to your value? Inside: ___________ Outside: _________ 3. Did the temperature on the wet-bulb rise or fall? _________ 4. What happens to the molecules of water on the cloth when the wet-bulb thermometer is placed in dry air? 5. What happens to the molecules of water on the cloth when the wet-bulb thermometer is placed in saturated air? 6. What was the purpose of spinning the psychrometer? What did the spinning psychrometer simulate? 7. Why did relative humidity vary at the two test sights? 8. Does a great difference in temperature between dry-bulb and wet-bulb thermometers indicate a high humidity or low humidity? Explain your answer.