Carefully read through these directions and the directions on the next page before you begin to work.

You may be part of a group for the first part of this exercise. If you are, the group should carry out the experiment and collect the data together, but each student must record the data in his or her own booklet. Be sure to record the data exactly as you observe them. After the data have been collected, each student should answer the questions independently.

After you have finished your experiment and have recorded all of the data, you will be asked to answer some questions about the experiment and the data you recorded. Your answers must be written in this test booklet in the space provided. Make sure that you understand each question before you begin to write. At any time while you are writing your answers, you may look back at the directions for the experiment and the data you collected. Be sure that your answers are written as clearly and neatly as possible.

Before you begin the exercise, read the list of materials given below and check to make sure that your group has everything listed.

Materials in the Kit cardboard hygrometer stand, Part A cardboard hygrometer stand, Part B one 5" piece of hollow shoestring (wick) 2 small o-rings (for holding wick on bulb) 2 thermometers with Fahrenheit scales Materials Supplied by the School ice 50 ml of iced water water at room temperature narrow masking tape or Scotch tape 2 100 ml beakers or equivalent paper, styrofoam, or plastic cups 1 pair of sharp, pointed scissors paper towels stopwatch or clock

The relative humidity of air compares the actual amount of water vapor in the air at a given temperature to the amount of water vapor that the air could hold at that temperature. Warm air can hold more water vapor than cool air. The human body is cooled in part by the evaporation of water from the skin, and the rate of evaporation is affected by air's relative humidity. Air's apparent temperature, a measure of how warm the air feels to a person, also depends on the relative humidity of the air that is in contact with the person's skin. Air with a high humidity has a higher apparent temperature than air with a low humidity, because the more humid air slows water's evaporation from the skin.

A hygrometer is a simple device that compares the air temperature measured by a dry bulb thermometer
(a thermometer with a bare bulb) to that measured by a wet bulb thermometer (a thermometer with a water-soaked wick attached to the bulb). The temperature measured by the wet bulb thermometer depends upon the rate of evaporation of the water from the wick, which depends in turn upon the air's relative humidity.

In this event, you will assemble a simple hygrometer, and you will use it to determine the relative humidity and apparent temperature of the air in your classroom. Be sure to read all of the directions before you begin this event.



Thermometer Calibration

• First, to correct for any defects in the thermometers, the thermometers must be calibrated. Label the thermometers A and B. Next, obtain some ice water from your teacher and add some ice to it. There should be ice in the water at all times. Using either thermometer, gently stir the ice and water mixture for a few seconds. Leave both thermometers in the ice water and wait 30 seconds. Then read the temperature of the iced water with each thermometer. For either thermometer, if the temperature reading is different from 32, find the correction factor. The correction factor equals the thermometer's reading minus 32. If you get a negative correction factor, record the factor with the minus sign. Record the correction factors for Thermometers A and B in the spaces below.
  
  Thermometer A correction factor = ____
  
  Thermometer B correction factor = ____
  
  
• If a correction factor is positive for one of the thermometers, whenever you measure a temperature with that thermometer, you must subtract the correction factor from that measurement.
  
  
• If a correction factor is negative for one of the thermometers, whenever you measure a temperature with that thermometer, you must add the correction factor to that measurement.
  
  
  
• For example, suppose Thermometer A (or B) gives a reading of 34 in the ice water. Then the correction factor is 34 - 32 = +2. Now, suppose the thermometer indicates an air temperature of 70. Since the correction factor for Thermometer A is positive, you must subtract 2 from this reading, so the air temperature is really 68.
  
  
• As another example, suppose Thermometer A (or B) gives a reading of 30 in the ice water. Then the correction factor is -2. Now, suppose the thermometer indicates an air temperature of 70. Since the correction factor is negative, you must add 2 to this reading, so the air temperature is really 72.
  
  
  
• Discard the ice water.
  

  (Note: (1) A proper calibration of these thermometers would include corrections at several additional temperatures, including one at the boiling point of water. Because of time constraints, we are omitting these additional corrections. (2) Each thermometer contains both Fahrenheit and Celsius scales.
  Make sure you read only the Fahrenheit scale.)
  

• Place Part A of the cardboard stand on a flat surface (table) and fold it where indicated along the dotted lines, as shown in Figure A.
  
  
  
  Figure A
  
  
  
• Pick up Part B and push out the 4 tabs with the holes. Then assemble Parts A and B by fitting the slots of Part B into the slots of Part A, as shown in Figure B.
  
  
  
  Figure B
  
  
• Push the two thermometers through the holes in the tabs, as shown in Figure C1.
  

  Figure C1		Figure C2

• Slide an o-ring onto one of the thermometers. (CAUTION: Do not bend the thermometer, as the glass may break and cause injury.)
  
  
• Slide the hollow shoestring wick over the bulb of the thermometer that has the O-ring, making sure the bulb is completely covered, as shown in Figure C2.
  
  
• To hold the wick in place, carefully roll the O-ring down over the wick, so that the O-ring is above the bulb of the thermometer. (The thermometer with the wick will be referred to as the wet bulb thermometer, and the other thermometer will be referred to as the dry bulb thermometer, as shown in Figure C1, above.)

You are now ready to begin collecting your data.

1. Obtain a cup of water at room temperature from your teacher.

2. Immerse the free end of the wick in the cup of water at room temperature and wait 30 seconds, or until the wick is saturated with water.

3. Remove the cup of water and, using a small paper fan or your hand, fan air over the wick and the bulb of the dry bulb thermometer for about 15 seconds.

4. Read to the nearest degree the temperatures shown on the two thermometers. Correct the temperatures using the results you obtained when you calibrated your thermometers (p. 2) and record your corrected results in the row labeled 'Trial 1' in Table 1.

5. Subtract the corrected wet bulb temperature from the corrected dry bulb temperature and record the temperature difference in the row labeled 'Trial 1' in Table 1.

6. For Trials 2 and 3, repeat Steps 3, 4, and 5. You will average the results of Trials 1-3.

7. To determine the relative humidity of the air in your classroom: Look up the dry-bulb temperature and the temperature difference that you recorded in Table 1 for Trial 1. Find your dry-bulb temperature on the left side of Table 2, page 21. Find your temperature difference on the top of Table 2. Using these 2 temperatures find the relative humidity for Trial 1 and record the relative humidity in Table 1. Repeat Step 7 to obtain the relative humidities for Trials 2 and 3.

8. Calculate the average corrected dry bulb temperature and the average relative humidity for the three trials (in Table 1), and record both averages in Table 1. In addition, write the average corrected dry bulb temperature and the average relative humidity above Table 3.

9. To determine the apparent temperature of the air in your room, use the averages from Step 8 in Table 3. Record the apparent temperature in the space indicated below Table 1.

Trial	Corrected Dry bulb temperature ()	Corrected Wet bulb temperature ()	Corrected Dry bulb temperature minus Corrected wet bulb temperature ()	Relative humidity (%)
1
2
3
Average

Apparent temperature () of the air in your classroom = ____

Table 2

Relative Humidity (%)

Difference Between Dry-Bulb and Wet-Bulb Temperatures ()




Table 3

Please answer the following questions by yourself.

1. A thermometer that is used to measure the temperature of a hot gas is calibrated in boiling water (the temperature of the boiling point at an atmospheric pressure of 1 atm = 212). Use the following information to calculate the actual temperature of the hot gas. Make sure to show all of your work.
   

   The temperature of the boiling water according to the thermometer = 215.

   The actual temperature of the boiling water = 212.

   The temperature of the hot gas according to the thermometer = 375.
   

   The actual temperature of the hot gas = ____ .
   
   
   
   
   
   

2. The information in Table 4 was collected at noon each day for 6 days at a given location. Based on the data in Table 4, estimate the noontime air pressure, percent cloud cover, and precipitation at this location if the air temperature is 86 and the relative humidity is 97%. Write your estimates in the empty cells in Table 5. Explain how you arrived at your estimates.

Day	Relative humidity (%)	Air temperature ()	Air pressure (inches of mercury)	Cloud cover (%)	Amount of precipitation (inches)
1	6	75	28.95	40	0
2	98	85	28.30	100	1.5
3	30	77	29.01	25	0
4	40	75	29.50	0	0
5	55	78	29.20	30	0
6	60	79	28.95	50	trace
7	95	85	28.25	100	1.3

Table 5

Relative humidity (%)	Air temperature ()	Air pressure (inches of mercury)	Cloud cover (%)	Amount of precipitation (inches)
97	86