WORKSHEET 21

Activity - Wet Air

Print and copy this worksheet for use in the classroom.
 

Purpose

 To measure the humidity in the classroom and school ground.

 
 

Equipment

  • two thermometers
  • cotton wool
  • rubber band
 

Safety

Handle thermometers carefully. Students shouldn't try to clean up the mess if a thermometer happens to break. Use a mercury spill-kit or throw sulphur over the mercury and clean it up wearing gloves and using a brush and pan. Remember not to touch it because mercury is a poisonous metal.

 
 

Procedure

  1. Wrap the cotton wool around the bulb of a thermometer. Secure it with a rubber band.

  2. Dip the wrapped bulb in water.

  3. Gently fan the wet-bulb thermometer until the temperature reaches a minimum.

  4. Compare the reading of the wet-bulb thermometer and the dry-bulb thermometer.

  5. Record the difference in the two readings.

  6. Read the relative humidity from the chart (see the humidity chart below). The numbers on the side of the chart represent the dry-bulb temperature. The numbers at the top represent the difference between the dry and wet bulb temperature. Measurements are in °C . The point at which the row and the column intersects is the percentage humidity in the air. For example, if the dry bulb temperature is 20°C, and the wet bulb depression is 5.5°C, then the humidity is 55 percent.

  7. Repeat the excercise elsewhere, such as the school ground.

 
 

Questions

  1. Why does the wet bulb thermometer register a lower temperature than the dry-bulb thermometer?
  2. When is the humidity higher?
    When the temperature difference between a wet-bulb and a dry-bulb thermometer is high?
    Or
    When the temperature difference between a wet-bulb and a dry-bulb thermometer is low?
 

Explanation

Humidity is a measure of the amount of water vapour in air. If air temperature is 20°C, each cubic metre of air can contain up to 17g of water vapour. If there is only 8.5g of water vapour per cubic metre at 20°C, we can say the relative humidity is 50%

Humidity can have a strong influence on how comfortable we feel. Combinations of high temperature and high humidity can be very uncomfortable. Physical activity and exposure to direct sunlight can also increase heat stress.

Humidity Chart

The numbers on the side of the chart represent the dry-bulb temperature. The numbers at the top represent the difference between the dry and wet bulb temperature. Measurements are in °C. The point at which the row and the column intersects is the percentage humidity in the air. For example, if the dry bulb temperature is 20°C, and the wet bulb depression is 5.5°C, then the humidity is 55 percent.
0.5 1 1.5 2 2.5
3_ 3.5 4 4.5 _5
 5.5 6 6.5 7 7.5 8_ 8.5 9 9.5 10 11 12 13 14 15 16

40

39

38

37

36

97 94 91 88 85

97 94 91 88 85

97 94 91 87 85

97 94 91 87 85

97 94 90 87 84

82 80 77 74 72

82 79 77 74 71

82 79 76 74 71

82 79 76 73 70

81 78 76 73 70

69 67 64 62 60

69 66 64 61 59

68 66 63 61 58

68 65 63 60 58

67 65 62 60 57

57 55 53 51 48

57 54 52 50 48

56 54 51 49 47

55 53 51 48 46

55 52 50 48 45

44 40 37 33 29 26

44 40 36 32 28 25

43 39 35 31 27 24

42 38 34 30 26 23

41 37 33 29 25 21

35

34

33

32

31

97 93 90 87 84

97 93 90 86 84

97 93 90 86 83

97 93 90 86 83

96 93 90 86 83

81 78 75 72 70

81 78 7572 69

80 77 74 71 69

80 77 74 71 68

80 77 73 70 67

67 64 61 59 56

66 64 61 58 56

66 63 60 58 56

66 62 60 57 54

64 62 59 56 53

54 51 49 47 44

53 51 48 46 44

52 50 47 45 43

52 49 46 44 42

51 48 45 43 41

40 36 32 28 24 20

39 35 30 26 23 19

38 34 29 25 21 16

37 32 28 24 20 16

36 31 27 22 18 14

30

29

28

27

26

96 93 89 86 83

96 93 89 86 82

96 93 89 85 82

96 92 88 85 82

96 92 88 85 81

79 76 73 70 67

79 76 72 69 66

79 75 72 69 65

78 75 71 68 65

78 74 71 67 64

64 61 58 55 52

63 60 57 54 52

62 59 56 53 51

62 59 56 52 50

61 58 55 51 49

50 47 44 42 39

49 46 43 41 38

48 45 42 40 37

47 44 41 38 36

46 43 40 37 34

34 30 25 21 17 13

33 28 24 19 15 11

32 27 22 18 13 _9

30 25 21 16 11 _7

29 24 19 14 _9 _5

25

24

23

22

21

96 92 88 84 81

96 92 88 84 80

96 92 88 84 80

96 92 87 83 79

96 91 87 83 79

77 74 70 67 63

77 73 69 66 62

76 72 69 65 62

76 72 68 64 61

75 71 67 63 60

60 57 54 50 47

59 56 52 49 46

58 55 51 48 45

57 54 50 47 44

56 52 49 46 42

44 41 38 36 33

43 40 37 34 31

42 39 36 33 30

40 39 36 33 30

39 35 32 29 26

27 22 17 12 _7 _3

26 21 15 10 _5

24 18 13 _8 _3

22 16 11 _5

20 14 _8 _3

20

19

18

17

16

96 91 87 83 78

95 91 86 82 78

95 91 86 81 77

95 90 86 81 77

95 90 85 81 76

74 70 66 62 59

74 70 65 61 58

73 69 65 60 56

72 68 64 59 55

71 67 62 58 54

55 51 48 44 41

54 50 46 43 39

52 49 45 41 37

51 47 43 39 35

50 46 41 37 34

37 34 30 27 24

35 32 29 25 22

34 30 27 23 20

32 2824 21 17

30 26 22 18 15

18 12 _6

15 _9_3

13 _7

10 _4

_8 _1

15

14

13

12

11

10

95 90 85 80 75

95 90 84 79 74

95 89 84 79 74

94 89 83 78 73

94 88 83 77 72

94 88 82 77 71

71 66 61 57 52

70 65 60 56 51

69 64 59 54 49

68 63 57 53 48

66 61 56 51 46

65 60 54 49 44

48 44 40 36 31

47 42 38 33 29

45 40 36 31 27

43 38 34 29 24

41 36 31 26 22

29 34 29 24 19

27 24 20 16 12

25 21 17 14 _9

23 18 14 10 _6

20 16 11 _7 _3

17 13 _8 _4

14 _9 _5

_5

_2

 

Extension Activities

How is humidity related to weather conditions?

Design an experiment to use wool to measure humidity. Test the experiment by comparing your measurements with the wool to humidity measurements from meteorological instruments.

 

Fact file

Cows lying down, appearance of large numbers of frogs and snails, wool swelling and straightening, pine cone scales becoming pliable, and your hair getting longer: theses may all be signs of high humidity.
   

 
This activity is courtesy of
the Cooperative Research Centre for Southern Hemisphere Meteorology

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