Page 237 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Measuring Relative Humidity AQuantitativeSOlUTiON and Dew-Point Temperature
how humid is it? Measuring the level of relative humidity is actu- ally a simple matter of reading the temperature simultaneously shown by two ordinary mercury-in-glass thermometers mounted side by side (Figure 7.12b). the first is the dry-bulb thermometer, a regular thermometer from which we read the temperature of the air. the other is the same type of thermometer, but with a cloth sleeve that can be wetted, giving the wet-bulb temperature. the thermometer with the wet sleeve measures a lower temperature than the dry bulb on any day when the air is less than saturated. this results because moisture evaporates from the cloth sleeve and cools the bulb. the lower the relative humidity is, the more rapid the evaporation, the greater the cooling, and the greater the difference in temperatures between the two thermometers.
Because the device described above works well only if air is moving rapidly past the wet bulb, a fan can be installed to create a strong air flow. In a sling psychrometer, two thermometers are
mounted side by side and are linked to a handle. after the user wets the cloth sleeve, the psychrometer is spun rapidly in a circle to produce the maximum evaporation rate. When the spinning is stopped, the two temperatures, one dry-bulb and one wet-bulb, are recorded and compared. the difference in temperatures is then used with the psychrometric tables in table aQS 7.1.
to use the tables to determine relative humidity (rh, %) and dew point (°C) for a pair of numbers from the sling psy- chrometer, calculate the difference between the wet bulb and dry bulb temperatures. this difference is called the wet-bulb depression.
the first two rows of table aQS 7.2 give some completed ex- amples of working out relative humidity and dew-point tempera- ture based on the differences between wet-bulb and dry-bulb readings. rows 3–6 are incomplete. Fill in the missing values (those marked with ?) using tables aQS 7.1a and 7.1b.
TaBLe aQS 7.1
psychrometric tables for (a) relative humidity and (b) Dew-point temperature
(a) Psychrometric Chart of relative Humidity (percent)
0.5
−20.0 70 41 11
1.0
Depression of the Wet Bulb (dry-bulb temperature minus wet-bulb temperature, C°)
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
7.5
10.0
12.5
15.0
17.5
20.0
22.5
25.0
−17.5
75
51
26
2
−15.0 79 58 −10.0 85 69 −5.0 88 77 0.0 91 82 5.0 93 86 10.0 94 88 15.0 95 90 20.0 95 91 25.0 96 92 30.0 96 93 35.0 97 93 40.0 97 94 45.0 97 94 50.0 97 95
38 18
54 39
66 54
73 65
78 71
82 76
85 80
87 82
88 84
89 86
90 87
91 88
91 89
92 89
24 10
43 32
56 47
65 58
71 65
75 70
78 74
81 77
82 79
84 81
85 82
86 83
87 84
21 11 1
39 31 23 15
−12.5
82
65
47
30
13
−7.5
87
73
60
48
35
22
10
−2.5
90
80
70
60
50
42
37
22
12
3
2.5
92
84
76
68
61
53
46
38
31
24
51 45 38 60 54 49 66 61 57 70 66 62 73 70 66 76 73 70 78 75 72 79 77 74 81 78 76 82 79 77
32 1
44 19
52 31 12
58 40 24 8
63 47 32 19 7
7.5
93
87
80
74
68
62
56
50
44
38
11
12.5
94
89
84
78
73
68
63
58
53
48
25
4
17.5
95
90
86
81
77
72
68
64
60
55
36
18
2
22.5
96
92
87
83
80
76
72
68
64
61
44
28
14
1
27.5
96
92
89
85
82
78
75
71
68
65
50
36
23
12
1
67 52 39 27 69 56 44 33 72 59 48 38 73 62 51 42 75 64 54 45
16 6
23 14
29 21
33 26
37 30
6
13 6
18 12 6
23 17 11
32.5
97
93
90
86
83
80
77
74
71
68
54
42
30
20
11
1
37.5
97
94
91
87
85
82
79
76
73
70
58
46
36
26
18
10
3
42.5
97
94
91
88
86
83
80
78
75
72
61
50
40
31
23
16
9
2
47.5
97
94
92
89
86
84
81
79
76
74
63
53
44
35
28
21
15
9
201
Dry-Bulb Temperature (air temperature, °C)