Humidity

Figure 2.14 Whirling hygrometerwith calculator.

Humidity is the amount of water vapour held in the atmosphere. In everyday language it is something described as 'close' (warm and sticky), 'dry' (little water in the air), 'damp' (cool and moist) and 'buoyant' (comfortable atmosphere). It is usually expressed more accurately in terms of relative humidity (see below).

The whirling hygrometer (psychrometer) is the most accurate portable instrument used for taking air measurements (see Figure 2.14). The wet and dry bulb thermometers are mounted such that they can be rotated around a shaft held in the hand, rather like a football rattle. Whilst the dry bulb gives the actual temperature of the surroundings, the wet bulb temperature is depressed by the evaporation of the water on its surface (the same cooling effect you feel when you have a wet skin). The drier the air, the greater the cooling effect, i.e. a greater wet bulb depression.

The relative humidity is calculated using hygrometric tables after the full depression of the wet bulb temperature has been found (see example below).

Hygrometers made out of hair, which lengthens as the humidity increases, are also used to indicate humidity levels. These can be connected to a pen that traces the changes on a revolving drum carrying a hygrogram chart. Other hygrometers are based on the moisture absorbing properties of different materials including the low technology 'bunch of seaweed'.

Relative Humidity

The quantity of water vapour held in the air depends on temperature, as shown below:

0°C 3 g of water per kilogram of air

The maximum figure for each temperature is known as its saturation point or dew point, and if such air is cooled further, then water vapour condenses into liquid water. One kilogram of saturated air at 20°C would give up 7 g of water as its temperature fell to 10°C. Indoors this is seen as 'condensation' on the coolest surfaces in the vicinity. Outdoors it happens when warm air mixes with cold air. Droplets of water form as clouds, fog and mist; dew forms on cool surfaces near the ground. If the air is holding less than the maximum amount of water it has drying capacity i.e. it can take up water from its surroundings.

One of the most commonly used measurements of humidity is relative humidity (RH) which is the ratio, expressed as a percentage, of the actual quantity of water vapour contained in a sample of air to the amount it could contain if saturated at the dry bulb temperature. This is usually estimated by using the wet and dry bulb temperatures in conjunction with hygrometric tables.

  1. If the absolute humidity for air at 20°C (on the dry bulb) is found to be 14 g/kg this compares with the maximum of 14 g that can be held when such air is saturated. Therefore, the RH is 100 per cent.
  2. It can be seen that RH falls to 25 per cent when the wet bulb depression shows only 3.5 g of water are present. This means that its drying capacity has increased (it can now take up 10.5 g of water before it becomes saturated).
  3. An example of working out the relative humidity from wet and dry bulb measurements is given below in Tables 2.5 and 2.6. In example 4, when the dry bulb reading is 20°C and the depression of the wet bulb is 2.5°C then the relative humidity is 78 per cent.
Table 2.5 Calculation of relative humidity from wet and dry bulb measurements

Example

1

2

3

4

5

Dry bulb reading °C (A)

25

25

25

20

10

Wet bulb reading 26°C

21

19.5

18

22.5

6.5

Depression of wet bulb °C (B)

4

5.5

7

2.5

2.5

Relative humidity (%)*

70

60

50

78

71

  • found from tables supplied with the hygrometer by reading along to the dry bulb reading row (A) then find where the column intersects the depression of the wet bulb figure (B) as shown in Table 2.6 below.
  • found from tables supplied with the hygrometer by reading along to the dry bulb reading row (A) then find where the column intersects the depression of the wet bulb figure (B) as shown in Table 2.6 below.

Table 2.6 Determination of relative humidity from the wet bulb depression Depression of wet bulb Dry bulb reading (in °C)

from Table 2.5

Table 2.6 Determination of relative humidity from the wet bulb depression Depression of wet bulb Dry bulb reading (in °C)

from Table 2.5

i n °C

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

0.5

94

94

94

95

95

95

95

95

95

95

96

96

96

96

96

96

1.0

88

88

89

89

90

90

90

90

91

91

91

91

92

92

92

92

1.5

82

83

83

84

84

85

85

86

86

86

87

87

88

88

88

88

2.0

76

77

78

79

79

80

81

82

82

83

83

83

83

84

84

84

2.5

71

72

73

74

74

75

76

77

77

78

78

79

80

80

80

81

3.0

65

66

68

69

70

71

71

72

72

74

74

75

76

76

77

77

3.5

60

61

62

64

65

66

67

68

69

70

70

71

72

72

73

74

4.0 54 56 57 59 60 61 63 64 65 65 66 67 68 69 69 69

Note: Example 4 from Table 2.5 illustrated in grey to show intersection of the dry bulb temperature of 20°C with the depression of the wet bulb by 2.5°C.

4.0 54 56 57 59 60 61 63 64 65 65 66 67 68 69 69 69

Note: Example 4 from Table 2.5 illustrated in grey to show intersection of the dry bulb temperature of 20°C with the depression of the wet bulb by 2.5°C.

Wind

Wind speed is measured with an anemometer, which is made up of three hemispherical cups on a vertical shaft ideally set 10 metres above the ground (see Figures 1.7 and 2.1). The wind puts a greater pressure on the inside of the concave surface than on the convex one so that the shaft is spun round; the rotation is displayed on a dial usually calibrated in knots (nautical miles per hour) or metres per second. An older and still much used visual method is the Beaufort Scale; originally based on observations made at sea, it is used to indicate the wind forces at sea or on land (see Table 2.7).

Wind direction is indicated with a wind vane, which is often combined with an anemometer. Decorative wind vanes are a familiar sight, but the standard meteorological design comprises a pointer with a streamlined vertical plate on one end mounted so that it can rotate freely. The arrow shape points into the wind and the movements over a minute or so are averaged. The direction the wind is coming from is recorded as the number of degrees read clockwise from true north, i.e. a westerly wind

Table 2.7 The Beaufort Scale

Force

Description for use on land

Equivalent wind speed

m/sec

approx miles/hour

0

Calm; smoke rises vertically

0

0-1

1

Light air; wind direction seen by smoke drift rather than by wind vanes

2

1-3

2

Light breeze; wind felt on face, vane moves, leaves rustle

5

4-7

3

Gentle breeze; light flags lift, leaves and small twigs move

9

8-12

4

Moderate breeze; small branches move, dust and loose paper move

13

13-18

5

Fresh breeze; small leafy trees sway, crested wavelets on lakes

19

19-24

6

Strong breeze; large branches sway, umbrellas difficult to use

24

25-31

7

Near gale; whole trees move, difficult to walk against

30

32-38

8

Gale; small twigs break off, impedes all walking

37

39-46

9

Strong gale; slight structural damage

44

47-55

10

Storm; trees uprooted, considerable structural damage

52

55-63

is given as 270, south-easterly as 135 and a northerly one as 360 (000 is used for recording no wind).

Light

The units used when measuring the intensity of all wavelengths are watts per square metre (W/sq m) whereas lux (lumens/sq m) are used when only light in the photosynthetic range is being measured. More usually in horticulture, the light integral is used. The light sensors used for this measure the light received over a period of time and expressed as gram calories per square centimetre (gcals/sq m) or megajoules per square metre (MJ/sq m). These are used to calculate the irrigation need of plants in protected culture.

The usual method of measuring sunlight at a meteorological station is the Campbell-Stokes Sunshine Recorder (Figure 2.15), a glass sphere that focuses the sun's rays on to a sensitive card; the burnt trail indicates the periods of bright sunshine (see p26). Another approach is to use a solarimeter, which converts the incoming solar radiation to heat and then to electrical energy that can be displayed on a dial.

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