Features Of The Evaporation Process

(a) Evaporation rates can be described either in terms of the rate at which the water level falls, or in terms of the rate at which latent heat is consumed in the process (Note 4.D).

(b) The rate of evaporation from a water surface is proportional to (e-e), and also to the speed of the wind blowing on the water surface. This relationship is named Dalton's equation (Note 4.E), after John Dalton (1766-1844).

(c) Evaporation is not the same as boiling; boiling always entails rapid evaporation, but evaporation does not necessitate boiling. Evaporation occurs at a liquid's surface at any temperature, provided the air is dry enough, whilst the bubbles characteristic of boiling form only when the boiling point has been reached. This is a temperature which depends on the local atmospheric pressure, for the following reason. Bubble formation requires a temperature high enough to make vapour molecules inside the bubble sufficiently energetic to exert an outwards pressure equal to the squeezing caused by the atmospheric pressure on the liquid. No

Table 4. 1 The effect of temperature (°C) on the saturation vapour pressure (svp: hPa) of an atmosphere above a water surface

Temperature (°C) Saturation vapour pressure (hectopascals}

Temperature (°C) Saturation vapour pressure (hectopascals}

Table 4. 1 The effect of temperature (°C) on the saturation vapour pressure (svp: hPa) of an atmosphere above a water surface

.0°C

,2°C

,4°C

,6°C

,8°C

0

6.1 hPa

6.2 hPa

6.3 hPa

6.4 hPa

6.5 hPa

1

6.6

6.7

6.8

6.9

7.0

2

7.1

7.2

7.3

7.4

7.5

3

7.6

7.7

7.8

7.9

8.0

4

8.1

8.2

8.4

8.5

8.6

5

8.7

8.8

9.0

9.1

9.2

6

9.3

9.5

9.6

9.7

9.9

7

10.0

10.2

10.3

10.4

10.6

8

10.7

10.9

11.0

11.2

11.3

9

11.5

11.6

11.8

11.9

12.1

10

12.3

12.4

12.6

12.8

12.9

11

13.1

13.3

13.5

13.7

13.8

12

14.0

14.2

14.4

14.6

14.8

13

15.0

15.2

15.4

15.6

15.8

14

16.0

16.2

16.4

16.6

16.8

15

17.0

17.3

17.5

17.7

17.9

16

18.2

18.4

18.6

18.9

19.1

17

19.4

19.6

19.9

20.1

20.4

18

20.6

20.9

21.2

21.4

21.7

19

22.0

22.2

22.5

22.8

23.1

20

23.4

23.7

24.0

24.3

24.6

21

24.9

25.2

25.5

25.8

26.1

22

26.4

26.8

27.1

27.4

27.7

23

28.1

28.4

28.8.

29.1

29.5

24

29.8

30.2

30.6

30.9

31.3

25

31.7

32.1

32.4

32.8

33.2

26

33.6

34.0

34.4

34.8

35.2

27

35.6

36.1

36.5

36.9

37.4

28

37.8

38.2

38.7

39.11

39.6

29

40.1

40.5

41.0

41.5

41.9

30

42.4

42.9

43.4

43.9

44.4

31

44.9

45.4

46.0

46.5

47.0

32

47.6

48.1

48.6

49.2

49.7

33

50.3

50.9

51.4

52.0

52.6

34

53.2

53.8

54.4

55.0

55.6

35

56.2

56.9

57.5

58.1

59.8

36

59.4

60.1

60.7

61.4

62.1

37

62.8

63.5

64.1

64.8

65.6

38

66.3

67.0

67.7

68.4

69.2

39

69.9

70.7

71.5

72.2

73.0

40

73.8

74.6

75.4

76.2

77.0

bubble could survive without such equality. So the saturation vapour pressure at the boiling point equals the ambient air pressure. For instance, the atmospheric pressure at sea-level (Section 1.5) is typically 1,013 hPa and that is the saturation vapour pressure at 100°C, which is hence the sea-level boiling point.

bubble could survive without such equality. So the saturation vapour pressure at the boiling point equals the ambient air pressure. For instance, the atmospheric pressure at sea-level (Section 1.5) is typically 1,013 hPa and that is the saturation vapour pressure at 100°C, which is hence the sea-level boiling point.

One result is that boiling point is lower at a high altitude, where the air pressure is less (Note 1.G). For example, water boils at 93°C on top of Mt Kosciusko (Australia's highest mountain, at 2,228 m above sea-level), and at about 70°C on top of Mt Everest.

(d) There is a reduction in the rate of evaporation from the sea, due to the dissolved salt. Salt attracts water molecules, so they less easily escape from the sea's surface. The result is that sea-water's vapour pressure is about 2 per cent less than that of pure water.

(e) Evaporation from impure water leaves the impurity behind. That is how salt is collected in vast shallow ponds by the sea in South Australia after the water has been evaporated away by exposure to the Sun. Likewise, polluted water becomes worse. And the salt in water brought to irrigate farmlands gradually poisons the soil by deposition during evaporation unless care is taken to flush the deposit away periodically, using excess irrigation and suitable drainage.

But condensation of the vapour yields pure water. The combination of evaporation followed by condensation is called distillation. It occurs naturally when water is evaporated from the oceans and later forms clouds and then rain. It can be done artificially in a 'still', such as that in

Figure 4.4.

(f) Evaporation from sea-spray leaves an increasingly concentrated solution of salt in

Figure 4.4 An arrangement for distilling drinkable water. A sheet of clear plastic is sealed completely over a hole in the ground, with moist material beneath. The Sun's heat evaporates water from the material, and then condensation occurs on the underside of the relatively cool plastic. A stone on the plastic makes a cone which focuses drips into a can, from which water can be sucked by a tube.

Figure 4.4 An arrangement for distilling drinkable water. A sheet of clear plastic is sealed completely over a hole in the ground, with moist material beneath. The Sun's heat evaporates water from the material, and then condensation occurs on the underside of the relatively cool plastic. A stone on the plastic makes a cone which focuses drips into a can, from which water can be sucked by a tube.

each droplet, steadily lowering its vapour pressure and therefore slackening the rate of evaporation. As a result, droplets survive long enough to be blown well inland.

(g) On the other hand, evaporation from a drop reduces its size, thus increasing the curvature of the surface, which enhances the rate of evaporation (Note 4.F).

(h) The evaporation rate from a lake, reservoir or rice-field can be greatly lowered by applying even a thin layer of a material such as an oil or acetyl alcohol, which impedes the escape of water molecules. Unfortunately, the layer is eventually degraded by the Sun and blown away.

(i) If world climates changed and the oceans became, say, 2 K hotter, their vapour pressure would be about 14 per cent more than now. (Compare values in Table 4.1 for 15°C and 17°C, for instance, i.e. 17.0 and 19.4 hPa, respectively.) Dalton's equation (Note 4.E) shows that this increase of es would initially promote evaporation. However, it also indicates that there is automatic negative feedback, as the extra evaporation would increase the atmospheric vapour pressure e, thereby reducing the difference.

(j) There is a close connection between evaporation from a vegetative crop like sugarcane and its growth, because both processes involve gases diffusing through the small openings in the surfaces of the crop's leaves, called stomates. Leaf evaporation involves water vapour diffusing outwards through these stomatal openings, from the wet tissue within the leaf, whilst photosynthesis within the leaf requires the diffusion of carbon dioxide from the atmosphere inwards. Therefore, any closing of the stomates reduces both water loss and photosynthesis.

The connection between evaporation and growth is strengthened by the fact that both depend on the prior rainfall, on the temperature and on the radiation input. As a consequence, the amount of evaporation is an index of crop growth, at least in areas where water availability is the factor mainly limiting crop yield (Figure 4.5, Note 4.G).

(k) Extended periods of high evaporation from grass, bark, twigs, etc. create dry tinder and therefore a bushfire hazard. This is important in rural and suburban Australia.

(I) Evaporation from the skin affects human comfort. If the rate of evaporation is less than the rate of perspiration, the skin becomes moist and one feels uncomfortable (Note 4.H).

(m) What is called the equivalent temperature remains unchanged even if condensation or evaporation takes place within a given parcel of air. This is the measured temperature plus the heating resulting from

Figure 4.5 The relationship between the yield of wheat in South Australia and the crop's evaporation.

condensing all the water vapour within the parcel. If there are x grams of vapour per gram of dry air, the heating is L.x/C degrees, where L is the latent heat of evaporation (Section 4.1), and C the specific heat of dry air (Note 3.A).

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