Variations Of Rainfall

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Average figures for rainfall fail to indicate the great differences from one period to the next, which occur in many places. The variability is important; a farmer would much prefer a reliable though modest rainfall to an irregular sequence of drought and flood with the same average, and most of the fluctuation of crop yields is due to rainfall variability.

Measures of Variability

Different ways of showing the considerable variability observed in practice are illustrated in Figure 10.8. In Sydney, for instance, the annual rainfall can be as low as 700 mm/a, or over 2,000 mm/a, so that the average of about 1,200 mm/a is only vaguely representative.

The variability of a series of values is often considered in terms of the deciles. These are values obtained after the set has been rearranged in order from smallest to largest. The first decile is the value lying 10 per cent of the way along this rearranged series, i.e. 10 per cent of the values in the set are smaller than the first decile. The fifth decile is the median value. The presentation of data from Santiago in Figure 10.8c gives the decile values directly.

The farmer is most interested in the 'dependable rainfall', often regarded as the amount exceeded in three years out of four (Note 10.K). This is called the '25th per centile', halfway between the second and third decile. Figure 10.8 shows that it is about 230 mm/a at Santiago, reading from 25 on the vertical axis.

Several ways of quantifying the variability of rainfall are discussed in Note 10.L. A good way is to divide the average departure from the mean by the mean itself, i.e. the relative variability. In general, a map of relative variability (Figure 10.9) is the inverse of a map of annual rainfall (Figure 10.3), being higher in drier regions. It exceeds 80 per cent on the west coast of South Africa, where the mean rainfall is low. Values in Australia are typically in the range 10-40 per cent, being highest in the deserts and least in the southwestern corner and Tasmania. Relative variability in South America is lowest in the Amazon basin and highest in the Atacama

Figure 10.8 Examples of the scatter of annual rainfalls, shown in various ways: (a) a 'time series' of falls at Sydney during the period 1836—1985; (b) a 'histogram' of those at Johannesburg during the period 1891—1990, and (c) a 'cumulative probability chart' of rainfalls at Santiago during the period 1867-1993.

Figure 10.8 Examples of the scatter of annual rainfalls, shown in various ways: (a) a 'time series' of falls at Sydney during the period 1836—1985; (b) a 'histogram' of those at Johannesburg during the period 1891—1990, and (c) a 'cumulative probability chart' of rainfalls at Santiago during the period 1867-1993.

Figure 10.9 The relative variability of the annual rainfall in the southern hemisphere.

desert. It can be seen that variability adds to the problem of water shortage for farmers in dry areas.

Particularly high relative variabilities at the latitudes of eastern Australia and South Africa (Chapter 16) appear to be related to the 'El Niño' phenomenon discussed in Note 10.M and the next two chapters.

Aspects of Variability

The variation noted in Darwin indicates a tendency towards alternating periods of almost equally high or low annual rainfalls, as though switching to and from two distinct regimes every few years (Figure 10.10). The same has been observed in records from Sydney. Annual rainfalls during a relatively dry time are almost uniformly below normal, so that the 'cumulative sum' curve declines as a roughly straight line, in a fashion that is remarkably consistent. And conversely during the wetter times. The alternation of dry and wet regimes suggests cycles of events created by feedback processes, and the constant conditions between switchings is a feature we call 'persistence', a tendency for periods of the same kind of weather to cluster, for a dry time to be followed by another (Note 10.N). Persistence is the opposite of variability.

Day-to-day persistence occurs because it takes several days for a given weather pattern to change. In the case of Melbourne, Table 10.2 shows that the nature of the previous day markedly affects the chance of rain on a particular day; a dry day yesterday in summer, tends to mean the same today, and likewise for a wet day in July. The chance of a dry day today is not increased by two dry days beforehand, in this example, so the so-called 'memory' of the process involved here is only one day. The degree of persistence depends strongly on the season in Pretoria (Figure 10.11), as elsewhere. In this case, there is a very strong persistence of dry days during JuneAugust.

A different kind of persistence is implied in the steadiness of abnormal rainfall at Darwin between 1965 and 1982 (Figure 10.10). An abnormality lasting so long suggests an explanation in terms of ocean circulations. These change only sluggishly because of the huge masses involved (Chapter 11), whereas the 'memory' of atmospheric processes is only a few weeks at most.

Rhythms of Rainfall

There have been many suggestions of rhythms in the amount of rainfall, perhaps linked to the

Figure 10.10 The variation of the cumulative sum of annual total rainfalls at Darwin. The 'cumulative sum' in any year is the sum of the departures of the annual rainfalls so far from the long-term mean. (So the first and last values of the sum are automatically zero.)
Figure 10.11 The variation from month to month of the chance of a dry day being followed by a wet day in Pretoria, South Africa.

Table 10.2 Probabilities of a dry day at Melbourne after either a wet day or one or two dry days in various months frequency of sunspots (Sections 2.2 and 10.7, Chapter 15) or phase of the moon. For instance, Francis Bacon suggested in 1625 that annual weather varies in a cycle of thirty-five years, named in 1887 after Eduard Bruckner, though no such variation appears significant. The only reliable rhythms are daily, seasonal and perhaps biennial, discussed below.

Daily Variations

Rainfall is more likely at certain times of the day in some regions (Sections 7.4 and 9.5). Convective rainfall often has a pronounced diurnal rhythm, especially on tropical islands. On land, it is most common in the afternoon after the surface has been heated by the Sun. Over the oceans, the diurnal cycle is much weaker, except over waters surrounded by land, as in the Indonesian Archipelago where convective rainfall peaks around dawn because of uplift due to the convergence of land breezes from the surrounding islands.

Stratiform rainfall occurs at any time of the day, although light rain or drizzle is slightly more common around dawn along the coast, when an onshore moist airflow is lifted slightly over the stable nocturnal boundary layer that formed at night over land (Section 7.6). Also, there may be less drizzle in the afternoon, when the surface relative humidity is low (Figure 6.4), because some of the small raindrops (Table 9.1) evaporate before reaching the ground.

Seasonal Variations

Seasonal variations reflect changes of solar radiation and wind direction (Chapter 12). Figure 10.3 indicated the variation in the southern hemisphere, and Figure 10.12 shows the great difference between the summer and winter patterns of rainfall in Australia, i.e. most rain at year's end in the north, and most in winter at higher latitudes. Places on the north coast, like Darwin, receive wet equatorial winds (Chapter 12) at year-end, during the period called 'The Wet', and dry winds from the interior in midyear. In contrast, wet winds from the Indian Ocean prevail in winter at Perth in the west of Australia, bringing rain, whilst summer winds are mostly easterly, coming from inland (Chapters 12 and 16) and therefore dry. Such a pattern of a dry summer and wet winter is called 'Mediterranean'. The pattern is not so evident on the east coast (Table 10.3), where the same prevailing easterly winds are onshore in summer, i.e. moist.

More generally, a slanting line across Australia, from 25°S on the west coast to 35°S on the east (Chapter 16), separates regions with a wet summer to the north from those with wet winters to the south, i.e. regions of chiefly convective and chiefly frontal rainfall, respectively. The same happens in southern Africa from 28°S on the west coast to 33°S on the south coast, and in South America, from 10°S on the Peruvian coast to 40°S in Argentina (Chapter 16). The slant is due to the difference between sea-surface temperatures to the west and east of a continent (Chapter 11).

The wettest month near the equator tends to occur a few weeks after the noonday Sun has passed overhead, i.e. once the ground has been most heated to promote convective rainfall. So there may be two rainy seasons if the Sun passes overhead twice in the year

Figure 10.12 Patterns in Australia of December—February and June—August rainfalls, respectively.

Table 10.3 The variation of the monthly median rainfall along the east coast of Australia

Monthly median rainfall (mm)

Table 10.3 The variation of the monthly median rainfall along the east coast of Australia

Monthly median rainfall (mm)

Place

Latitude (°S)

Jan

Feb

Mar

Apl

May

Jun

July

Aug

Sept

Oct

Nov

Dec

Thursday Island

11

399

393

321

168

20

11

8

4

2

1

12

163

Cooktown

15

290

334

340

169

55

40

22

18

11

15

33

14

Town svi lie

19

249

245

169

42

15

14

5

3

5

11

21

82

Rockhampton

23

121

109

83

39

28

32

21

13

17

34

58

92

Brisbane

27

127

123

115

58

43

41

36

28

41

61

75

107

Port Macquarie

31

115

150

154

135

112

98

80

63

65

74

79

100

Nowra

35

71

57

73

77

50

53

71

36

41

50

42

67

Flinders Island

40

38

47

40

62

79

56

80

73

59

49

54

48

Cape Bruny

43

54

49

60

74

80

79

91

77

70

76

72

67

(Section 2.2). For example, the Sun passes over in March and September at Quito (0.2°S, in Ecuador) and the wettest months there are April and October. The two wet periods merge into a single period at the Tropic of Capricorn, and even near the equator the tendency towards a double maximum is often overridden by other processes.

Wet summers in the north of Australia mean that bushfires there occur naturally in winter (mid-year), whereas they are summertime hazards in the south. Also, variations of rainfall determine the growing season for crops in areas where temperatures are adequate (Note 3.1). There, growth occurs while the soil is at least half full of available moisture (Note 4.G).

There is relatively little seasonal variation of the frequency of precipitation over the southern oceans, but a steady increase with latitude. There is a sharp maximum of convective rainfall around 38 degrees in winter.

Biennial Variation

One feature of rainfall variability is the evidence from many places, including south-east Australia, that years tend to be alternately wet and dry, especially in the tropics. For instance, rainfalls in Victoria during the period 1913-76 tended to be about 10 per cent less than average each 2.1 years or so. More than two-thirds of Australia had subnormal rain in 1957, 1959, 1961, 1964, 1965, 1970 and 1972, for instance. Similar slight 2-3-year rhythms have been found at Fortaleza in Brazil, in New Zealand and South Africa, and in the date by which a quarter of Adelaide's annual rain has fallen. An almost biennial rhythm occurs in the flooding of the Nile, monsoonal rains in India, rainfall in New Zealand, snowfall in Australia (Section 10.8), and in sugar-cane harvests in Queensland, for instance. These may reflect the 'Quasi-Biennial Oscillation' of winds in the equatorial stratosphere (Chapter 12).

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  • daniel
    What causes rainfall variation?
    4 years ago
  • jarno
    What causes variations in rainfall?
    1 year ago
  • elvia
    What ccauses variaiton in precipitation?
    10 months ago
  • meeri lenkkeri
    What causes variations in precipitation?
    7 months ago
  • Dora
    What causes daily variations in the weather?
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