Elevation

Elevation is associated with less atmospheric moisture because of the colder temperatures and greater distance from the sea. Figure 6.8 shows that an increase of elevation by 1 km around 30-40°S in South America has the same effect on dewpoint as an increase of latitude by about

July

Figure 6.7 Global distribution of monthly mean vapour pressure (hPa) near the surface in January and July.

Table 6.2 Monthly mean vapour pressure (hPa) measured at 9 a.m. at various places in Australia; all places are coastal, except Alice Springs (546 m elevation), Canberra (570 m) and kiandra (1,395m)

Place

Latitude (°S)

January

April

July

October

Darwin

12

31.1

27.0

17.6

27.7

Townsville

19

26.1

22.1

14.1

19.7

Alice Springs

24

11.9

10.1

6.5

6.8

Perth

32

14.8

13.4

10.9

11.7

Sydney

34

18.8

15.0

9.6

13.0

Adelaide

35

11.9

11.3

9.4

10.1

Canberra

35

13.1

10.3

6.6

9.7

Kiandra

36

11.1

7.6

4.7

7.3

Hobart

43

11.0

10.0

7.6

9.1

Figure 6.8 The variation of the annual mean dewpoint with latitude and elevation in South America. The crosses indicate surface stations.

South North

Figure 6.8 The variation of the annual mean dewpoint with latitude and elevation in South America. The crosses indicate surface stations.

Figure 6.9 Lines joining places with equal dewpoints (a) and winter (right).

Western Australia, and (b) eastern Australia in summer (left)

8 degrees, i.e. a reduction by about 5 K, though the variation is less nearer the equator.

Values from places at various elevations in Australia also illustrate the fall in vapour pressure. It is only 4.7 hPa (at 9 a.m. in July) at Kiandra (New South Wales) at 1,395 m, and 6.6 hPa on average at Canberra (at 570 m), compared with about 9 hPa in Melbourne and Sydney, beside the sea (Table 6.2). Likewise for the surface mixing-ratio at Pretoria in South Africa (Table 6.3). An analysis of data from places over the whole of Australia shows that the average rate of fall of dewpoint with elevation is between 3 K/km in July and almost 6 K/ km in May, which resemble the annual-mean value of 5 K/km from Figure 6.8 for similar latitudes in South America. Figure 6.8 also indicates that an increase of elevation by 1 km around 30-40°S lowers the average dewpoint as much as an increase of latitude by about 8 degrees.

Figure 6.9). This is shown more dramatically by changes of the dewpoint temperature or saturation deficit than by figures for the relative humidity (Table 6.4). Dewpoints in eastern Australia are below 2°C in winter along the Dividing Range in south-east Australia, but above 22°C in northern Queensland in summer (Figure 6.9). Mixing ratios in South Africa also demonstrate great changes seasonally, as well as the effects of latitude, elevation and remoteness from the sea (Figure 6.10). Comparison with Figure 6.9 shows a similar decline of humidity with latitude, the equivalent dewpoints in Figure 6.10 ranging from around 27°C to 4°C, like the 25°C to 2°C in Australia. In both continents, the decrease of moisture is less rapid from the east coast, where low-latitude easterly winds (Chapter 12) extend the sea's humidity inland. Particularly dry conditions obtain in winter, because of the air's coldness, especially inland (Chapter 3).

Season

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