Seasonal Changes

There is an annual swing of temperature due to the tilt of the Earth's axis, either toward the Sun (summer) or away (winter), as shown in Figure 2.3. However, there is no single well-defined season of relatively high temperatures between

3000

3000

temperature: °C

Figure 3-8 The effect of annual mean temperature on the 'net primary productivity', which is the measured total amount of photosynthesised organic material less what is lost in respiration, assuming optimal conditions of soil moisture and nutrients, etc.

temperature: °C

Figure 3-8 The effect of annual mean temperature on the 'net primary productivity', which is the measured total amount of photosynthesised organic material less what is lost in respiration, assuming optimal conditions of soil moisture and nutrients, etc.

the Tropics, which includes the northern 40 per cent of Australia. At the equator, the midday sun is well above the horizon in every month, so the seasonal changes of temperature are less than 3 K. Consequently, seasons between the Tropics tend to be defined by rainfall rather than temperature. For instance, there is 'the Wet' in the 'Top End' of Australia (to use the local terminology) starting around year's end and lasting about three months (Chapter 10). At the opposite extreme, a polar 'night' of several weeks allows long uninterrupted cooling and hence a large annual range of temperatures in Antarctica (Table 3.4). Monthly mean temperatures at the

South Pole vary from -25°C in January to -60°C in July. A strong dependence on latitude is evident in South America (Figure 3.9), but the annual range is strikingly unaffected by the elevation of the Andes mountains.

Annual ranges in the south are less than in the northern hemisphere, because there is more ocean to cushion changes of temperature. The average range of monthly mean temperatures is about 30 K at 60°N compared with 13 K at 60°S, and 20 K at 40°N compared with 10 K at 40°S. The role of latitude is discussed further in Note 3.G.

Distance from the Sea

The annual range of temperature is increased by remoteness from the sea (Table 3.1, Note 3.G, Figure 3.10). Table 3.5 shows ranges to be 14.8 K inland but only 8.5 K at the coast. The effect is commonly called continentality, though that is merely a label not an explanation. The explanation is that water changes temperature only slowly, compared with a land surface, for the following reasons:

(a) the absorption and sharing of radiation heat over a substantial depth;

(b) evaporation cooling, which compensates for any increase of radiation input (Chapter 5);

Table 3.4 Effect of latitude on the annual range of temperatures; the mean temperatures are averages of values from the same latitudinal band and month

Latitude (°S)

January mean (°C)

July mean (°C)

Range (Kj

Equator

26.4

25.6

0.8

10

26.3

23.9

2.4

20

25.4

20.0

5.4

30

21.9

14.7

7.9

40

15.6

9.0

6.6

50

8.1

+3.4

4.7

60

+2.1

-9.1

11.2

70

-3.5

-23.0

19.5

80

-10.8

-39.5

28.7

90

-13.5

-47.8

34.3

Figure 39 The difference between January-mean and July-mean temperatures in South America. The lines labelled zero separate places where January is hotter than July from those in the north where the reverse is true.

(c) a comparatively large specific heat (Note 3.A);

(d) the dispersion of heat by oceanic advection between hot and cold regions (Chapter 11); and

(e) the mixing of surface heat into the bulk of the water through the stirring caused by waves.

As a consequence, the annual variation of seasurface temperatures is below 1 K at the equator and only about 5 K near 30°S in the Pacific and Indian Oceans, and around Australia.

The difference between the hottest and coldest months is 5 K at the tip of Cape York Peninsula in the north of Queensland, where the measuring site is almost surrounded by sea. By contrast, the annual range of monthly mean temperatures is as much as 18 K over the western interior of Australia, and even more on the much larger land masses of Eurasia and northern America. It exceeds 40 K in parts of monsoonal

Figure 310 The effect of distance inland at 27°S in Queensland on the annual and daily ranges of temperature.

Table 3.5 Effect of distance from the sea on the daily and annual temperature ranges; the comparison is between two places at 35°S on the east coast of Australia, i.e. Jervis Bay at 77 m and Canberra at 571 m

Table 3.5 Effect of distance from the sea on the daily and annual temperature ranges; the comparison is between two places at 35°S on the east coast of Australia, i.e. Jervis Bay at 77 m and Canberra at 571 m

regions (Chapter 12). Australia has Siberia, China and Canada.

Wind Direction

The yearly range is also governed by any variation of the prevailing wind direction, as in relatively small annual ranges, as the lack of any great north-south corridor in the lee of high mountain ranges like the Rockies or the Andes means that there is no obvious route for large invasions of polar air in winter. Such inflows occasionally lower temperatures in Dallas (east of the Rockies at 33°N) to below -20°C, compared with -2°C as the lowest temperature ever observed at Hobart at 42°S.

Temperatures in Adelaide yield a larger annual range than in other Australian cities (i.e. 11 K, between a monthly mean of 22°C in February and 11°C in August) because summer winds tend to come from the hot interior, whilst the prevailing wind in winter is from the polar south.

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