Screen Temperatures

Air scatters visible light, but absorbs very little, so that it is not appreciably heated by the Sun directly. Instead, solar radiation heats the ground surface and then the ground heats the nearby air. Likewise, surface air is cooled by the ground at night. In both cases, screen temperatures follow the ground-surface temperatures. However, the lag is negligible near the times of the daily extremes at dawn and early afternoon, when temperatures change only slowly (Section 3.4).

Heating and cooling of the air by the ground lead to screen temperatures varying with elevation, time and space. To consider spatial variation across the globe, it is convenient to link places which have equal surface temperatures by lines on a map called isotherms. The positions of isotherms depend on several factors—(i) latitude, (ii) elevation, (iii) the advection of heat in the wind, (iv) advection in ocean currents, (v) distance from the sea, (vi) orientation of the ground to the Sun, (vii) cloudiness, (viii) urban warmth and (ix) wetness of the ground. Some of these are discussed in what follows, and the rest in other chapters.

Latitude

The latitudinal effect on temperature is dominant and is chiefly a response to the amount of net radiation received (Figure 2.19, Note 3.B). Table 3.1 shows how monthly mean temperatures decrease towards the south, especially in summer; the table indicates a fall by about 1 K for each extra degree of latitude in New South Wales (NSW). Temperatures fall by around 0.7 K for each degree of latitude in Australia as a whole, so that Sydney at 34°S averages about 5 K warmer than Hobart at 42°S. Globally, there is little change between the equator and the Tropics, and a most rapid variation (of about 1.5 K/degree) between 4555° of latitude. There is sea-ice at latitudes beyond 55°S, which forms at -2°C and cuts off the surface air from the relative warmth of the ocean, so that there is then a dramatic lowering of air temperatures.

The frequency of hot days depends somewhat on the latitude at the coast, but much more on the distance inland (Table 3.2).

Distance from the Sea

As one travels inland in Australia the average temperature tends to rise (Table 3.1 and Table 3.2), because summers at the coast are cooled by sea breezes (Chapter 14). This is quite different from the pattern in the huge land mass of Eurasia, where July mean temperatures are about 14°C at both Narvik (at 68°N on Norway's coast) and also at Verkoyansk at the same latitude, far inland in Siberia, whereas the respective January means are -5°C and -50°C,

Table 3.1 Effects of the season, latitude, elevation and distance inland (km), on temperatures at places in Australia between 29-37°S.

Factor

Latitude: (°S)

Elevation (m)

Distance inland (km)

Effect

29-33

34-37

25-400

600-1800

25-200

300-1100

Mean

Jan.

24.4

21.5

24.4

18.2

21.4

25.8

July

9.9

8.0

10.2

4.9

8.7

9.6

Daily range

Jan.

14.1

13.8

13.9

13.9

12.6

15.7

July

12.5

10.9

11.9

10.7

11.8

12.1

Annual range

14.5

13.5

14.0

13.2

12.6

16.2

Table 3.2 Effect of latitude and distance from the sea on the temperatures exceeded for 7.5 hours in January and July, respectively.

Place

Latitude

Distance inland (km)

January temp, exceeded ( C)

July temp, exceeded (°C)

Alice Springs

24°S

900

40.7

25.1

Giles

26°S

700

41.0

25.0

Broken Hill

32°S

400

39.4

18.6

Canberra

36°S

100

33.8

13.6

Darwin

12°S

coast

33.4

31.8

Townsville

19°S

coast

33.9

26.2

Port Hedland

21 °S

coast

41.2

30.0

Perth

32°S

coast

39.0

20.0

Adelaide

34°S

coast

37.0

16.8

Sydney

34°S

coast

32.4

19.8

Melbourne

38°S

coast

35.8

16.1

so that the inland place is about 22 K colder than the coast over the whole year.

Coastal temperatures are influenced by that of the nearby ocean surface, which in turn is affected by the advection of heat in ocean currents (Chapter 11), either from lower latitudes (bringing warmth) or from higher latitudes, cooling the shore. The directions of the main ocean currents (Chapter 11) tend to make the western coast of a continent cooler than the eastern, as seen in Figure 3.4. For instance, the cold current off Lima in Peru limits monthly mean daily maximum temperatures to 28°C, even though it is at 12°S, the same as Darwin, where the equivalent is 34°C. The Gulf Stream from the Gulf of Mexico towards Europe raises the mean temperature of Dublin (at 54°N) about 4 K above that of Tierra del Fuego (at 54°S), at the tip of South America.

As regards Figure 3.4, the obvious similarity between the pattern of temperature and that of incident solar radiation (Figure 2.11) shows the close connection between sunshine and warmth.

Elevation

Average temperatures from many places show that they tend to decrease by about 4.2 K per kilometre extra elevation (Figure 3.5). (This average rate for surface temperatures is less than the rate of about 6.5 K/km measured during ascent in free air, see Note 1.L.) An analysis of data from Australia alone, and therefore over a smaller range of heights, shows a variation between 4.5 K/km in January and 6.5 K/km in November. The coolness at higher elevations is the result of the expansion of ascending air (Section 1.6), which spreads the air's warmth over a larger space, reducing the amount of sensible heat in unit volume, i.e. lowering the temperature. A similar cooling is observed in air escaping from a car tyre, and a corresponding warming results from the compression within a bicycle pump.

The combined effect of latitude and elevation is seen in Figure 3.6, which shows the variation with latitude of the summertime snowline; snow persists throughout the year above this line. Consequently, there is snow on the top of Mt Kilimanjaro (5,895 m), even at 3°S, and a glacier at about 5,000 m on Mt Carstenz at 4°S in West Irian Jaya (Plate 3.1). The Quelccaya ice cap is at 5,670 m at 14°S in Peru. Elsewhere in South America, the snowline is at 5,180 m at 17°S in Bolivia, 4,500 m at 33°S in Chile and 1,140 m at Tierra del Fuego (54°S). It is highest around the Tropic, where solar radiation is at its greatest (Figure 2.11).

Wind Direction

Advection of heat in winds may affect surface temperature, especially when the winds flow directly from high latitudes, for instance. Advection is insignificant near the equator where all winds have much the same temperature, but is an important cause of day-to-day changes of temperature at midlatitudes.

Extremely High Temperatures

The highest shade temperature yet recorded is 58°C at Al'Aziziyah in Libya, in 1922. In the southern hemisphere, 48.9°C has been recorded at Rivadavia in Argentina and 53.1°C at Cloncurry in Queensland. The records in Melbourne and Sydney, are 44.7°C and 45.3°C, respectively, and at Adelaide 47°C. A particularly hot place is Marble Bar (Western Australia), with a long-term mean daily maximum of 35.3°C and temperatures above 37.8°C (100°F) on each of 162 consecutive days during a heatwave in 1923-24. (Sometimes a 'heat wave' is arbitrarily defined as a series of consecutive days with maximum temperatures above 32.2°C, i.e. 90°F.) Marble Bar is hot because it lies in the north-west of Australia, i.e. towards the equator, and on the downwind edge

Figure 3 4 Comparison of the global patterns of isotherms in January and July, respectively.

July

Figure 3 4 Comparison of the global patterns of isotherms in January and July, respectively.

Figure 3.5 The effect of the elevation of a place on the difference there between the latitudinal-mean long-term-average sea-level temperature and the observed mean temperature. The data come chiefly from places in South America and Africa (south of the equator) and North America. The slope of the line is 4.2 K per kilometre.

Figure 3.5 The effect of the elevation of a place on the difference there between the latitudinal-mean long-term-average sea-level temperature and the observed mean temperature. The data come chiefly from places in South America and Africa (south of the equator) and North America. The slope of the line is 4.2 K per kilometre.

Figure 36 Effects of latitude on the elevation of the snowline, shown by black dots, and on the average height of the treeline, shown by the letter T, above which all monthly mean temperatures are too low for trees to thrive, i.e. below 12°C.

Plate 31 The glaciers on Mt Carstenz (5,029 m), above the snowline, near the equator (i.e. at 4°S). The two photographs were taken respectively in 1936 (a) and in 1991 (b), showing the same scene but from opposite ends of the valley. The area of snow decreased appreciably over the half-century in accordance with the global warming discussed in Chapter 15. In other words, this plate suggests that global warming has not been occuring only at high latitudes, as sometimes reckoned.

of the continent (with respect to the prevailing easterly winds), at the greatest distance from the relatively cool Pacific Ocean. Proximity of the sea everywhere in New Zealand limits the record maximum there to 35.8°C (at Oxford in the South Island). The record at Auckland (37°S) is only 32.5°C, since it is surounded by water. New Zealand's highest temperatures occur on the south-east side of the South Island; this happens when north-westerly winds subside from the mountains (Chapter 7).

The hottest places in the world are joined by a curving line called the thermal equator. It fluctuates during the year from near the geographical equator in January (with excursions of 20 degrees latitude south within the South American, African and Australian continents) to about 10°N over the oceans in July, but 35°N in North America, 20°N in Africa and 30°N across

Asia. The average is about 5°N, so the Earth's surface temperatures are asymmetric about the equator, because of the different amounts of land in the two hemispheres. The asymmetry is like that of solar radiation (Section 2.2).

Highest screen temperatures in the southern hemisphere tend to occur sometime after December 22 (Section 2.2). The lag is only two or three weeks inland, but a month or more near the sea. It is greatest for a small island, again because of the sea's slowness to change temperature (Section 3.3). Of course, variations of cloud and weather complicate matters, and so the hottest month is November in the north of Australia, but later in the south.

The pattern of the energy needed for air conditioning in Australia (Figure 3.7) reflects the latitude, the distance inland and the distance downwind of the east coast, where the dominant winds impinge in summer (Chapter 12).

One important effect of a heatwave is its impact on human mortality (Note 3.C and Table 3.3), even though the effect is partly offset by adaptation and acclimatisation (Note 3.D). As a result, increased urban heating (Section 3.7), air pollution and global warming (Chapter 15) will worsen conditions for a population which is ageing and becoming more urban.

Low Temperatures

The counterpart of a heatwave is a cold spell, a period of unusually low temperatures. The lowest daily minimum ever observed was -88.3°C at Vostok in Antarctica, due to the high latitude (78°S) and elevation (3,420 m), the relative absence of surface winds from warmer climates, remoteness from the sea's influence (being about 1,400 km inland), and the permanent snow cover,

Figure 3.7 Electrical energy needs for domestic space cooling in Australia, in units of gigajoules per person annually, given current building standards and air-conditioning equipment.
Table 3.3 Effect of daily temperatures on mortality from various causes during the 1966 heatwave in the New York metropolitan area

Date

Daily temperatures (°C) Max.. Mean Min.

Stroke

Causes of death * Heart attack

Other

June 30

34

29

23

89%

83%

97%

July 1

31

27

23

109%

94%

134%

2

38

30

22

99%

91%

114%

3

39

31

24

214%

162%

147%

4

37

31

25

302%

258%

390%

5

31

27

22

219%

178%

173%

6

33

27

22

156%

129%

107%

7

34

29

23

172%

120%

108%

8

33

26

20

115%

117%

66%

9

33

26

19

94%

106%

103%

Total heat-related deaths!

109

372

548

* The numbers shown are the mortality rates expressed as a percentage of the normal mortality rate at that time of the year, due to the causes listed.

t The number of heat-related deaths was estimated by subtracting the normal death rate from the death rate during the ten-day heat wave.

* The numbers shown are the mortality rates expressed as a percentage of the normal mortality rate at that time of the year, due to the causes listed.

t The number of heat-related deaths was estimated by subtracting the normal death rate from the death rate during the ten-day heat wave.

with its high albedo reflecting solar energy away (Table 2.3). Such temperatures cannot be measured with a thermometer containing mercury, which freezes at -39°C.

Australia is warm by comparison; the record low is -23°C, measured on 29 June 1994 at Charlotte Pass, at 1,759 m near Mt Kosciusko (NSW). The effect of such low temperatures on people is aggravated by wind, and in cold climates one refers to the windchill temperature, which combines the effects of temperature and wind (Note 3.E).

Long-term Variation

Annual mean temperatures change from one year to the next. In Sydney, for instance, the difference between the annual means of consecutive years has been as much as 1.2 K. The scatter of annual mean values may be expressed in terms of the 'standard deviation' (Chapter 10); it is 0.5 K at Sydney. That is comparable with the apparent climate warming this century (Chapter 15), making it still hard to discern long-term trends with certainty.

Several factors which affect the year-to-year variations are discussed elsewhere. These include the temporary cooling due to the veil of dust from a large volcanic eruption (Note 2.G), changes of ocean temperatures (Chapter 11) and fluctuations of wind patterns (Chapters 12-14). The variations are partly responsible for differences of crop yields. Plant growth tends to vary as shown in Figure 3.8 for optimal soil, radiation and water-supply conditions, often increasing with temperature up to 30°C or so (Note 3.F). At higher temperatures net growth is reduced by increased respiration, the opposite of photosynthesis (Note 1.B).

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