Daily Changes

Screen temperatures do not rise and fall over equal periods of the day; the cooling period lasts longer (Figure 3.11). This can be explained as follows. Heating of the ground occurs when the net radiation is positive downward, which is the situation between dawn and mid-afternoon (Figure 3.12). Solar radiation declines after noon (Figure 2.4), but the gradual rise of ground temperature leads to a continued increase in the loss of terrestrial radiation (Note 2.C). As a result, the net radiation on a sunny day changes from positive to negative about two hours after solar noon—slightly earlier in winter than in summer—and the surface temperature is then at a maximum. This timing is little affected by cloudiness, which reduces both the shortwave radiation gain and the compensating longwave loss. The moment of daily maximum temperature is followed by a cooling till dawn next day (Note 3.H), i.e. there

Figure 3-11 Typical daily variations of temperature at Marsfield, Sydney in various seasons. The ranges are higher in winter because the air is drier and the sky less cloudy. (The Eastern Standard Time (EST) is close to the local solar time, i.e the Sun is highest at noon.)

-1-1-1-1-r net shortwave net shortwave

midnight sunrise noon sunset

Figure 312 Typical daily variations of the radiation input and loss, at the ground. The shortwave input is Rs (1-a), where Rs is the solar radiation, and a is the ground's albedo. The net longwave loss is the difference between the relatively constant sky radiation and the upwards terrestrial radiation. Part of the net radiation (i.e. the difference between net shortwave gain and net longwave loss) is used to heat the ground.

midnight sunrise noon sunset

Figure 312 Typical daily variations of the radiation input and loss, at the ground. The shortwave input is Rs (1-a), where Rs is the solar radiation, and a is the ground's albedo. The net longwave loss is the difference between the relatively constant sky radiation and the upwards terrestrial radiation. Part of the net radiation (i.e. the difference between net shortwave gain and net longwave loss) is used to heat the ground.

may be sixteen hours of cooling, for instance, and only eight hours of warming each day.

Daily Maximum

The average daily maximum screen temperature in any month can be identified by a diagram such as that in Figure 3.13. The maximum tends to be higher when either the Sun is high in clear skies, when the surface air is damp (which obstructs the loss of terrestrial radiation—Section 2.7), the ground is dry (which reduces both the conduction of heat into the ground and any evaporative cooling— Chapter 4), a warm air mass has settled over the area (Chapter 13), or there is little wind, since wind shares the ground's heat with the atmosphere. Also, warm places tend to be well inland (away from cooling sea-breezes— Chapter 14) and at a low elevation.

Measurements over oceans, and over land recently wetted by extensive rain, show that daily maximum temperatures there do not exceed 33°C. This is due to cooling by evaporation, which increases as temperatures rise (Chapters 4 and 5).

The time of the daily maximum screen temperature typically lags behind the maximum surface temperature by 10-30 minutes on a calm day. The time of maximum on a windy day depends on the direction and warmth of the wind. The time is typically closer to noon near the coast, because of the frequent onset of a sea breeze in the afternoon (Chapter 14).

However, changes in the weather may drastically alter the diurnal temperature cycle. For instance, afternoon temperatures may not

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Thermo Isopleths
Figure 3.13 An isogram, or 'thermo-isopleth diagram', illustrating the times of day and months of the year when particular temperatures are normally to be expected. This one applies to Belem, at 1°S in Brazil.

exceed that at 6 a.m. if a vigorous cold front passes through at 7 a.m. (Chapter 13). Likewise, the daily maximum temperature at Darwin is often reached at 11 a.m. during the Wet, because clouds and thunderstorms occur in the afternoon.

Daily Minimum

Nocturnal cooling is promoted by the same factors, except that latitude is unimportant, and winter 'nights' at places above 67 degrees latitude are days or weeks in duration, so that there is lengthy cooling. A dry atmosphere permits an unobstructed loss of terrestrial radiation, so it accelerates nocturnal cooling. Cloud has a particularly great effect (Section 2.7).

The daily minimum temperature is reached around dawn. It varies across a region much more than the maximum does, since it is lower in hollows, where cold air settles. Table 3.5 shows that minima in Canberra in winter are 9.5 K lower than those 100 km away at the coast. Differences of a few degrees are measured within the city. The minimum is greatly raised by even light winds, which stir in warm air and prevent the cooling which takes place below a 'radiation inversion' (Chapter 7). So a sudden onset of wind during the night is usually accompanied by an abrupt warming.

Daily Range

The difference between the maximum and minimum defines the daily (or 'diurnal') range. It is governed by latitude, elevation, distance from the sea, season, cloud and wind.

The daily range of temperatures is comparatively small near the equator, because of considerable humidity and cloud (Chapters 6 and 8) but it is more than the annual range (Section 3.3) between the Tropics, i.e. at less than 23 degrees latitude. For instance, the daily range is 8.6 K in February and 14.5 K in August (in the dry season) at Cuiaba (at 16°S in central

Brazil), whilst the annual range is only 4.3 K. The daily range is greatest at places inland around latitudes of 30 degrees, where relatively cloudless skies allow considerable warming from the day's strong sunshine and a dry atmosphere permits appreciable cooling at night. At mid-latitudes, the range is reduced by cloud and a low Sun. At latitudes above 67 degrees (within the Polar Circle) the 'daily range' has little meaning when a 'day' or 'night' may last weeks.

A high elevation tends to increase the daily range, because of less air and moisture above, to attenuate both the daytime input and the nocturnal loss of radiation. Figure 3.14 shows how ranges at 14-17°S in Peru increase with elevation from about 6 K at sea level to more than 20 K. Likewise, the range is 5.3 K at the coastal town of Antofagasta, at about 23°S in Chile, but 22 K at the equally dry Calama, located 140 km inland, at an altitude of 2,260 m.

The daily fluctuations at the surface of the sea are usually less than 1 K, and less than 3 K at the surface of a lake. This thermal stability has a great effect on coastal climates, creating sea breezes (Chapter 14) and moderating changes of screen temperature locally (Table 3.2).

Figure 3.10 shows that the daily range is roughly independent of the distance inland,

Figure 314 Effect of elevation in Peru on the mean temperature and daily range during fifteen days.

except close to the coast, where sea breezes penetrate (Chapter 14). These may occasionally reach several hundred kilometres inland in arid flat regions bordered by a relatively cold ocean, such as south-west Australia, but arrive after sunset, too late to affect the daily maximum temperature. This late arrival applies also in Canberra, which is 100 km inland and elevated (Table 3.5). In Queensland, the daily range is reduced within 70 km of the sea in winter but 160 km in summer, when sea breezes are stronger.

The diurnal range in Australia tends to be greatest in late December (the summer solstice) and least in late June, i.e. winter. However, there is a lag in the centre of the country; the range at Alice Springs typically changes from a minimum of 14 K in February to a maximum of 18 K in September. Also, the daily range in the north is less in the wet season than in the dry, on account of more cloud. Measurements in Sydney show that the range is about 12 K when there is no cloud, but 3 K when the sky is totally overcast.

Wind reduces the range by stirring the air, moderating the extreme temperatures at the surface.

The range throughout the world appears to be slowly decreasing, chiefly on account of an increase of daily minimum temperatures, i.e. reduced nocturnal cooling. An example is given in Figure 3.15. The decrease is consistent with the increased atmospheric concentration of greenhouse gases (Section 2.7), but may also be affected by more nocturnal cloud (Chapter 8) or urbanisation around the weather stations (Section 3.7).

Daily Mean

Daily mean temperature should be calculated by adding hourly measurements and dividing by 24, but is normally taken as the average of the maximum and minimum values. This convenient approximation usually overestimates

Figure 3 15 Recent changes of the daily range of temperature in South Africa.

the average slightly, because the daytime maximum is more peaked than the nocturnal minimum. The mean is expressed occasionally in terms of the difference from some reference temperature, selected as important in agriculture (Note 3.I) or in assessing human comfort (Note 3.J).

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