The amount of radiant energy received or emitted every second at a surface of one square metre is properly known as the radiance, though we lazily refer to it simply as the 'radiation'. The radiance on a surface facing the Sun, just outside the Earth's atmosphere, is called the solar constant, equal to 1,367 W/m2. This is more than the output of a kilowatt electric heater on

Figure 2.4 The solar elevation at various times of day and in various months, at Sydney (34°S). For instance, the Sun is about 40° above the horizon in September at 10 a.m.

Figure 2.5 Solar elevation and the amount of light entering a window on the equatorward side of a house. The upper diagrams show the Sun's path at 34°S, at mid-summer and at mid-winter, respectively. The Sun's noontime elevation is 78° 27' (i.e. 90-34+23° 27') at mid-summer, and 31° 33' at mid-winter (i.e. 90-34-23° 27'.

The consequences are shown in the lowest diagram. The angle a (the departure from vertical of a line from the window-sill to the eaves) needs to be more than [f-23°], where f is the latitude, to exclude radiation in summer, and 8 has to be less than f+23°], to maximise the intake of sunshine in winter.

Figure 2.5 Solar elevation and the amount of light entering a window on the equatorward side of a house. The upper diagrams show the Sun's path at 34°S, at mid-summer and at mid-winter, respectively. The Sun's noontime elevation is 78° 27' (i.e. 90-34+23° 27') at mid-summer, and 31° 33' at mid-winter (i.e. 90-34-23° 27'.

The consequences are shown in the lowest diagram. The angle a (the departure from vertical of a line from the window-sill to the eaves) needs to be more than [f-23°], where f is the latitude, to exclude radiation in summer, and 8 has to be less than f+23°], to maximise the intake of sunshine in winter.

each square metre of surface facing the Sun. The 'constant' was 30 per cent less when the Earth initially formed 4.5 billion years ago, assuming that the Sun developed like other stars of its size and composition. Also, there are variations due to the elliptical orbit of the Earth each year, to Milankovic changes of orbit over millenia, and (by less than 1 per cent) to sunspots, discussed below.

The energy onto a horizontal surface on top of the atmosphere, parallel to the ground, is called the extra-terrestrial radiance (Figure 2.6). It is necessarily less than the solar constant because it is the radiation onto a surface oblique to the Sun's rays. It depends on the solar constant and the orientation of the ground to the Sun (which are both known for any chosen time of the year), so it can be calculated and tabulated for various latitudes and months (Note 2.F).

The combination of the tilt and the elliptical orbit of the Earth leads to a minimum of seasonal variation of extra-terrestrial radiation at a latitude of 3.4°N, which may be called the radiation equator. It happens to be close to the average latitude of the 'thermal equator' (Chapter 3).

### Daylength

The daylength varies greatly with latitude and month, as shown in Table 2.1. Daylength is 24 hours in midsummer within the south Polar Circle (i.e. at latitudes above 66° 33'S), with continual sunshine over several days. Conversely, there are 24 hours of darkness each day in midwinter. At a midlatitude city like Dunedin (48°S), daylength is about 16 hours in midsummer, but less than 9 hours in midwinter.

Figure 2.7 Sunspots on the Sun. (Note that it is dangerous to try to observe them directly with the naked eye, doubly so with binoculars, and trebly with a telescope. It is safer to look through very dark glass at the Sun's reflection in a pool.)

Table 2.1 Effect of latitude and season on the fortnightly mean daylength

Lat Hours of daylight in the northern hemisphere

January February March April May June July August September October November December

Table 2.1 Effect of latitude and season on the fortnightly mean daylength

Lat Hours of daylight in the northern hemisphere

January February March April May June July August September October November December

 70° 0.5 2.2 4.5 7.0 9.8 12.7 15.5 18.0 20.3 22.4 24.0 24.0 24.0 22.4 20.2 17.8 15.1 12.2 9.5 6.9 4.5 2.3 0.3 0.0 60° 6.3 7.3 8.5 9.9 11.0 12.5 13.8 15.0 16.5 17.7 18.6 18.8 18.5 17.5 16.4 15.0 13.6 12.2 10.9 9.5 8.2 7.0 6.2 5.9 50° 8.3 8.9 9.7 10.6 11.3 12.4 13.3 14.2 15.0 15.8 16.2 16.4 16.2 15.6 14.9 14.1 13.2 12.2 11.3 10.3 9.4 8.7 8.3 8.1 40° 9.5 9.9 10.4 11.0 11.5 12.3 13.0 13.6 14.2 14.6 14.9 15.0 14.9 14.5 14.1 13.5 12.8 12.1 11.5 10.9 10.3 9.8 9.5 9.3 30° 10.3 10.6 10.9 11.4 11.7 12.2 12.7 13.1 13.5 13.8 14.0 14.1 14.0 13.8 13.4 13.0 12.6 12.1 11.7 11.3 10.8 10.5 10.3 10.2 20° 11.0 11.2 11.4 11.7 11.9 12.2 12.5 12.7 13.0 13.2 13.3 13.3 13.3 13.1 12.9 12.7 12.4 12.1 11.9 11.6 11.3 11.1 11.0 10.9 10° 11.6 11.7 11.8 11.9 12.0 12.1 12.3 12.4 12.5 12.6 12.7 12.7 12.7 12.6 12.5 12.4 12.3 12.1 12.0 11.8 11.7 11.6 11.6 1 1.5 0° 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 July August September October November December January February March April May June

Lat Hours of daylight in the southern hemisphere

Lat Hours of daylight in the southern hemisphere

In contrast, it fluctuates only between 11.7 to 12.8 hours at Darwin (12°S).