Environmental Engineering Reference
In-Depth Information
SEASONAL VARIATION
Seasonal variations in insolation arise from the changing axial tilt of Earth relative to the
sun throughout the year (Figure 2.11) and the eccentricity of Earth's orbit. The orbit is an
ellipse, not a circle, so that Earth is slightly nearer the sun (147 million km) on 3 January
and at its farthest distance (152 million km) on 5 July. The variation in distance means
that the amount of energy received also varies. The variation in energy received is ±3·5
per cent, which does make a measurable difference in total insolation received in the two
hemispheres (Figure 2.12). Being nearer the sun means that the radiation input will be
slightly higher. Earth is closest to the sun (perihelion) in the northern hemisphere winter
and farthest away (aphelion) in the southern hemisphere winter at the present time.
Because of changes in the shape of Earth's orbit, to be discussed in Chapter 9, these
relationships are constantly changing.
As Earth orbits the sun with its axis of rotation pointing in a constant direction, the
area that is illuminated by the sun and the angle between the sun's rays and the top of the
atmosphere will change. At the June solstice the sun is above the horizon throughout the
twenty-four hours for all latitudes north of the Arctic Circle, while south of the Antarctic
Circle the sun would not be visible. Between the autumn equinox (22 September) and the
winter solstice (22 December) the latitude at which the midday sun is overhead gradually
moves southward from the equator to the Tropic of Capricorn (23·5° S). By 22 December
insolation will be at a maximum at that latitude and zero north of the Arctic Circle.
Between 22 December and 21 March the sequence is reversed, and in the period leading
up to the summer solstice the latitude of the overhead sun moves northward from the
equator to the Tropic of Cancer, insolation increases in the northern hemisphere, and the
South Pole is thrown progressively into shadow (Figure 2.13).
If you stand with your back to the North Pole in the northern hemisphere, the altitude
of the sun is the angle between the horizon and the sun at noon. Navigators used to use a
sextant to measure this angle. Altitude can be calculated with the following formula:
Declination is the latitude at which the sun's rays are vertical at noon.You add
declination if you are in the same hemisphere as the sun, subtract if the sun is in the
opposite hemisphere. For example, the altitude of the sun at solar noon in Hong Kong
(latitude 22° N) would be:
22 December
Altitude = 90° −22° −23·5°
Altitude = 44·5°
