Figure 6.2 shows how the amount of solar radiation received annually varies over the Earth's surface. Intuitively, you might expect that the contours would be parallel to lines of latitude. This is clearly not the case.
Figure 6.2 The amount of solar radiation received at the surface of the Earth, in W nr2, averaged over the course of a year. (Contours over high ground have been omitted.)
In general, at a given latitude. Lire contour values greater over the oceans or over the continents 1 Why might this be '
Insolation - the amount of incoming solar radiation reaching the Earth's surface - is generally greater for continental areas than for the oceans. As discussed in Chapter 2. the atmosphere over the oceans contains a large amount of water, particularly in low latitudes. Along with gases such as C02 and SOi. water vapour and clouds absorb on average about 20% of incoming solar radiation (cf. Figure 6.3). Clouds especially have a marked effect as they not only absorb incoming radiation but also reflect it back to space. Over land, the atmosphere tends to be less cloudy and drier, particularly in tropical and subtropical areas.
incoming solar radiation I tOO unitsj
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