Albedo

Figure 2.12 The relationship between average annual yields of rice in various countries and the rates of photosynthesis, calculated on the assump-tion of total use of the photosynthetically active radiation within the available solar radiation during the four summer months when the crop grows. For instance, the actual yield of each year's crop of Australian rice is about 6 tonnes per hectare, which is only 5 per cent of the 120 t/ha that is theoretically possible there.

Figure 2.12 The relationship between average annual yields of rice in various countries and the rates of photosynthesis, calculated on the assump-tion of total use of the photosynthetically active radiation within the available solar radiation during the four summer months when the crop grows. For instance, the actual yield of each year's crop of Australian rice is about 6 tonnes per hectare, which is only 5 per cent of the 120 t/ha that is theoretically possible there.

Some of the shortwave radiation reaching the ground is reflected upwards. The ratio of upwards to downwards fluxes is called the albedo (or shortwave reflectivity), denoted by the symbol a. Note that the albedo is a ratio, not a flux.

The higher the albedo of an object, the more light is reflected, making the object appear brighter. Also, a surface with a high albedo (like a white car) does not become as hot as one with a low albedo (like a black car), because more radiation is reflected away (Chapter 5). Aluminium foil facing the Sun may reach 46°C in air at 28°C, whereas black paper reaches 75°C, for instance.

The albedo is the average reflectivity for white light as a whole, but in fact an object's reflectivity usually varies with wavelength, as shown in Figure 2.13. This determines the object's colour, which depends on the wavelengths that are particularly reflected. Thus, a leaf appears green in white light because green wavelengths (of

wavelength: |im

Figure 2.13 The variation with wavelength of the reflected, absorbed and transmitted parts of radiation onto a leaf. It may be seen that most of the visible solar radiation is absorbed for photosynthesis and warming of the leaf. But most near-IR radiation is either transmitted or reflected.

wavelength: |im

Figure 2.13 The variation with wavelength of the reflected, absorbed and transmitted parts of radiation onto a leaf. It may be seen that most of the visible solar radiation is absorbed for photosynthesis and warming of the leaf. But most near-IR radiation is either transmitted or reflected.

about 0.55 pm) tend to be reflected, whilst others are absorbed or trans-mitted (Figure 2.13). But a leaf illuminated by a beam of red light appears black as red light is not reflected.

Satellite measurements of the radiation in two bands from surface vegetation provide a means of assessing its health. It is flourishing if there is more radiation reflected in the IR range 0.720.98 pm, but the ground is dry and the vegetation desiccated if there is more radiation in the visible range 0.578-0.70 pm.

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