Radiation at the Earths surface

The radiation received at the Earth's surface consists of direct and scattered (plus reflected) short-wavelength radiation plus long-wavelength radiation from sky and clouds, originating as thermal emission or by reflection of thermal radiation from the ground.

Figure 2.12. Net radiation flux E0 at the top of the atmosphere (NCEP-NCAR, 1998). Direct and scattered radiation

Direct radiation is defined as radiation which has not experienced scattering in the atmosphere, so that it is directionally fixed, coming from the disc of the Sun.

Scattered radiation is, then, the radiation having experienced scattering processes in the atmosphere. In practice, it is often convenient to treat radiation which has experienced only forward scattering processes together with the unscattered radiation, and thus define direct and scattered radiation as radiation coming from or not coming from the direction of the Sun. The area of the disc around the Sun used to define direct radiation is often selected in a way depending on the application in mind (e.g. the solid angle of acceptance for an optical device constructed for direct radiation measurement). In any case the "direct radiation" defined in this way will contain scattered radiation with sufficiently small angles of deflection, due to the finite solid angle of the Sun's disc.

The individual scattering and absorption processes will be discussed further below, but in order to give a first impression of the modifications in the spectral distribution, due to the passage through the atmosphere, Fig. 2.13 shows the amount of radiation surviving at sea-level for a clear day with the Sun in zenith. A large number of absorption lines and absorption bands can be seen in the low-frequency part of the spectrum (corresponding to wavelengths above 0.7 x 10-6 m). They are due to H2O, CO2, O2, N2O, CH4 and other, minor constituents of the atmosphere. At higher frequencies the continuous absorption bands dominate, in particular those of O3. Around 0.5 x 10-6 m, partial absorption by O3 produces a dip in the spectrum, and below a wavelength of 0.3 x 10-6 m (the ultraviolet part of the spectrum) or a frequency above 9.8 x 1014 s-1, the absorption by ozone is practically complete.

Wavelength (I0"s m)

Frequency (I014 s"1 )

Figure 2.13. Frequency spectrum of solar radiation at the top of the atmosphere and at the surface (sea-level and minimum air mass, corresponding to the Sun in zenith on a clear day). Also shown are typical spectra of scattered radiation in the extremes of pure sky radiation (cloudless day) and pure cloud radiation (completely overcast). (Based on NASA, 1971; Gates, 1966.)

Frequency (I014 s"1 )

Figure 2.13. Frequency spectrum of solar radiation at the top of the atmosphere and at the surface (sea-level and minimum air mass, corresponding to the Sun in zenith on a clear day). Also shown are typical spectra of scattered radiation in the extremes of pure sky radiation (cloudless day) and pure cloud radiation (completely overcast). (Based on NASA, 1971; Gates, 1966.)

Figure 2.13 also shows the scattered part of the radiation on a clear day (dashed line). The scattered radiation is mainly composed of the most energetic (high frequency) radiation, corresponding to the blue part of the visible spectrum. The sky on a clear day is thus blue. The scattered light is also po larised. The radiation from clouds, i.e. from a completely overcast sky, is also shown in Fig. 2.13. It has a broad frequency distribution in the visible spectrum, so that the cloud light will appear white.

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