Scattering

The air's molecules and aerosols scatter the Sun's rays, which increases their paths within the atmosphere and hence their absorption. The scattering of radiation by gas molecules and the smallest aerosols is called Rayleigh scattering. This occurs with particles which are between a hundredth and a thousandth the size of the wavelength and scatter incoming radiation according to the inverse of the radiation's wavelength. In other words, blue light (with a relatively short wavelength) is scattered ten times more than red light. That explains the blue sky above (Figure 2.10). But on the Moon, where there is no atmosphere, the consequent absence of Rayleigh scattering makes the sky always black, even in sunshine. Back on Earth, blue light in solar radiation is notably scattered away by air pollution or fine dust, which makes the remaining direct sunlight seem red, especially when the Sun is low and the ray's path through the atmosphere is long (Chapter 15). Beautiful red sunsets occur in the desert after a hot day allows dried dust to rise in the air. Bushfires and other air pollution also create red sunsets. Large volcanic eruptions produce a veil of sulphuric-acid aerosols in the stratosphere and these cause red sunsets covering much of the attenuation over this distance

Figure 2.9 The twofold effect of latitude in reducing the amount of solar radiation reaching the ground. Firstly, the distance travelled through the atmosphere is greater at high latitudes because of the oblique angle of the rays; secondly, fewer rays impact on unit area of the ground.

attenuation over this distance

Figure 2.9 The twofold effect of latitude in reducing the amount of solar radiation reaching the ground. Firstly, the distance travelled through the atmosphere is greater at high latitudes because of the oblique angle of the rays; secondly, fewer rays impact on unit area of the ground.

Figure 2.10 Effect of Rayleigh scattering of the Sun's radiation by atmospheric molecules and aerosols. Preferential scattering of blue light makes the sky seen by the person on the left appear blue, whereas the Sun seen by the person on the right appears red.

Earth

Figure 2.10 Effect of Rayleigh scattering of the Sun's radiation by atmospheric molecules and aerosols. Preferential scattering of blue light makes the sky seen by the person on the left appear blue, whereas the Sun seen by the person on the right appears red.

sky (Note 2.G). Sunsets are never so red when viewed from an aircraft flying above the troposphere, which contains most of the air and aerosols.

Larger particles and droplets, of a size like the light's wavelength, cause Mie scattering, where light of all wavelengths is equally deflected, giving the sky a hazy white appear ance. This kind of scattering is in all directions, but primarily forward and backward. Also, it is much more intense than Rayleigh scattering, which explains the bright whiteness of clouds.

The main effects of clouds on solar radiation are twofold—to increase reflection to space and to reduce the transmission of light to the ground. On average, only about a quarter of extraterrestrial radiation reaches the ground when the sky is overcast, instead of about 75 per cent with a clear sky. Other properties of clouds are discussed in Chapter 8.

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