Solar radiation enters the atmosphere. About 18 percent of the incoming shortwave radiation is absorbed by ozone near the top of the stratosphere and in the troposphere by clouds, water vapor, and aerosols. Absorption by oxygen (O2) and ozone (O3) produces the ozone layer by the reactions:
M is a molecule of any substance, but usually nitrogen. This sequence of reactions absorbs ultraviolet radiation and when governments acted to halt and reverse the depletion of the ozone layer it was because of fears that exposure to ultraviolet radiation is harmful to human health. The reactions also have a climatic effect. Incoming energy that is absorbed high in the stratosphere is energy that does not reach the surface. Effectively, the oxygen-ozone reactions reduce the amount of solar radiation. If the ozone layer is depleted so that it is less effective as an ultraviolet filter, more
Incoming radiation and the ozone layer
energy will penetrate the troposphere, where it has a climatic warming Effect of scattering effect. The effect is quite small, but it exists.
Clouds, the air, aerosols, and the surface reflect about 35 percent of the incoming radiation back into space. Although the Earth has an overall albedo of about 0.31, a large part of its brightness is due to clouds. Pictures of Earth taken from space show that the clouds and polar ice sheets are brilliantly white, but that the remainder of the surface is fairly dark.
Sunlight is also scattered by air molecules and aerosols, some of it back into space, so that much of the sunlight we receive arrives in a very diffuse form. Scattering sends light in all directions. It allows light to penetrate inside a room, for example. On the Moon, which has no atmosphere, all of the sunlight is direct. Shadows are always very sharp and if there were buildings with windows, the lights would have to remain switched on even in the middle of the day because, as the drawing shows, light would not reach into every part of the room. Away from area illuminated by the direct sunlight the room would be in total darkness. It is also this scattering of light by the Earth's atmosphere that gives a clear sky its blue color, produces red dawns and sunsets, and that makes a dust-laden
As sunlight passes through the atmosphere, it strikes air molecules. These collisions scatter the radiation, but the way it is scattered depends on the size of the particles compared with the wavelength of the radiation. Lord Rayleigh (1842-1919), the English physicist (born John William Strutt), discovered that shortwave radiation is scattered most efficiently if the particles are much smaller than the wavelength of the light. This is called Rayleigh scattering. Air molecules average 0.0004 ,um across and the wavelength of light is most intense at about 0.5 ,um.
The shortest wavelengths are scattered first. Violet and indigo wavelengths are scattered and absorbed high in the atmosphere, so they do not contribute to the color of the sky Blue is scattered next. It is light at close to the peak intensity, so there is more blue light in the solar spectrum than there is light of any other color. Rayleigh scattering sends blue light in all directions, but more is scattered forward than back or to the sides. Diffuse blue light reaches our eyes from all sky directions and gives the lower layers of the sky its blue color.
Larger particles also scatter light, but they scatter all wavelengths equally. This was discovered in 1908 by the German physicist Gustav Mie (1868-1957) and is called Mie scattering. Because they scatter all wavelengths, these particles make the sky appear white. The sky is white when the air carries a large amount of dust, and a blue sky is a deeper color after a shower of rain has washed dust to the surface. The sky is deeper blue in the middle of the day than it is in the early morning or late afternoon because, at these times of day, sunlight travels a greater distance through the air and more of it is affected by Mie scattering.
The sky often appears red or orange when the Sun is close to the horizon. This is because light must then travel through a much greater thickness of air than it does when the Sun is higher, and for a great deal of this distance the light is moving through the lowest part of the atmosphere, often close to ground level. It encounters water vapor, dust, salt crystals, and particles of many other types. These scatter all wavelengths equally, but air molecules continue to scatter blue light, so blue light is scattered more than the other colors and most of it is scattered out of the line of sight. This leaves the orange and red wavelengths predominant.
sky white (see the sidebar). The remaining incoming radiation is absorbed by the land and ocean surfaces.
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