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Source: Adapted from Goody and Yung, 1989.

Source: Adapted from Goody and Yung, 1989.

At the top of the atmosphere, the net energy output is determined by the incident shortwave radiation from the sun minus the reflected shortwave radiation. This difference determines the net shortwave radiation flux at the top of the atmosphere. To balance this inflow of shortwave energy, the earth-atmosphere system emits longwave radiation to space.

Satellite observations of the top of the atmosphere have made fairly accurate estimates of the global mean energy budget. According to these estimates, the global mean annual outgoing longwave radiation is 235 W m-2 and the annual mean absorbed shortwave flux is 238 W m-2. Hence, the measured top-of-atmosphere budget balances to within 3 W m-2. A part of this imbalance could be associated with the buildup of greenhouse gases and a part is probably associated with El NiƱo events (Kiehl and Trenberth, 1997).

Incoming Shortwave Radiation

Solar radiation that encounters matter, whether solid, liquid, or gas, is called incident radiation. Interactions with matter can change the following properties of incident radiation: intensity, direction, wavelength, polarization, and phase. Radiation intercepted by the earth is absorbed and used in energy-driven processes or is returned to space by scattering and reflection (Figure 2.1). In mathematical terms, this disposal of solar radiation is given by the equation

Qs = Cr + Ar + Ca + Aa + (Q + q)(1 - a) + (Q + q) a (2.8)

where Qs is the incident solar radiation at the top of the atmosphere; Cr is reflection and scattering back to space by clouds; Ar is reflection and scattering back by air, dust, and water vapors; Ca is absorption by clouds; Aa is absorption by air, dust, and water vapors; (Q + q) a is reflection by the earth; (Q + q)(1 - a) is absorption by the earth's surface, where Q and q are, respectively, direct beam and diffused solar radiation incident on the earth and a is albedo. The global disposal of shortwave radiation (W m-2 per year) is given in Table 2.4.

About a quarter of the solar radiation is reflected back to space by clouds. On average, the reflection is greatest in middle and high latitudes and least in the subtropics. A small portion of the incident radiation is scattered back to space by the constituents of the atmosphere, mainly air molecules, dust particles, and water vapors. About 30 percent of the radiation is scattered downward. Atmospheric scattering results from multiple interactions between light rays and the gases and particles of the atmosphere. The two

FIGURE 2.1. Daytime radiation balance over the earth's surface
TABLE 2.4. Disposal of solar radiation

Solar energy

W m-2

Incident on the top of the atmosphere

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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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