Atmospheric Attenuation. Annual Global Mean Energy Budget

As solar radiation passes through the atmosphere, some is absorbed, primarily by water vapor and clouds, and some is scattered, both upwards to space and downwards onto the surface, by clouds, air molecules, and particles suspended in the air. This downward scattered radiation, known as diffuse radiation, emanates from all directions of the sky. In contrast, direct beam radiation is not scattered and originates from the Sun’s position in the sky.

Mathematically, the solar radiation on a horizontal surface at the top of the atmosphere (S_H) is attenuated at the surface (S ↓) as:

where τ^т is atmospheric transmittance. Typical values for τ range from 0.6 to 0.7 for clear skies. The optical air mass (m) is:

where P is air pressure and P_s is pressure at sea level. Optical air mass increases, so that transmittance decreases, with greater zenith angle. The longer path length through the atmosphere causes less radiation to reach the ground. Altitude also affects solar radiation by reducing the path length that solar radiation travels through the atmosphere. With greater height in the atmosphere, there are fewer air molecules and air pressure decreases. Because there is less air mass at higher elevations, solar radiation is less likely to be scattered or absorbed as it passes through the atmosphere and more radiation reaches the ground. For a given zenith angle, optical air mass decreases with elevation (as pressure decreases). Consequently, atmospheric transmittance increases with higher elevation.

Diffuse radiation is most important when scattering is high. On overcast days, all the radiation is diffuse. The fraction of total radiation that is diffuse is also high when the low solar altitude angle leads to a longer path through the atmosphere, such as in mornings, evenings, winter, and at high latitudes. When the Sun is directly overhead, solar radiation travels through less of the atmosphere than when it is at an angle and hence less is scattered. Consequently, diffuse radiation may account for 25-50 percent of the total radiation when the Sun is low on the horizon, but only 10-20 percent when the Sun is high in the sky. The proportion of the total radiation that is diffuse increases, often by a factor of two, for cloudy, overcast, or polluted skies. Increasing amounts of aerosols in the atmosphere, by altering the proportion of direct and diffuse radiation, can affect plant photosynthesis.

Annual Global Mean Energy Budget. Although Earth as a whole balances annual solar and longwave radiation at the top of the atmosphere, the geographic distribution of net radiation is unequal (Figure 4.9). Latitudes near the tropics, between 30° S and 30° N, generally absorb more than 275 W m^-2 solar radiation annually; latitudes closer to the poles absorb less radiation. The major exception to this is North Africa, where the bright desert soils reflect a large portion of the solar radiation. Tropical latitudes, because they are warmer, generally emit more longwave radiation than high latitudes.

Fig. 4.9. Annually averaged radiative fluxes as observed by satellite. Fluxes poleward of latitude 60° are unreliable and are not shown. (a) Annual absorbed solar radiation. Regions absorbing more than 275 W m^-2 are shaded. (b) Annual outgoing longwave radiation. Regions losing more than 250 W m^-2 are shaded. (c) Annual net radiation (absorbed solar minus outgoing longwave). Regions with a net loss of radiation are shaded. Data provided by the National Center for Atmospheric Research (Boulder, Colorado)

Annual outgoing longwave radiation at the top of the atmosphere is generally greater than 250 W m^-2 between latitudes 30° S and 30° N. Regions of high precipitation, which have low longwave fluxes because of the deep, cold clouds associated with precipitation, are an exception. Between latitudes 30° S and 30° N, this heat loss is less than the heat gained from solar radiation. In general, there is an annual excess of solar radiation gain over longwave radiation loss in the tropics and a deficit at latitudes poleward of 35°-40°. This unequal geographic heating is an important determinant of Earth’s macroclimate at continental to global scales.