Energy and Radiation

Shortwave radiation

Shortwave radiation from the Sun speeds through space to the outer edge of the atmosphere unimpeded by the vacuum of outer space. Once solar radiation penetrates the outer edge of the atmosphere a portion is absorbed and some is reflected or scattered back to space before it reaches the Earth's surface. 

Figure 4.* Scattering by particles in the atmosphere causes a beam of light to be broken into several weaker beams of light.

Over a quarter of the solar radiation available at the top of the atmosphere is reflected or scattered back to space by particulates and clouds before it reaches the ground. Gases of the atmosphere absorb another 21% of incoming solar radiation. Forty five percent of the energy available at the top of the atmosphere is absorbed by the surface as direct and diffuse solar radiation. Direct solar radiation (S) is shortwave radiation able to penetrate to the surface without being affected clouds, particulates or other atmospheric constituents. Diffuse radiation (D) is shortwave radiation that has been scattered by gases, particulates, and aerosols in the atmosphere. Scattering is a process whereby a beam of radiation is broken down into many weaker rays and redirected in other directions. Together, direct and diffuse shortwave radiation accounts for the total incoming solar radiation or insolation (K↓). In equation form:

K↓ = S+D 

A portion of incoming solar radiation is absorbed by the surface and a portion is reflected away. The proportion of light reflected from the surface is the albedo (a). Albedo ranges from 0 for no reflection to 1 for complete reflection of light striking a surface (see Table ER.1 for sample albedos). Albedo can be expressed as a percentage (albedo multiplied by 100) that for some is easier to understand. For instance, grass has an albedo of about .23, or 23% of the light striking it is reflected away. On the other hand, highly reflective surfaces like snow have an albedo as high as .87, or 87% of sunlight is reflected away. The amount of reflection (K↑) is given by the following equation:

K↑ = (S+D)a

Figure 4.** Darker urban surface absorb more heat during the day causing the urban heat island effect.

The amount of reflection largely depends on surface color, darker surfaces reflect less than lighter colored ones. For some surfaces like water, the sun angle affects albedo. If you have been out on a lake you might have noticed that sunlight reflects off the water more when it is lower in the sky than it does when it is overhead. 

Net shortwave radiation is the difference between incoming and outgoing shortwave radiation, or the amount of absorbed shortwave solar radiation. Net shortwave radiation is expressed as:

K*= (S+D) - (S+D)a

During the day, K* is a positive value as incoming always exceeds outgoing shortwave radiation. At night, K* is equal to zero as the Sun is below the horizon.     

Previous | Continue