Shortwave Radiation Basic Concepts

Extraterrestrial Radiation

The shortwave solar radiation at the top of the earth's atmosphere is called extraterrestrial radiation. Extraterrestrial Radiation is a theoretical amount of solar radiation that would be available at the Earth's surface in the absence of the attenuation by the atmosphere.  Variation in extraterrestrial radiation is caused by two sources, the first being the variation of radiation emitted by the sun and the second being the distance from the sun to the top of Earth's atmosphere. Earth is closest to the sun on January 3rd and farthest from the sun on July 4th. The figure below shows that extraterrestrial radiation varies between 1400 $//<![CDATA[ \begin{array}{l}W/m^2\end{array} //]]>$  and 1330 $//<![CDATA[ \begin{array}{l}W/m^2\end{array} //]]>$ based on the time of the year.

The extraterrestrial radiation at the atmosphere's surface a mean distance from the sun and perpendicular to the direction of radiation is known as the solar constant and is generally accepted as  1366.1 $//<![CDATA[ \begin{array}{l}W/m^2\end{array} //]]>$. The extraterrestrial radiation, Gon, on any day of the year can be estimated with the equation below:

where,

$//<![CDATA[ \begin{array}{l}G_s_c\end{array} //]]>$  is the solar constant in W/m^2

N is the day of the year measured from January 1

On a cloudless day, about 70% of the extraterrestrial radiation reaches the earth's surface. With dense cloud cover, about 25% of the extraterrestrial radiation can still reach the land surface in the form of diffuse energy.

Shortwave Radiation and the Energy Budget

Downwelling short-wave radiation is composed of direct solar beams from the sun and diffuse components in the atmosphere. Upwelling short-wave radiation comes from reflection at the surface. The ratio of upwelling reflected back to the total radiation that reaches the earth's surface is albedo and is typically about 30% of the total radiation.

Spatial and temporal variations impact the surface energy balance of shortwave radiation's absorption and reflection. Open water absorbs nearly all solar energy that reaches it. Green vegetation absorbs most of the visible spectrum and is reflective in the near-infrared spectrum. Dead or dormant vegetation reflects up to 25% of the visible spectrum. Freshly fallen snow is the most highly reflective naturally occurring surface cover and reflects over 80% of all solar radiation however, this reflective potential decreases as the snowpack ages.

The portion of solar radiation absorbed heats the earth's surface and ultimately results in one of the following phenomena:

• heat is conducted into the soil column
• heat is radiated to atmosphere as thermal radiation
• sensible heating of atmosphere by direct contact with air
• latent heating of water via evaporation