There are three (basic) atmospheric processes that modify the incoming solar radiation passing through our atmosphere, at around 340W/m2. These processes (Scattering, Reflection, Absorption), act on the radiation when it interacts with gases and suspended particles found in the atmosphere.
Scattering occurs when small particles and gas molecules diffuse part of the incoming solar radiation (in random directions) without any alteration to the wavelength of the electromagnetic energy, but does, however, reduce the amount of incoming radiation reaching the Earth’s surface. A significant proportion of scattered shortwave solar radiation is redirected back to space. The amount of scattering that takes place is dependent on two factors: wavelength of the incoming radiation, and the size of the scattering (dust, ice, water) particle or gas molecule.
In Earth’s atmosphere, the presence of a large number of particles (with a size of about 0.5 microns), results in shorter wavelengths being preferentially scattered. This factor also causes our sky to look blue because this colour corresponds to those wavelengths that are best diffused. If scattering did not occur in our atmosphere the daylight sky would be black.
Some gases and particles in the atmosphere have the ability to absorb incoming insolation, (angle, power received at the Earth’s surface per unit area on a horizontal area).
Absorption is defined as a process in which solar radiation is retained by a substance and converted into heat energy. The creation of heat energy also causes the substance to emit its own radiation. In general, the absorption of solar radiation by substances in the atmosphere results in temperatures that get no higher than 1800° Celsius. According to Wien’s Law, bodies with temperatures at this level or lower would emit their radiation in the longwave band. Moreover, this emission of radiation is in all directions so a sizeable proportion of this energy is lost to space.
Reflection is a process where the photon is redirect back into space after it strikes an atmospheric particle (as opposed to scattering), resulting in a one hundred percent (100%) loss of the insolation. Most of the reflection in our atmosphere occurs in clouds when light is intercepted by particles of liquid and frozen water. The reflectivity of cloud can range from forty (40%) to ninety (90%) percent.
Sunlight reaching the Earth’s surface unmodified by any of the above atmospheric processes is termed Direct Solar Radiation. Solar radiation that reaches the Earth’s surface after it was altered by the process of scattering is called Diffused Solar Radiation. Not all of the direct and diffused radiation available at the Earth’s surface is used to do work (photosynthesis, heat generation, evaporation, etc.). As in the atmosphere, some of the radiation received at the Earth’s surface is redirected back to space by reflection.
The reflectivity or albedo of the Earth’s surface varies with the type of material that covers.