The spectrum of the Sun's solar radiation is similar to that of a black body with a temperature of 5,800 K. Roughly half of it lies within the range of the visible shortwave portion of the electromagnetic spectrum. The other half resides mostly in the near infrared portion, just beyond the visible wavelengths. A small amount lies in the ultraviolet range.
Incoming solar radiation, also referred to as insolation, is a measure of the solar radiation energy that is received on a specific surface area for a specific amount of time. It can be expressed as average irradi-ance in watts per square meter (W/m2) or kilowatt-hours per square meter per day (kW-h/m2-day). The surface in this case would be the surface of the Earth. When the insolation strikes a surface, some of it will be absorbed, causing radiant heating of the object, and the rest will be reflected. Each surface reacts differently, depending on its orientation, texture, reflectivity, and other physical properties. The amount of radiation that gets either absorbed or reflected depends on the object's albedo. An object with a high albedo will reflect most of the isolation reaching it; an object with a low albedo will absorb most of the isolation reaching it. Surfaces with a high albedo usually remain cooler, such as an ice sheet. Objects with a low albedo usually heat up faster because they absorb more energy (in the form of heat), such as asphalt roads in urban areas.
The direction, or orientation, of an object also influences how the insolation reacts with it. Insolation is greatest when a surface faces the Sun. Surfaces at a large angle from the Sun result in less insolation. The larger the angle, the less insolation reaches it without being reflected off. This is why the polar areas are colder than the equatorial areas.
Several interactions can occur to insolation once it enters the Earth's atmosphere. If it does not have to interact with any particulates, gases, or other components in the atmosphere, then much of the energy will make it to the Earth's surface as "direct insolation." If, however, there are a lot of particulates in the atmosphere, which interfere with the incoming radiation, it will cause the solar radiation to be scattered and reflected in a process called "diffuse insolation."
The incoming solar radiation travels as wavelengths at the speed of light. Over a year, the average solar radiation arriving at the top of the Earth's atmosphere is about 1,366 W/m2; and the radiant power is distributed across the entire electromagnetic spectrum. Although the Sun's radiant power is distributed across the entire electromagnetic spectrum, most of it is centered in the visible portion of the spectrum. When the Sun's rays enter the atmosphere, they are attenuated—or weakened—so that they are roughly 1,000 W/m2 for a surface that is perpendicular to the Sun's rays at sea level on a clear day.
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