Solar radiation refers to the electromagnetic radiation that reaches the Earth from the Sun. At an average distance of 150 million kilometres from the Sun, the outer atmosphere of Earth receives approximately 1367 W/m² of insolation (World Meteorological Organisation). This varies by around ±2% due to fluctuations in emissions from the Sun itself as well as by ±3.5% due to seasonal variations in distance and solar altitude.
The radiation from the Sun is spread over a wide frequency range. As discussed in the colour temperature topic, the greater the temperature of an object, the shorter the wavelength of its radiant emissions. Solar radiation contains electromagnetic wavelengths as short as 0.2mm (ultraviolet) with maximum energy centred at around 0.4mm (visible blue light).
Figure 1 clearly shows that the majority of solar radiation occurs in the short-wave visible and ultraviolet portions of the electromagnetic spectrum. There is some long-wave component of infrared, however large bands of this are absorbed by gasses and particles within the upper atmosphere. For information on how this energy is generated, see the topic on the Sun.
Ultraviolet (UV) radiation makes up a very small part of the total energy content of insolation, roughly 8%- 9%. The visible range, with a wavelength of 0.35mm to 0.78mm, represents only 46%-47% of the total energy received from the sun. The final 45% of the sun's total energy is in the near- infrared range of 0.78mm to 5mm. In addition to the spectrum of solar radiation there is a spectrum of terrestrial radiation that fills out the far-infrared range spanning from 3 to 75mm. These are basically the heat radiating from the surfaces of materials that have been warmed by the sun.
As solar radiation passes through the earth's atmosphere, some of it is absorbed and scattered (25%) by air molecules, small airborne particles, water vapour, aerosols and clouds. Some of the radiation is reflected straight back out into space (usually around 20% but much more with increased cloud cover) whilst the rest arrives somewhere on the Earth's surface. Once the radiation arrives at the surface, some of it is immediately reflected back into the sky. This amount depends on the nature of the actual surface - fresh snow can reflect up to 95% whilst desert sands reflect 35-45%, grasslands 15-25% and dense forest vegetation 5-10%.
It is the scattered component that makes the sky look bright and provides the ambient diffuse daylighting used in buildings. Without it, the sky would look just as black as it does at night, with the sun being a very-very large and bright star occasionally passing through it. It just happens that most of the particles in the atmosphere that are responsible for scattering are around 0.5 microns in size. As radiation with longer wavelengths simply ignores these particles, higher frequency (shorter wavelength) radiation tends to be scattered more. This is what makes the sky appear blue - as lower frequency red and yellow light pass almost directly through whilst blue light is bounced about all over the place.
If you look closely at Figure 1 you will notice significant differences between the spectral content of the radiation reaching the outer atmosphere and that actually reaching us on the surface. This is due to the absorption of some of the radiation when a gas molecule or particle retains some of this energy as heat. There are noticeable dips in the solar spectrum that coincide with the absorption characteristics of different gasses. Whilst some of this absorbed heat finds its way to the surface as long-wave radiation, the vast majority is simply re-radiated back out into space.
The Earth's orbit around the Sun is not circular but elliptical, meaning that it is closest to the Sun in late Summer and farthest away in late Winter. However, this has only a slight effect on the intensity of solar radiation. Of more importance is the axial tilt of the Earth at 23.45°. Both mean that the path of the Sun through the sky changes significantly throughout the year.
This has an effect because the lower the Sun is in the sky, the more of the Earth's atmosphere the solar radiation has to pass through in order to reach the surface, thus the more scattering and absorption it is subjected to. Also, the greater the angle the direct radiation makes with the ground, the greater the surface area its energy is spread over, reducing its intensity and its heating capacity as per the Cosine Law.
Of even greater significance is the effect of cloud cover. Whilst a cloudy sky can actually increase the amount of diffuse solar radiation, a heavy rain cloud can reduce the direct component to almost zero. As there is generally an increase in cloud activity during the colder or wetter months, these factors combine to produce a significant seasonal variation in available solar radiation.
The solar radiation that passes directly through to the earth's surface is called Direct Solar Radiation. The radiation that has been scattered out of the direct beam is called Diffuse Solar Radiation. The addition of the direct component of sunlight and the diffuse component of daylight falling together on a horizontal surface make up Global Horizontal Solar Radiation.