As discussed above, atmospheric absorption and scattering are the main cause why the solar spectrum on the earth surface –
the terrestrial spectrum – differs from the spectrum above the atmosphere – the AM0 spectrum.
The figure on the left summarises the variable influences on the terrestrial solar spectrum. The solid lines represent the transmittance
given under standard atmospheric conditions, while the dotted curves give an impression of its variability:
- the ozone density is reduced in the ozone hole over Antarctica,
- NO2 is shown in medium and heavy pollution (NO2 gives smog the brownish color),
- Water vapor absorbs more under humid, tropical conditions,
- Mie scattering is shown on a very clear day or a very hazy day.
The figure on the right shows the influence from the air mass, i.e. in dependence on the sun's elevation angle over the horizon.
The haziness and other atmospheric properties are kept constant in these calculations.
It is apparent from the two figures that the largest variability of the terrestrial spectrum comes from the air mass and from Mie
scattering (at water droplets and dust particles, generally called aerosols), then followed by humidity and pollution.
Note the reduced blue part of the spectrum at higher air mass. This is more apparent in the lower half of the figure on the right,
where the ratio of the photon flux to the AM1 spectrum is plotted.
Many of these factors act on specific regions of wavelengths, so the terrestrial solar spectrum varies over a day not only by a
general scaling factor, but in shape.
This puts high demands on defining a reference spectrum, as will be outlined on the next page.