Comparing Eg from Different Sites

When comparing sites it is convenient to consider annual totals of Eg#, since seasonal variations can be large and vary greatly areally. However, Eg# varies with altitude and latitude. One way to normalize data from different sites is to determine the transmission of a unit atmosphere, which is similar to turbidity [6,23]. Yearly means of Eg# are converted to atmospheric transmittance, tm, by dividing by integrated yearly extraterrestrial solar irradiance on a horizontal surface (So) computed for the latitude of the measurements, that is,

Transmittance is also an exponential function of the optical thickness of the atmosphere k, and the vertical non-dimensional air mass, m, such that tm = exp(—km)

For a unit air mass (m =1) Eqn (2) yields t1 = exp(-k) = exp(ln(tm)/m) (3)

Values of t1, which expresses the yearly average transmittance of a unit atmosphere at the site, are computed for each yearly mean of Eg#, where m is computed from site altitude using a simple altimetric relationship like:

or and A is site altitude (m) (after [6]). A second method to normalize data from different sites is multiple regression of Eg# on time and site parameters, where the influence of altitude is taken as linear, but site latitude (F) is taken as cos3(F) [23].

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