Illuminance on Tilted Surfaces

[1] The daily total of radiation balance is computed for all 24-h intervals avail­able. If at least one hourly total value is missing, the daily total is not calcu­lated. For a missing data symbol, “-” is entered in the table instead.

[2] In computing daily totals, sunrise and sunset hour intervals are specially treated to avoid undue reduction in daily totals due to the gaps in the data for those hours:

If observational data are missing for the mentioned hour intervals on a number of days, the solar elevation angle e is computed for those hours.

[3] Site coordinates (latitude, longitude, elevation, time zone)

• Station pressure (calculated from site elevation and latitude if unknown)

producing the ai… di coefficients as a function of epsilon bin (1, 2, … , 8) are given in Table 8.4.

If precipitable water vapor is not available, it can be estimated using the Garrison approximation based on relative humidity (Equations 3.14 to 316 of Section 3.4.1, Chapter 3), or Perez proposed the following approximation based on dew point tem­perature Td (°C):

W = e(007 Td – 0 075) atm-cm (8.9)

In the same fashion as for the irradiance version of the Perez model, computing the diffuse illuminance on a tilted surface depends on the application of the “anisotropic coefficients” Fj and F2 described in Section 6.4, Equation 6.12 and repeated here:

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