Bird Clear Sky Model  
The Bird Clear Sky Model, authored by Richard Bird, is a broadband algorithm which produces estimates of clear sky direct beam, hemispherical diffuse, and total hemispherical solar radiation on a horizontal surface. The Excel^{@} Spreadsheet implementation was done by Daryl Myers. The model is based on comparisons with results from rigourous radiative transfer codes. It is composed of simple algebraic expressions with 10 user provided inputs. Model results should be expected to agree within ± 10% with rigourous radiative transfer codes. The model computes hourly average solar radiation for every hour of the year, based on the 10 user input parameters; however variable atmospheric parameters such as Aerosol Optical Depth, Ozone, Water vapor are fixed for the entire year.
ERROR NOTE:
BEFORE 2101 GMT 08162012 Users who downloaded the BIRD CLEAR SKY model spreadsheet before 08/16/2012 should correct Column H, the HOUR ANGLE CALCULATION. Cell H3, and all others in column H, where Hour angle is calculated, needed rearrangement of parameters in the equation in order to calculate solar noon correctly. [NOTE: the value 12.5 represents shift of solar geometry calculations to midpoint of the hour ending at hour indicated]
The incorrect formula reads:
To correct a version downloaded before 18:00 GMT on 08/16/2012: UNPROTECT the sheet (Password BIRD); edit H3 by inserting the correct equation, and copying down the column. Then Reprotect the sheet (best to use the same password, BIRD). Our apologies for any inconvenience.

Note:
Mac users may find that the download fails. If this happens, doubleclick on the file icon in the Downloads window.
This applies Mac users running Firefox, byut may also apply to others.