Bird Clear Sky Model

The Bird Clear Sky Model, authored by Richard Bird, is a broadband algorithm that produces estimates of clear sky direct beam, hemispherical diffuse, and total hemispherical solar radiation on a horizontal surface.

Bird Clear Sky Model


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The model is based on comparisons with results from rigorous 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 rigorous 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, and water vapor are fixed for the entire year.


Simplified Clear Sky Model for Direct and Diffuse Insolation on Horizontal Surfaces, NREL Technical Report (1981)