# 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.

## Publications

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