Perez All-Weather Sky Model

The Perez All-Weather Sky Model is a mathematical model used to describe the relative luminance distribution of the sky dome and has become the de facto standard model for daylighting calculations, as it uses real data gathered from weather stations all over the world.

The data for any given date and location isn't necessarily the average for that date at that location, but is instead the data that best represents the conditions on that date in that location. For example, data at a given weather station may have been collected over a 20-year period. The data for May 20 may be from 2008, as it best represents the weather and sky conditions on May 20, but the data for May 31 might be from 2002, as a best representation for the conditions on that date.

The two parameters that the Perez Model uses are delta (representing sky brightness) and epsilon (representing sky clearness). These parameters are determined from the measured diffuse horizontal and direct normal irradiance values for specific sites and date/time combinations. Diffuse Horizontal Irradiance is from the sky alone, measured horizontally. The units are watts per square meter. Direct Normal Irradiance is from the sun alone, measured by an irradiance meter aimed directly at the sun. Its units are also watts per square meter.

The weather database also includes the surface dew point temperature, which is considered by the Perez Model. This parameter is associated with relative humidity and influences the quantity and scattering of light as it penetrates through the atmosphere.

Sunrise, Sunset and Twilight: See Daylighting Overview.

Downloading the Weather Database: See Weather Database - Procedures.