Overview

This app lets you interactively experiment with the latest Perez All-Weather Sky Model, which is the basis for the CIE Standard General Sky methodology described in both ISO 15469:2004(E) and CIE S 011/E:2003. This is basically a mathematical model for calculating representative spatial distributions of daylight over the sky dome for a range of different weather conditions.

The daylight distribution is governed by five (5) distinct sky coefficients, each of which you can change in real time to see their effect. The standard also defines a set of 15 standard sky types that represent different sky conditions - from overcast to cloudless and with varying levels of direct sunlight. An additional 16th type is given that is based on a much simpler mathematical model for overcast skies defined and used by CIE prior to this standard. Each sky type is generated by a specific configuration of these five coefficients.

The values for each sky coefficient can also be calculated from measured values of direct and diffuse illuminance and/or irradiance. Direct beam refers to sunlight coming directly from the Sun and its corona, as measured on a surface whose normal points directly at it. Diffuse horizontal refers to skylight coming from the rest of the sky dome as measured on a completely unobstructed horizontal surface. This means that annual hourly weather data can also be used to simulate dynamic sky conditions at different times within the year.

Background

This app was developed as part of my work on a series of daylight and solar analysis tools, primarily to help visually verify and debug my implementation of the CIE standard. The other key aim was to understand just how smoothly the model transitions between sky types when iterating through time using actual measured hourly weather data. It has already provided several useful insights that have helped me significantly refine the process of interpolating between skies and aggregating at sub-hourly time intervals.

© Dr. Andrew J. Marsh, 2018.