Sky Cover

Clear, partly cloudy or overcast — how the sky changes over the day and year · Brown & DeKay, Technique A8
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About the presets: these load TMYx weather served from the CODEX epw-data/ folder. If a file isn't reachable, the tool shows clearly-labelled synthetic sample data so the method is still demonstrable.
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Sky Cover Monthly Normals
Share of days in each month that are clear, partly cloudy, or overcast (classified by mean daytime sky cover)
Mean Hourly Sky Cover

How to read this

A day counts as clear when its mean daytime sky cover is 0–2 eighths, partly cloudy at 3–6 eighths, and overcast at 7–8 eighths — classified over daylight hours only, since that is what matters for daylighting and solar gain. The monthly normals answer “which condition predominates?”; the heatmap shows how cloudiness shifts across the day and the seasons.

This is the data side of the Light section: pair it with the Daylight Availability tool. The overcast share sets your worst-case (CIE overcast) daylighting design sky, while the clear share tells you how much clear-sky solar and direct-sun control to expect — which in turn feeds the shading question.