standard overcast sky

A completely overcast sky whose luminance is defined by the Commission Internationale de l’Eclairage (CIE), used in tests of the adequacy of lighting. The light from a CIE standard overcast sky is non-directional in relation to the points of the compass. It is brightest at the zenith (vertically overhead) and gets duller towards the horizon (to 1/3 of the value at the zenith).

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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