The state or quality of being cloudy; the degree to which clouds cover the sky or the degree to which something is unclear or obscured.
From 'cloudy' (cloud + -y, 'full of clouds') plus the abstract noun suffix '-ness' (from Old English), which turns adjectives into nouns representing the state of being that adjective.
Scientists measure cloudiness in 'oktas'—a sky is divided into 8 parts, and the cloudiness is how many of those parts clouds cover, so you can have exactly '3 oktas' of cloud cover if you want to be precise about a partly cloudy day!
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