An instrument used to measure the intensity or degree of blueness in the sky, historically used in meteorology and atmospheric studies.
From cyano- (Greek kyanos 'dark blue') + -meter (Greek metron 'measure'). This 18th-19th century scientific instrument emerged when meteorologists wanted to quantify sky color as an indicator of atmospheric conditions.
In the 1700s, scientists like Horace-Bénédict de Saussure actually thought sky blueness could predict weather—the cyanometer was their tool, though it turned out sky color wasn't as reliable as they hoped for forecasting.
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